AudioCircle
Audio/Video Gear and Systems => Open Baffle Speakers => Topic started by: scorpion on 9 Dec 2006, 10:03 pm
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Ever since MJK answered my question about air cavity resonances between the 12" parallell backwings in his OB-design I have been
thinking over his answer, http://www.audiocircle.com/index.php?topic=32919.110
For the 'regular U-baffle (Box with no back) at least with dimensions covering one or two units, 12" in my case, there is no doubt that you
will have a well accented resonance coinciding with the lengt of the U-baffle. In the same time H-baffles do no have as pronounced rersonance
as U-baffles. But in both cases these are measurable with simple RS Sound Level Meters. I do not think that you will need more sophisticated equipment.
The intention here is to start a discussion about what happens if we allow another degree of freedom, that is to open up the top.
Does this fundamentally change the situation ? Or do we have to calculate carefully with our dimensions ? As I see it if we can have more knowledge
in this respect it could give us important guides to OB-design especially with wings.
I, myself as a layman is not the man to answer these questions, but I think there are members with good theoretical and pratical knowledge that can contribute
a lot in trying to answer these questions. :)
/Erling
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A good question, and having built an H baffle with neither top nor bottom, strictly for bass, I can tell you that it works fine.
I am sure that a proper H baffle with both top and bottom would produce different bass, but the H I used with my 12 inch fosgate in it worked fine.
I would guess that the baffle is "smaller" effectively speaking, but with careful consideration for overall design, I am sure a workable compromise could be sorted out.
(http://www.audiocircle.com/image.php?id=2617)
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Mark,
Yes, this is a good illustration, 'topless' but not 'bottomless'.
/Erling
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Right, floor provides the bottom, correct.
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Scorpion,
If it's only in the augmenter's section, resonances are easily cured with some stuffing
without ill effects. If you run U's for your main driver, then you have to contend with
1/4 wave as well as standing wave resonances. MJK threw me for a loop when he said
angled sides couldn't prevent resonances, because I've built a number of resonance free
U shaped cabs with main drivers within the cavity. I've also built open backed straight
pipes out of wood for main drivers that required no stuffing to eliminate resonance (pm
me if interested).
To avoid resonances, I never sweep the sides back 90 degrees and I always use slightly
different depth sides, even if they are tapered. I also start wings as close to the driver
cutout as possible. Since I started building winged OB's this way, the only time I have
resonance problems is with the cabs backed up very close to wall, which results in a
hollow resonant sound, apparently due to reflections between the wall and the cavity, but
if that placement is required some stuffing in the cavity works fine.
I have tried some of these U shaped (more V shaped) cabs without any top cap, but
the only time I didn't like the sound better with a top was an OB line array. There's something
about the shortest distance being over the top that is bothersome to my ears, but it's hard
to explain.
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Not exactly relevant to topless U's but you may find this of interest.
http://www.musicanddesign.com/NaO-II-U-frame.html
It's about how to design a U-frame woofer and how to tune the damping of the 1/4 wave resonance for optimum performance.
Simply removing the top of the U will not only alter the resonance behavior but also significantly impace the low frequency performance.
John k...
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Thank you John,
I am familiar with your work on U-baffles and have been playing around quite a bit with your and MJK's spreadsheet for
U- and H-baffles.
My interest in this question with regard to bassreproduction is that I would like to have some kind of general dipole-bass
of as small dimensions as possible beeing usable up to 250-300 Hz. But still going down real low into the 30ies and with as few things as possible to correct for.
You wrote in the 'Edge diffraction' thread and published measurements comparing flat and winged baffle rear response.
Bass was quite augmented with the winged baffle but I could not trace any resonance phenomena in your measurements.
So I think there is a case for my question.
/Erling
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Erling,
Some brought that Jussi brought up in his OB thread at DIYAudio is that there is a
sonic difference (not just response difference) in the bass region between U's and
dipoles. It relates to placement and the difference in reflections due to the very
different polar response of the two. He mentioned that U's tend to sound better
with placement near the front wall, and that dipoles sound better than U's if placement
is near the side walls. Playing around with placement recently of some of my U's, I'd
have to agree with Jussi.
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Not to contradice Jussi, but the difficulty in comparing U's and conventional dipole woofer is that a dipole woofer is pretty much a dipole woofer, whether it be on a flat baffle, and H frame, W frame, etc. But the performance and polar response of a U is highly dependent on the damping of the rear wave. Either system will have a 6dB increase in SPL if place near a side wall.
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Had to get out of the wifes way for some time today. So I started to clean up my harddisk a bit and found this:
(http://rudolffinke.homepage.t-online.de/audio/GBS515/H_frame0.gif)
It´s a comparison between a H baffle with opened and closed top. Mic position was in the red circle (center of back opening). Black curve is with closed top, red with open top (as shown). Interpretion is up to everybody himself. :scratch:
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Rudolf,
I have seen that there has been some dispute over the GBS 515's at diy-Audio. However the measurements are
innovative, it is a pity that SL didn't do the same for his Protos, but we will go further. :D
/Erling
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I don't know why I posted this in another thread, but I'll repeat it here.
"I was reading the thread on Open top U-frames and made some quick measurements comparing the rear radiation from a flat baffle dipole (11" wide) and one with 8.5" deep side wings, straight back. These are pretty deep wings, but the point is that once you start adding wings things become more complex, for better or worse. Green is rear w/o wings, red with wings."
(http://www.musicanddesign.com/pubimages/Addiocircles1.gif).
DYI, the baffle and wings are about 40" tall.
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John,
What's causing the 10db higher SPL in the rear radiation with wings below 300hz?
Are they very nearfield measurements? That's the kind of graph I would expect
on the front side.
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Seems pretty obvious to me. It's because "D" is larger.
The front and back responses will look somewhat different, but the "winged" baffle will show more SPL because the cancellation from front/back is less effective.
Cheers,
Davey.
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Davey,
That was my knee-jerk reaction too, until I read JohnK's post again and realized
he said measured in the rear. I'm hoping John actually measured in front, otherwise
I am confused. I like those measurements if they are from the front, because they
would really make the case for wings on bass augmenters (+10db essentially for free
up to 300hz is about equal to 3 times as many drivers).
Measuring from the backside 90 degree wings don't have any effect on D. That's
why a perfect U-Baffle has the null to the rear, since the front and rear wave travel
distances are closest to equal along the rear axis. Wings do act as a waveguide
containing the rear radiation for their length, which would distort very nearfield
measurements.
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No, I didn't knee-jerk. I read it a couple of times.
I am assuming these measurements are from John's NaO Mini system with and without the wings. I also believe the measurements were not very close or nearfield because of the roll-off in both measurements which is approximately 6db/octave.
Whether the measurements are taken in front or back (back in this case) "D" is larger for either measurement when the wings are attached and thus the SPL output will be larger and the "Fequal" point changed.
I'm sure John will elaborate the next time he stops by this forum.
Cheers,
Davey.
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The only point of the measurement was to see if the wings changes the radiation impedance seen by the back side of the driver. The winged measurements were taken in the exit plane of the wings. I forget where the unwinged measurement was taken. But the point here is that there is an obvious resonance at about 550 Hz in the winged rear SPL which is absent from the unwinged response. The conclusion is that there must be a change in radiation impedance due to the wings even though the top is opened. I'll try to look at it again and maybe with different length wings, but right now I'm rather busy with other projects.
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Davey,
Nearfield explains the increased bass SPL primarily because the output has
been constrained, so there's naturally more pressure. "D" is the difference
between the front and rear waves' travel distance to the listening position.
eg You can cut the front of a typical H frame baffle all the way back to
the driver mounting baffle, and D is unchanged. The radiation pattern
changes from dipole toward cardiod.
wrt The link to SL's discussion of D that you gave someone at DIYaudio,
one of SL's diagrams is misleading, the one for an H baffle. While in a
symmetrical H baffle D happens to equal the depth, the proper measure of D
is from the back the driver around the rear edge and back forward to the
point even with the baffle plane. That single diagram has prevented most
from understanding how folded baffles affect bass. That's why H baffles
are common and U's aren't, yet H's are much more compromised, starting
with double the floor space in a wasted manner.
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John,
With all due respect, I think you need to read the information on Siegfried's and John K's pages more carefully.
And start learning to take some measurements to confirm your theories. SoundEasy (one option) is a really nice program with extensive testing suite and John K. is a wizard with it so you will have expert help.
Cheers,
Davey.
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Davey,
I am not sure where you found John to need reading SL more carefully. I believe John is right in regarding SLs H baffle diagram as misleading:
(http://rudolffinke.homepage.t-online.de/audio/Dipol/SL_H-baffle.gif)
In part "a" and "b" the distance "D" is following the cone positions (source of "+" and "-"). SL should have followed this in "c" too (like the red pathway I drew in his diagram).
Instead he connected "D" with the front and back of the H baffle (and transfering "+" and "-" to those positions too). And this, I believe, has been a source of some misinterpretation.
John is quite right in his remark "You can cut the front of a typical H frame baffle all the way back to
the driver mounting baffle, and D is unchanged. The radiation pattern changes from dipole toward cardiod." This implies, of course, that you are only looking at the front of the baffle.
Where I can´t follow John is his comment "That's why H baffles are common and U's aren't, yet H's are much more compromised, starting with double the floor space in a wasted manner." Cardioids and dipoles are different kettle of fish and it is no use to plainly regard one better than the other. It strictly depends on the individual design goals.
Rudolf
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Rudolf,
Try it with a measurement. Use diagram "c" for one configuration and then diagram "c" with the front portion cutoff for another measurement. There's going to be a noticeable difference in the measurement. I've done this myself.
"This implies, of course, that you are only looking at the front of the baffle." There you go! You have put your finger right on it.
Yes, the radiation pattern has changed because it's now asymmetrical. I understand that.
I think we might be getting wrapped up in semantics here unfortunately. I understand that "D" could be considered (depending upon how you look at) unchanged, but it's really not. It's now "D/2" and for John to state that the "H"-style baffle could simply have its front half removed is not correct. It's now a different configuration and the operation has been changed. I believe I understand John's thinking (there's a scary thought) and why he's having difficultly getting his brain wrapped around this one, but I believe it would be more clear to him if he did some testing of his own a real-world example. A person learns much more by doing than reading on an internet forum. :)
http://www.musicanddesign.com/u_frame.html
Interesting thread, but a lot of speculation and subjective-only comment. :)
Cheers,
Davey.
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U's advantages over H's:
1. Essentially half the floor space for the essentially same bass response.
2. U's have better very deep bass extension, more like a box, but H's are limited
by the room dimensions. See JohnK's tech studies regarding room response.
3. Less wood, so less cost.
4. Bracing for proper construction is easier to hide with U's. I've never
seen an H construction that addressed panel vibrations.
5. Typically U's can be used higher in frequency because resonances are
addressed by damping in the cavity. This may be possible with H's yet I've
never seen damping of H's on the backside, much less on the frontside
which would also be required.
The only advantage I see for H baffles is that they are easier, since with U's
they don't sound right until you tune them with damping in the cavity. In-room
response is different, but I find that U's still enjoy most of the same benefits
over boxes that dipole's are known for. Also, I believe that U's have a wider
area of prime listening in front than dipoles.
Something I touched on in a post elsewhere is that U's seem to sound better
than dipoles with a wall close in back. Jussi brought this up over at DIYaudio
and he speculated that this results from the difference in dispersion pattern and
reflections and how they bounce around within the room boundaries in front of
the listening position. Jussi mentioned that U's sound more true to life than
dipoles near the front wall, but that dipoles sound more natural than U's with
placement nearer the side walls. I've noticed the same, but never had an
explanation until Jussi brought it up.
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Davey,
If you've done measurements that support your contention that the panels
extending in front of the driver mounting baffle in an H configuration affect
SL's "D", then
1. You measured in the nearfield, which is invalid for bass response purposes.
2. You don't really understand the cause of the 6db/oct rolloff of OB bass, which
is determined by the phase relationship of the front and rear waves reach your
ears.
The front part of an H has no effect on the travel distance of either wave front,
or using JohnK's terminology, it does nothing to "delay" the rear wave from reaching
your ears, and it obviously doesn't affect the front.
BTW, I take real exception to you inference that I read rather than do. I've built
literally hundreds of variations of OB's and I supplement my real world experience
with online research to gain an understanding and explanation for what I hear. I
believe that in my case it would have been a mistake to get tangled up in measurements
too early on, because, just as you proved, measurements can lead to incorrect
conclusions without really understanding how OB's function. Now that I'm trying to
explore some areas of design for which research is a waste of time, I'll have to get
more formal in my approach.
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Interesting thread, but a lot of speculation and subjective-only comment. :)
Could you post a link to your measurements please? Thanks :)
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The front part of an H has no effect on the travel distance of either wave front,
or using JohnK's terminology, it does nothing to "delay" the rear wave from reaching
your ears, and it obviously doesn't affect the front.
Yes, I understand that. But remember we're talking about more listening and measuring locations than just the "front" where your ears might be located...(at distance "L.") I'm talking about the whole picture. When you "front-think" about the situation from a single point in space it does seem rather simple and all the diagrams make much more sense.
By the way, I take real exception to your inference that my measurements have led to incorrect conclusions. :) My measurements (outdoors/indoors/near/far) have in most cases confirmed the analysis/measurements that SL and John K. have put forward regarding these woofer configurations. Since these two fellas are acknowledged experts in speaker design I assumed that my measurements were valid and probably not incorrect.
Cheers,
Davey.
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Interesting thread, but a lot of speculation and subjective-only comment. :)
Could you post a link to your measurements please? Thanks :)
John,
What would you like to see?
Here's one that I just took this morning for another (different type of) project that happens to be on my desktop:
(http://home.comcast.net/~dreite/Temp/dpl.jpg)
I'll dig some out that highlight what we're talking about here and move them onto my web-space.
Actually, better yet. Here's a photo of an el-cheapo "H"-baffle prototype enclosure that I can mount a couple of ten inch drivers in. I wouldn't mind cutting the front half (or top) off of this. If you fellas tell me exactly what measurements you'd like to see I'll set this up and take a series and then cut the fronts off and take some more.
(http://home.comcast.net/~dreite/Temp/IMG_0071.JPG)
Cheers,
Davey.
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The front part of an H has no effect on the travel distance of either wave front,
or using JohnK's terminology, it does nothing to "delay" the rear wave from reaching
your ears, and it obviously doesn't affect the front.
Yes, I understand that. But remember we're talking about more listening and measuring locations than just the "front" where your ears might be located...(at distance "L.") I'm talking about the whole picture. When you "front-think" about the situation from a single point in space it does seem rather simple and all the diagrams make much more sense.
By the way, I take real exception to your inference that my measurements have led to incorrect conclusions. :) My measurements (outdoors/indoors/near/far) have in most cases confirmed the analysis/measurements that SL and John K. have put forward regarding these woofer configurations. Since these two fellas are acknowledged experts in speaker design I assumed that my measurements were valid and probably not incorrect.
Cheers,
Davey.
By all means go ahead and bring up off axis response differences in an attempt to justify your incorrect statement(s).
I can't wait to see your measurements that support them, especially since they will be in contradiction to the work
that both SL and JohnK have so generously shared.
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Actually, they won't contradict.
I'm trying to make an effort here John. Do you want to see some measurements or not? (I'm having a little trouble decoding your snide tone.)
Aren't you even curious to see me proven wrong by my own measurements? :)
Davey.
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Davey,
Those your 11" x 23" baffles ? Very generous to destroy them for the sake of our knowledge.
To reconnect with my thread start and also to JohnK's measurement and comment and Rudolf's measurement may I suggest the following
measurements:
1) H as it stands.
2) H without top.
3) U with same D as H and with top.
4) U without top.
I hope that you could manage this with the help of glue and clamps.
Please state the dimensions and speakers used. I would suggest nearfield measurement possibly at the back exit plane of the wings.
That would expose resonances I suppose and be of help in designing bass-dipoles.
What do you all say ?
/Erling
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Davey,
I'm sorry. I withdraw my tone. Please keep in mind that mine was just a
response to your post at the top of page 3. I took similar crap from Thorsten
some time ago in a discussion regarding folded baffles where he couldn't admit
being wrong, yet amazingly he's seen the light and is using them in some designs.
I'm sure all of us would appreciate measurements showing a real world comparison
of U's and H's, but please don't waste time with nearfield. Neither D nor the depth
of the H cavity in front should be significant in relation to the measuring distance
unless you want to include nearfield as well to demonstrate how it skews results.
Outdoors if possible, because placement has a significanty different impact for
dipoles vs U's.
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Scorpion,
Why the request for nearfield? I thought you and JohnK already demonstrated that
once you fold the sides back that we have to worry about resonances, and that they
are different with the top on or off. I don't think anyone disputes that having a driver
in a cavity causes resonances.
To me it would be beneficial to the entire community to put the whole dipole vs U debate
to rest. Even John Sherrin gave it a shot, but came up with invalid results by using
nearfield measurements.
If Davey declines, this is something on my long list of comparisons that I intend to measure,
along with the effects of backside cavity shapes, edge geometries, room placement, and
much more.
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I don't know if you have seen this or not. It's pretty buried in my site with no direct link from the Tech section. It's a clear description of how the U-fame works and how it is damped correctly.
http://www.musicanddesign.com/NaO-II-U-frame.html (http://www.musicanddesign.com/NaO-II-U-frame.html)
The point is that at very low frequency, well below the U resonance, the UNDAMPED U behaves as a dipole of the same D. As the frequency rises the response departs from a dipole because of the lack of symmetry between the front and rear SPL due to the U resonance. What you actually get is an ovulated radiation pattern with no nulls at the side or rear. When damping is added the low frequency response become more cardioid like with a null to the rear. This may not be a complete null, but it can be made pretty deep by correct tuning. And the on axis SPL is now what would be expected from a dipole of length 2D. The beauty of the H frame is the simplicity. The draw back is relatively high monopole = dipole frequency for a reasonable length. The beauty of the U is the 6dB improvement in sensitivity for the same "D" which translates into one driver doing what two do in an H of similar length. The draw back is the need to correctly damped. But that really isn't as hard as some would have you believe if you have some measurement gear.
John k...
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JohnK,
Thanks for stepping in, because the negative group delay of the undamped U needs to
be addressed to equal the bass performance of the double sized H. Damping the U corrects
this. WRT the double driver H baffles in Davey's pic, are those open enough in comparison
to a pipe-like U, to negate much of negative group delay compared to a pure U of similar
depth?
I remember John Sherrin mentioning that the propagation delay in a pipe is far less, and now
I think I understand what he was talking about. It's essentially the negative group delay that
you mention in your tech study about U's. It doesn't affect H's in the manner because the
negative GD happens with the front radiation too.
Davey,
The negative group delay may be the cause of the performance difference you've measured
between U's and H's in the past, if you didn't properly damp the U that was used.
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JohninCR, JohnK and others interested,
JohnK, yes I am familiar with your work on U-baffles which is important and innovative. In this regard I have two questions:
1) How accurate is the MathCad spreadsheet (FRD Consortium) you and Martin J King constructed in simulating U-baffle behaviour and with regard to damping material stuffing ?
2) Would you consider using U-baffles (box with no back) for bass up to 300 Hz ?
JohninCR, You mean me not Rudolf. I don't think bass quantity is an issue. I am more interested in quality and easiness of baffle-
construction. I think that nearfield measurement also will give some hints regading effeciency and absolutly reveal resonances.
MJK did argue that his topless U-baffles lack resonances, that might not be true and if true also might depend on a number of different things. You, JohninCR, cited an advice regarding wing construction that should help to avoid resonances, open a pair of wings 1 inch over 6 inches and that should do the trick.
My view is that measurement will help to establish some thruth in this case.
As threadstarter my interest is to be able to build as simple a bass-dipole as possible and not having to correct its performance electrically other than for dipole 6 dB fall. I personally also have an interest of using this baffle up to 300 Hz to be able to pair it with about any speakerchassis there is.
I invite all interested to express their view on this issue. :thumb:
/Erling
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Scorpion,
I agree with you, although a resonance should show up no matter where you measure,
unless with an H the resonances front and rear cancel each other out at the listening
position, which is an important consideration. :scratch:
Also, I'm always surprised how quickly +6db of U's is dismissed. We're talking about half
the drivers or half the size being required for the same output. This cuts the what I see as
the biggest problem with fully OB systems in half. :o If a side benefit of proper construction
of U's is that the resonance(s) of the cavity are sufficiently damped to permit use at a
higher frequency, then that is another big plus, especially since it doesn't entail much
construction effort, and as you mentioned, we can use most anything on top. :wink:
I believe the concept of JohnK's negative group delay is an avenue requiring further
scrutiny, especially when added to John Sheerin's statement that below the 1/4 wave
resonance the air in a pipe moves in unison, making the terminus the source. If my
understanding is correct then this is the cause of the negative GD. It seems intuitive
that an expanding pathway has the potential for reducing or eliminating this effect.
I've already found it to, at a minimum, spread or reduce, resonant behavior without
damping. Getting rid of the negative GD is important too, because we need the full
benefit of propagation delay for the rear wave from the rear of the driver around our
construction. Is stuffing the only way to skin this negative GD cat? :scratch:
This stuff is infinitely more interesting than boxes. :thumb:
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I want to make a couple of comments here. I think things are clear here in regard to cutting off the front of an H and its performance. First of all, SL's figures are all correct. I would hope the references to my two web pages would clear up the confusion about cutting off the front of an H and propagation distances. The point there was that while from the front the physical propagation distances are the same for the H and the cut off H, physical distance isn't the only issue. The acoustic delays associated with loading the driver with a duct have to be considered and these are different when the he front of the H is removed to form a U.
JohninCR,
"WRT the double driver H baffles in Davey's pic, are those open enough in comparison
to a pipe-like U, to negate much of negative group delay compared to a pure U of similar
depth?"
I really can't answer that question accurately, but my guess is that the answer is no. In Davey's H a horizontal divider could be placed between the two drivers with no effect. It's a plane of symmetry. So it's not really as open acoustically as it appears physically due to the vertical height.
Erling,
1) History of the MathCAD worksheet: When I stated looking at U's I was fooling around with TL models because the rear radiation of a U is just that of a TL tuned well above the driver Fs. I used Martin's TL codes and SoundEasy to generate the rear SPL and then I used a number of tools from the FRDC to merge and sum front and rear responses. So I approached Martin with the idea of making one of his TL sheets into a U, H simulator. I also approached the developer of SoundEasy to do the same thing. Both responded and Martin and I worked together on the MathCAD side. Martin did all the work and deserves much of the credit. I just provided guidance and assistance in defining what needed to be done. Now to your question. I found the the MathCAD sheet does provide a reasonable representation of what is going one. However I don't use it very often because a) I'm not patient enough to wait for it to run. It just takes too long to look at different cases, at least on my PC. 2) I've never been successful in translating the the results for damping into what is required for actual damping. The result is that when I look at U design I start with the ideal case to determine the maximum performance obtainable. Then I build the U and test, adjusting the damping by measurement as I outlined in the web page I referred to before. http://www.musicanddesign.com/NaO-II-U-frame.html (http://www.musicanddesign.com/NaO-II-U-frame.html)
2) Trying to use an H or a U to 300 Hz is going to be difficult. My recommendation is that the response peak (dipole peak) should be a minimum of 1/2 octave above LP crossover point. That would place the dipole peak at 424 Hz and the monopole = dipole frequency at 141 Hz (a 16" H). Now assuming you have a driver with Qts and Fs such that no additional eq is required other than the dipole eq, say Qts = 0.7 Fs = 30. Then you need about of about 19dB eq. But the response will roll off steeper than 2nd order and ultimately drop to 3rd order. If you start with a driver with low Qts and Fs, like Qts = 0.2 and Fs = 20 and can accept an Fc = 30 with Qtc = 0.5 you would need about 23dB total gain. See figure belwo. Trying to use a U to 300Hz would be more difficult. The U would have to be about 8" deep and at that point you would have to start to consider the propogation distance around the front baffle since the waffle width would be wide compared to the length and the wave length at 300 Hz is getting to the point where baffle shadowing (baffle step type effects) would need to be included. You would also need suitable woofers. I would have concernes about the Peerless XLS series due to where their break up is. Something like a pair of 10" SLS woofer migh work, but they would have lower SPL limits than the XLS.
(http://www.musicanddesign.com/pubimages/Addiocircles2.gif)
(http://www.musicanddesign.com/pubimages/Addiocircles3.gif)
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JohnK,
What are the effects of overstuffing the U? ie Is there an easy way to tell when we have too much damping
and obtain acceptable tuning without measurements?
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I believe the concept of JohnK's negative group delay is an avenue requiring further
scrutiny, especially when added to John Sheerin's statement that below the 1/4 wave
resonance the air in a pipe moves in unison, making the terminus the source. If my
understanding is correct then this is the cause of the negative GD. It seems intuitive
that an expanding pathway has the potential for reducing or eliminating this effect.
I've already found it to, at a minimum, spread or reduce, resonant behavior without
damping. Getting rid of the negative GD is important too, because we need the full
benefit of propagation delay for the rear wave from the rear of the driver around our
construction. Is stuffing the only way to skin this negative GD cat?
The old acoustic approach; low frequency, lumped parameters.... Noting wrong with that. The negative GD is a result of several things (which can be analyzed using lumped, or distributed parameters as the frequency gets higher). The duct of a U or H has capacitance and inductance and an impedance mismatch at the terminus. This all leads to the resonant behavior of the duct which leads to the negative GD. Just set up a Q resonance circuit is your cad software, Fc = 150, Q = 5, boost = 15dB or so and look at the GD below the resonance. Expanding the duct reduces the impedance mismatch and changes the C and L which can reduce the effect of the resonance and reduce the GD. Stuffing damps the re sonace and also acts as an LP filter. The damping reduces the negative GD and the LP filter effect introduces a positive GD to help offset the negative. I'm looking into other approaches as well, but unfortunately they all have one draw back. Things get physically bigger, something a stuffed U tends to avoid.
Anyway, I have to get some work done. :banghead:
Later.... :wave:
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I'm looking into other approaches as well, but unfortunately they all have one draw back. Things get physically bigger, something a stuffed U tends to avoid.
Maybe they don't have to get bigger if taming the resonance also cures the negative group delay OR dimensions are such that the pipe resonances coincide with the dipole nulls.
Here's something I did about a year ago. I was waiting to do measurements, but it's pertinent here. My approach was to hopefully prevent resonances and create different driver to edge differences around the baffle. I built a 5 sided pipe (no parallel surfaces) 16.5" deep and put a 15" coax woofer on the end. I tapered the terminus by 2" and in the end of the shortest panel I cut out a Karlson shaped "V" for an even earlier start of some pressure release. With cross bracing from the 5 corners to near the center of the opposing panel, the cab with no damping is almost free of resonance, and response ripples are quite small even though the woofer runs up well past 1khz. I also tried an identical construction just over 18" deep in an attempt to reach even deeper bass, but the deeper cab sounded terrible, so I know that dimensions play a big role, but I'm clueless wrt optimizing the dimensions. Below is a pic.
(http://1stlines.net/JCsKslot.JPG)
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Here's something I did about a year ago. I was waiting to do measurements, but it's pertinent here. My approach was to hopefully prevent resonances and create different driver to edge differences around the baffle.
I tapered the terminus by 2" and in the end of the shortest panel I cut out a Karlson shaped "V" for an even earlier start of some pressure release.
(http://1stlines.net/JCsKslot.JPG)
Once again I am reminded of the tops of the Mississippi river boat funnels, only now I am becoming convinced the old timers were doing this not so much for appearances but to detune organ pipe resonances in the interest of reducing apparent noise.
The funnels I am talking about had this sort of "Karlson V" all around the end of the huge vertical smoke stacks, then the tips were splayed out.
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I agree with JohninCR that this is fascinating stuff. And I think we are a good way through. U-baffles (Box without back) would be an obvious choice for Subs preserving the good things from dipoles and adding performance benefits. Just to get damping correct and I suppose that this could be determined with relatively simple measurements, even with our RS SPL-meters.
However I hoped that we could advance a little more with regard to 'topless U-baffles' beeing it fixed or movable wings and their advantages.
A flat baffle of course is no problem going up to 300 Hz, only dimensions and equalization. What surplus could we get from folded baffles ?
Some comparative measurements would be nice even if only bringing light on one or two cases.
/Erling
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Russell,
When I built them I wasn't thinking about 1/4 wave resonances, I just wanted to vary the
distance around the baffle without creating to big of a shortcut at the shortest dimension.
You may be right about the riverboat decorative pipe ends.
Scorpion,
These may be more like what you're wanting to measure. I need to redo the manifolds to
replace the cheapies 6"ers with something that makes real bass, but these don't resonate
at all. I tried the layout without a top, but didn't like the sound of the shortcut it created
for the main driver's output. I would caution against moveable wings with anything that
makes real bass. Even these 1" wood baffles needed bracing subsequent to these pics.
Dan named them my Flintstone cabs, since the top caps look like the roofs of houses in the
cartoon. These are going to get measured too as part of the measuring frensy I'll soon start.
(http://1stLines.net/Flintstones.JPG)
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my OB/RLH's, which remain the world's only single drive unit OB speaker
of reasonable size capable of full range at meaningful SPLs.
I can't wait to see the results for this one when measurement day comes. Hopefully you have applied for a patent on OB/RLH which I take to mean Open Baffle Rear Loaded Horn? Looks like a dandy.
Great output down to the 50's from a B200 OB/RLH. Wow. John you are a speaker designing wild man down there in CR. World class speakers popping up left and right. With a little luck we'll see some measurements soon.
cheers,
AJ
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Once again I am reminded of the tops of the Mississippi river boat funnels, only now I am becoming convinced the old timers were doing this not so much for appearances but to detune organ pipe resonances in the interest of reducing apparent noise.
The funnels I am talking about had this sort of "Karlson V" all around the end of the huge vertical smoke stacks, then the tips were splayed out.
Very interesting observation Russell. I'm impressed with the connection of riverboats and John's creation.
Very interesting indeed.
Bob
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AJ,
Once again a misapplied quote taken out of context. I was disappointed with the extension of the B200 version. The 15" version works much much better and was the subject of that quote. They will both get measured, and yes OB/RLH is "Open Baffle / Rear Loaded Horn". If Dan Wiggins comes up with a coax 10" for OB, like I asked, then I may be able to get solid performance down near 30hz using a single drive unit, OB sound, and without EQ in a size that's much more room friendly than my 15" version. A patent isn't out of the question, but there's no use going to the time and expense for something not commercially viable. The 15" is too big to be commercial, since its lack of power handling makes it inapplicable for pro use.
Since you chimed in I'll direct Davey's inapplicable comment toward me and add to it for you. Instead of being only a potshot sniper, at which you aren't very good since you pick targets that shoot back and have better aim, why don't you try to come up with something original on your own?
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Given John's amazing productivity in building baffles, I'm starting to become concerned with the deforestation issues in Costa Rica :wink:
Cool stuff, John/Russell/JohnK
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Brad,
Don't worry there are plenty of tree huggers here. Actually CR is pretty strict with its timber resource management. I try to avoid waste and make my OB's as small as possible. I even recycle my driver cutouts into stuff like my diffraction rings baffle.
Cheers,
JohninCR (living very green)
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My apologies John. But now I am even more confused. In the B200 thread http://www.audiocircle.com/index.php?topic=35192.msg313188;topicseen#new
You have this quote above this picture
B200 solo in an OB-RLH, great output down to the 50's
(http://1stlines.net/B200obrlh.JPG)
You had great output down into the 50's, but disappointed with the extension of the B200 version? :scratch:
AJ,
If Dan Wiggins comes up with a coax 10" for OB, like I asked, then I may be able to get solid performance down near 30hz using a single drive unit, OB sound
:scratch: A coax is a single drive unit? I have over a dozen coaxials and they all have a tweeter (HF driver) mounted on the central axis of a woofer (LF driver). That's two (2) drivers I'm counting on every one of them. When you said my OB/RLH's, which remain the world's only single drive unit OB speaker
of reasonable size capable of full range at meaningful SPLs.
I foolishly interpreted that as a single driver, like for example, the very B200 that you used in the Open Baffle Rear Loaded Horn. Again, my apologies for misunderstanding that a coax is a single driver (same as a midrange like the B200).
A patent isn't out of the question, but there's no use going to the time and expense for something not commercially viable. The 15" is too big to be commercial, since its lack of power handling makes it inapplicable for pro use.
You've lost me again. I was talking about a patent for Open Baffle Rear Loaded Horn itself, not a specific application of this alignment using for example, a 15" driver. Have you discussed this alignment with Wiggins or John K here? It is far beyond my expertise, so perhaps you or maybe John K could explain to this yahoo how such an alignment works? I think it's safe to say that I would not be the only one interested to find out how an open baffle can simultaneously be rear horn loaded. *Some electro-acoustic models would be wonderful. Thanks in advance.
Cheers,
AJ
* I fully understand if you would rather not post any electro-acoustic models of such a unique, patent pending design, but figured this is very much a DIY crowd and as such, would appreciate it.
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AJ,
For me 50's extension isn't full range because it still needs a sub. I said "single drive
unit" for lack of a better term to encompass single drivers and coax's, since it's not
really a single point source due to there being some output from the mouth. 2 drivers
sold and used as a single unit makes the term reasonable to me. If you come up with
something better let me know.
I tried soliciting help some time ago to no avail, so I'll wait until I at least have response
and impedance measurements.
It has nothing to do with topless U-baffles, although the top section is a U-baffle, so I
won't discuss it further here. Why don't you try contributing instead of splitting hairs?
It would make a good New Year's resolution.
Happy New Year!
John
PS- Here are some pics and drawings, since my FE108EZ baffles are topless U's (sorry
about the "Z" AJ. My keyboard doesn't have a Sigma)
(http://1stlines.net/OBs.JPG)
(http://1stlines.net/OBRLH.JPG)
(http://1stlines.net/OBRLH3.JPG)
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Happy new years John.
AJ,
For me 50's extension isn't full range because it still needs a sub. I said "single drive
unit" for lack of a better term to encompass single drivers and coax's, since it's not
really a single point source due to there being some output from the mouth. 2 drivers
sold and used as a single unit makes the term reasonable to me. If you come up with
something better let me know.
Well for many, "great output" down to the 50's would represent sufficient bass for lots of music, not to mention splendid performance from a 8" midrange driver in any type alignment, much a dipole :o
2 drivers sold and used as a single unit = single drive unit? Why not accurately use coaxial (2 drive unit) and widerange/fullrange (1 drive unit) when those terms already exist? I would not have confused the world's only single drive unit OB speaker of reasonable size capable of full range(d) at meaningful SPLs as a fullrange if you had said "coaxial".
It would be tough for me (or anyone else in the world) to come up with something better if
a) "reasonable size" is undefined dimensionally (no measurements)
b) "full range"(d) is undefined (no specifications - or measurements)
c) "meaningful SPLs" is undefined (no specifications - or measurements)
Just thought I'd let you know.
Are those drawings your electro-acoustic models of OBRLH? Do the arrows represent the horn loading of the rear wave or was this measured at some distance from the loudspeaker to confirm your theory? Or perhaps even with the Fletcher-Munson curve you could simply hear the presence of the bass horn loading with your self-trained ears? I'm not really seeing any horn loading happening there in a U-baffle open back enclosure with a hole in the bottom, so perhaps I could be enlightened as to how this was occuring. Perhaps with some actual measurements of the system now that you have this capacity? Thanks.
It has nothing to do with topless U-baffles
This thread is about topless U's, so a hole in the bottom U might not be too far off in behavior IMHO.
cheers,
AJ
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The expanding pathway is loaded in the same manner a Helmholtz panel bass traps bass, and
I used the bass trap formula as a guide to dimension the slot. The placement of the slot is at
the area of highest pressure making it more efficient at capturing some of the lowest bass than
a single slot trap would be out in the room. I tried loading the pathway via a bunch of properly
dimensioned holes in the bottom of the U, but too little was captured. It's not a Helmholtz
resonator, although it turns into one with horrible sound by closing off the mouth. With the mouth
closed and stuffing in the pathway it becomes just a trap, and not much sonic difference from a
plain U.
Not much output comes out of the mouth, but what does come out is only very low in frequency.
Keep in mind that what comes out of the mouth has about a 2m delay compared to the front wave,
so it's dipole effect is partial reinforcement of the front wave down to an Fequal point below 30hz
instead of dipole cancellation. The 15" version has an F3 in the 30hz range with no EQ or filtering,
which is impossible with a dipole without using a baffle so big that it would behave more like IB than
OB, an "unreasonable" size.
While an F3 in the 50's might be marginally acceptable for a box due to room gain, with OB it's
pretty anemic even though it may be acceptable for a narrow range of music. The reason I posted
the B200 version was because smaller size and better extension than the 80hz the poster complained
about is possible if he was interested in giving it a shot.
Regarding my use of terminology and somewhat ambiguous wording, in comparison to what most
speaker manufacturers say, my statements were pretty straight forward.
Happy New Year!
ps- What happened to your new year's resolution that your posts would contribute instead of nitpick?
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Thank you, JohnK for your answer about my interest of having bass going up to 300 Hz. This is because I have a lot of interesteing OB-projects coming up some of which certainly need woofer assistance at least to 250 Hz.
However what is your opinion about MJK's answer to my question about his OB and resonances: http://www.audiocircle.com/index.php?topic=32919.110 . My opinion is that this would be relatively easy to measure even with crude measurement. At least this is my opinion with some experience from U-baffles with no top and U-, W- and H-baffles. The U-baffle was very easy to recognise the resonance as also the W-baffle. The H-baffle however, only 22 cm deep did not show any resonances (but probably was not of very good help anyhow) and
the toppless U didn't show any resonances although in this case not very pressed.
I would appreciate an answer. :)
/Erling
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Hello Erling,
I'm not sure which comments of MJK you are referring to. There is some speculation by him which is just that so I won't speculate further. But he also states that, "When the wings are extended the low end of the bass is improved. I am not sure how to determine the equivalent baffle width with the 12'' sides projecting back from the front baffle. But there is a noticeable improvement with the wings rotated out to form a bigger front baffle area."
Assuming this refers to his 48 x 24 baffle with 12" wings then simple math tells you that with the wings folded back the propagation distance to an on axis listener is always greater then with the wings extended. This would suggest that bass would be stronger, not weaker as Martin seems to report. The reason is that there must be a cavity resonance, even with the open top, which introduces negative GD in the rear response so that effectively the propagation distance is actually shorter. This is consistent with a closed U frame. When the wings are folded back there is certainly a restriction of the propagation of the rear radiation which will alter the acoustic impedance. Even if this is only a change in the resistive component of the acoustic impedance it will change the relative strengths of the resistive and reactive components and result is some resonance. The question is how strong is the resonance and what is the Q? For these types of resonances it is actually possible that a lower amplitude, low Q resonance can introduce greater negative GD than a higher amplitude, higher Q resonance. Thus for an open U with large height to depth and width ratio it is possible that the cavity resonance is much less audible in itself, but still has a greater impact with regard to reducing the effective propagation delay, resulting in reduced low frequency output with folded wings. The thing is winged baffles and U's, open top or closed just don't function the way one might expect from a purely geometrical point of view.
Another way of looking at this is from the diffraction point of view. The sound wave is propagation outward from the back of the driver and is confined between the two wings. When it reaches the end of the wings it is free to expand around them, a form of diffraction, or turning of the wave. The means that a wave of the opposite family must be created and reflected back at the source. That will set up some type of resonance between the wings. Again the unknowns are the strength and the Q.
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Hi John,
Martin has a fixed topless U on the backside. In addition he has the folding wings.
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johnk,
Thank you for the answer. Yes, MJK's answer was a speculation. His baffle is like JohninCr states. I assume that you cannot escape the laws of Physics. To me the flat baffle and the H-baffle does seem to have less apparent resonances than the other types.
/Erling
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I don't understand why an H-baffle would have any less resonance than a U-baffle, although I guess there could be some dipole cancellation of the resonances. The interesting thing is the original topic of your thread, and that Martin wasn't concerned about resonance with his 12" deep topless U. While JohnK demonstrated through actual measurement that resonance exists without a top, I lean towards Martin's thinking that this shape will lessen resonance compared to a standard U, and tha's why he left it without a top.
It would be nice to expand the thread topic, and with the help of experts like JohnK, MJK, and Rudolph explore other variations of the basic U shape in order to predict results that will lessen, broaden, and/or increase the frequency resonances, as well as eliminate the negative group delay phenomenon that negates the size benefit of U-baffles over dipole shapes. I'm pretty sure that some of my designs result in some or all of these effects, but it would be much better to have some real guidelines, so we aren't left to intuition and guesswork.
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I think there is too much concern over the resonances in H and U's used fro woofer systems. In an H the resonance is typically not an issue because it is usually placed well above the LP cut off of a woofer system an they are well attenuated by the LP filter. If a U were designed to have the same on axis sensitivity as an H the resonance would be at the same frequencies as the H and the over all length would be 1/2. However, even though the the resonances would be attenuated just as well for the U, the damping is still required to restor the internal delay. These are no resonances in the pass band and should effect the performance of a correctly designed system. If you don't want to deal with resonances go with flat baffles.
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JohnK,
I completely understand your point, however, 1st order XO's seem to work very well for the bass augmenter of simple systems, resulting in a wide audible passbands. Also, eliminating resonances without damping increases the performance potential of smaller simple OB designs to which beginning DIYers and hard core minimalists are drawn. More flexibility and WAF appeal of simple designs will continue to increase the popularity of OB's as a whole, leading to more demand for the more complex/less compromised systems like your Nao.
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As a follow up to the speculations I had made with respect to the acoustic behavior of the 12" sides on my Lowther OB speakers, I modeled this air cavity to see what the resonant frequencies and standing waves are for this geometry and then assessed what the potential was for exciting these standing waves. To perform the study, I modeled the geometry using the acoustic analysis options in the ANSYS finite element package. The air was modeled in this 12" x 23.5" x 48" cavity and rigid boundary conditions were applied on four of the sides while the last two sides were left open to the atmosphere. Simple acoustic boundary conditions were applied to the open back and top surfaces of this volume.
The ANSYS program then calculated the resonant frequencies and mode shapes. The first 10 resonant frequencies were found at 291 Hz, 353 Hz, 410 Hz, 452 Hz, 456 Hz, 536 Hz, 570 Hz, 639 Hz, 647 Hz, and 683 Hz. The mode shape of the fundamental mode at 291 Hz showed a pressure maximum along the bottom front edge and a pressure minimum along the back and top surfaces, almost like a spherical quarter wavelength standing wave between the bottom front edge and the open air at the back and top of the volume. There were no sub 200 Hz resonances and associated standing waves.
So the question becomes will these resonances be excited by the drivers enough to produce a SPL response anomaly at the listening position. If you consider the area of the free surface boundary condition and calculate the equivalent (2 x k x a) value that is used to plot acoustic impedance at the mouth of horns, a rough estimate can be made of how much acoustic damping will exist.
Area = 23.5 in x (12 in + 48 in) = 1410 in^2
Equivalent radius = a = (Area / 3.1415)^0.5 = 21 in
2 x k x a = 2 (2 3.1415 291 Hz / 344 in/sec) 0.538 m = 5.719
This calculation ignores any increase in the acoustic impedance due to reflections off the floor. The (2 x k x a) = 5.719 value indicates that the acoustic impedance is purely resistive so sound energy is not reflected back into the cavity once a sound wave reaches the open back and top. Typically (2 x k x a) = 2 is defined as the transition from resonant transmission line to damped horn behavior for an expanding pipe geometry. Without a reflection of sound energy back into the cavity, there will not be any significant standing wave resonances. Every potential mode is extremely highly damped.
My conclusion is that for the baffle geometry I built, standing waves will not be excited in the volume between the 12" deep sides. The 12" deep sides do in effect lengthen the path the rear wave must travel to produce dipole cancellation of the front wave. This will raise the bass somewhat in the same way rotating the wings outward to extend the horizontal size of the front baffle.
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This will raise the bass somewhat in the same way rotating the wings outward to extend the horizontal size of the front baffle.
Hi Martin,
The conclusions I made were based on you comment that you notice a reduction in the low frequency perfromance whent eh wings were folded back. Geometrically, the path length from the rear of the driver to the listener will be longer with the wings folded back than when they are positioned as a flat baffle. So, if your comment is accurate, that the low bass perfromance was reduced with wiings folded it must be a result of a shorter effective path length which can only arise due to the acoustic interaction of the rear wave with the folded wings. As I mentioned, a low amplitude, low Q resoance (well damped) can have greater negative GD at low frequency than a significantly larger, high Q resonance. Thus, the audibility of a resonance may be nill but there can still be a significant effect on the propogation delay. In absolute terms, folding the wings back could have maximum effect of reducing the path length diffreance at low frequenct from 1/2 the baffle width to the depth of the wings.
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I ran some simulations my self to see what the response directly behind various open configurations looked like. (http://www.musicanddesign.com/pubimages/MJK_composite.gif)
Starting at the top are a series to 3 simulations to provide confidence in the last two results. The top is a simulation of an 18" U-frame using SoundEasy's Enclosure design. This is a limped parameter model based on the work of Backman, IIRC. The next plot is an undamped U-frame 18" long by 9.5" wide x 9.5" high, opened at the back simulated using SoundEasy's 3-D finite element code. The plot is the response at the center of the read exit opening. The third figure is the result for the same 18" u frame using the King, Kreskovsky (mostly King) MathCAD worksheet. Note that all three give very similar results. The 4th plot down is for a SoundEasy 3d finite element simulation of a 48" tall, 24" wide, 12" deep U with only the back opened. This would be close to Martins baffle with folded wings and the top closed. The driver is assumed to be centered on the 48" x 24" front baffle. The response is ",measured" in the center of the rear opening, directly behind the driver. The last figure is the same as the 4th but with the top opened. Frequency range is 50 to 1k Hz.
I think the lower two results are consistent with (http://www.musicanddesign.com/pubimages/Addiocircles1.gif) where I measures the response directly behind one of my NaO Mini baffle with (red) and without (green) "wings".
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If I look a little closer at plots, I see a couple of interesting features that should help us understand better. In the top three plots, the smaller U frame, looking at the MathCad result I believe the impact of the acoustic impedance boundary condition at the open end are easily seen, the successive peaks become less and less pronounced. I can only conclude that the SoundEasy simulation does not include this type of frequency dependent boundary condition so all the peaks exhibit a much higher Q. Also, the first peak in the MathCad model is a little lower in frequency due to the contribution of acoustic mass at lower frequencies from this same boundary condition.
Keeping these differences in mind, when we look at the next two plots from SoundEasy I have to wonder if the peaks and dips are overstated. If there is significant damping applied at the much larger opening for this geometry I would not expect these peaks and dips to exist and the response would become more uniform. The second plot should show higher damping compared to the first due to the larger opening. This is typical of FE solutions, the acoustic boundary conditions tend to be limited to fixed, in other words frequency independent, constraints on pressure or velocity or in some cases the ratio of the two. My conclusion is that the SoundEasy results probably predict the frequencies correctly but are calculated with too little acoustic damping and therefore overstate the response.
Returning to the discussion of deeper bass with the wings folded back or extended. In simple terms the bass response of a dipole is directly related to the baffle size and shape. Picturing the size of the baffle as setting the distance between two simple sources, one positive and one negative, then in simple terms if we calculate the distance traveled from the center of the front source to the center of the rear source this might help decide which geometry produces deeper bass. Looking at the two cases :
Wings Folded Back
d = 12 + 24 + 24 = 60 inches (assuming the rear wave is planer in the rear cavity)
Wings Extended
d = 12 + 24 + (24^2 + 12^2)^1/2 + 12 = 74.8 in (same assumption as before)
I would conclude that the wings extended would produce deeper bass. This somewhat makes sense when you think about the front wave traveling to the edge of the front baffle then wrapping around and picture in your mind the baffle step response one would expect if this was a monopole system. The wide baffle would have a baffle step loss much lower in frequency. But whatever logic is offered, sitting and listening to the Lowther OB system I have the impression of deeper bass with the wings extended.
I have also modeled U Frames in my latest MathCad worksheets. These worksheets include the calculation of group delay. I don't see negative group delay in my results, I don't understand the concept of negative group delay. Physically how does one end up with a phase plot that has a slope such that a negative group delay results? At this point negative group delay does not make physical sense to me.
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JohnK,
can Soundeasy simulate a U-baffle with a hole in the bottom leading into an expanding, folded TL like JohninCR has built? http://www.audiocircle.com/index.php?topic=34399.msg313229#msg313229 (http://www.audiocircle.com/index.php?topic=34399.msg313229#msg313229)
It would be fascinating to see the predicted response vs JohninCR's (eventual) measurements.
Strong output into the 50's using a driver with a Mms of only 9.7g seems to suggest greater potential than a simple U, stuffed or not.
cheers,
AJ
p.s. MJK, nice to have you back. Hope you had a Happy New Years.
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JohnK & MJK,
I need to help get you guys on the same page.
John, Martin's baffle has a fixed U. In addition, it has wings that he can fold out to end up with a big wide front plus a narrower U in the back, not just a wide flat baffle. Martin, John is saying that some damping in the U should result in even deeper bass extension.
Regarding this negative group delay thing. A negative GD, to me, means sound is getting somewhere before it is supposed to. The only explanation that seems to hold water is that the air in the cavity is behaving as a "lumped mass", so you don't get the full propagation delay from the back of the driver to the rear edges of the U shape. Instead the rear wave source is moved closer to the rear edge of an undamped U. Does this lumped mass behavior have a direct correlation with resonance, or is it coincidence that damping cures both? Could this have relevance to the argument about damping's affect on the speed of sound in a TL over on the Decware forum?
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Damn it. I had a response type in here but it was lost. I'm not going to retype it but I will summarize. For the U SE and the MathCAD worksheet shows qualitative agreement. The amplitudes may be different, but the effects are similar. Therefore I feel confident that the results for the 48 x 24 x 12 U with and without open top are also qualitatively correct. Amplitudes may vary.
I'm not particularly concerned with Martins exact configuration, just whether or not resonances in a widely open U with/without top can be excited.
Negative GD. It's just a ways of looking at this.
(http://www.musicanddesign.com/pubimages/neg_GD.gif)
Acoustic wave propagates down a duct at the speed of sound. That is the physics. So the rear radiation should be delayed by L/C compared to the front. But look at this figure (http://www.musicanddesign.com/images/NaO_II_U_frame1.gif)
The phase of the rear response of this undamped U, measured at the rear exit is the same (except for noise) as the phase of the front response measured at the dust cover up to about 75 Hz. Given that the acoustic waves from the rear of the driver must travel a distance of L to the rear exit you would expect that they would be delayed by L/C relative to the front. But they are not. This is because of the negative GD of the resonances. If you look at the first figure above the negative GD is constant at low frequency. What than means is that the phase is increasing with frequency in a linear manor. The propagation delay yield a phase which decrease linear with frequency. So we have Phi = aF at low frequency from the resonance canceled by Phi = -aF from the propagation delay.
Martin, you can run your worksheet for an undamped U frame and look at the polar response well below the resonance and you will see a dipole response. How ever you want to reason it out in you mind, that result means any internal propagation delay associated with the length of the U doesn't appear in the rear SPL response. You won't see negative GD because I am using that in the sense that the rear response is just the front response, delayed, with the duct transfer function (the resoannce) superimposed on it. It is supposed to be a simplistic way to look at this. Maybe it's only simplistic to me. ;-) Obviously you wont' see this in the net response a the rear of the U. I'm just trying to break it down in simple terms; Driver response x propogation delay x duct transfer function = output. The driver's volume velocity is the same front and rear so the difference in the front and rear response, in simplified terms, is due to the delay and duct TF.
AJ, Not sure. I'm have to try and set it up. But I'm not going to do that.
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John,
I understand a resonance causing phase shift, at least I think I do, however, playing content with frequencies only below that first resonant frequency there is no resonance, so what causes this constant negative GD? I want to understand the cause, so I can figure out alternatives to combat it.
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you can run your worksheet for an undamped U frame and look at the polar response well below the resonance and you will see a dipole response. How ever you want to reason it out in you mind, that result means any internal propagation delay associated with the length of the U doesn't appear in the rear SPL response. You won't see negative GD because I am using that in the sense that the rear response is just the front response, delayed, with the duct transfer function (the resoannce) superimposed on it.
OK, I think I understand what you are calculating. At low frequencies (a relative term) the air in the pipe acts like a lumped mass and moves together as a rigid body with the back of the driver's cone. So the air motion in front of the driver's cone and at the open end of the pipe have the same magnitude but are exactly 180 degrees out a phase, a classic textbook dipole. As the frequency increases, the air in the pipe starts to "flex" due to the distributed mass and stiffness of the air column and a magnitude and phase difference starts to appear as rigid body motion transitions to wave motion eventually leading up to the fundamental resonance. But I think at some point the same phenomenon will occur for a bass reflex, a longer TL, and a back loaded horn but at much low frequencies. The only unique thing about the U or H frame is the higher frequencies that the rigid body motion of the air slug continues to occur due to the relatively short lengths involved. My MathCad models include both behaviors and the transition is accounted for in the simulation. I think this also explains the magnitude and phase curves in your plotted data.
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Martin,
Thank you very much. That even helps me understand how that fundamental resonance is
created, since as the flex starts, the change in phase causes a buildup kind of like a sonic
boom as a jet passes through the speed of sound. Is that at least semi-accurate, except
this is more like the speed of sound slowing down through the speed of the jet?
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I don't know anything about jets and passing through the speed of sound.
I like to think about a short slinky that you hang from your hand and then move your hand up and down. If you start out slow the slinky moves with your hand and does not stretch, this is rigid body motion of the slinky. As you hand starts to move faster the slinky starts to stretch a little, now the dangling end is not 100% in phase with your hand. Keep moving your hand faster and the stretch gets more pronounced, the phase difference increases. At some rate of moving you hand up and down, the free end of the slinky will move a lot and it is the maximum response. Move you hand up and down a little slower or a little faster and the motion of the free end of the slinky decreases. The maximum motion occurs at the fundamental resonance and the rate at which you hand moves up and down is the resonant frequency. You can learn a lot about transmission lines playing with your kid's slinky.
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Martin,
Perfect analogy thanks. :bowdown: Short pipe, so short slinky and it happens at higher frequencies. The slinky thing may even cover the higher harmonics. Now I have to figure out how to stiffen the same length slinky. :dunno:
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The length of the pipe will determine the resonant frequency, assuming S0 = SL then the equation
L = c / (4 x f)
will size the length. If the pipe is expanding or tapered this rule of thumb equation does not work and you really need a computer solution to set the length. The stiffness of the pipe is controlled by the cross-sectional areas S0 and SL. Bigger areas softer spring, smaller areas stiffer spring. No rocket science required.
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OK, I think I understand what you are calculating. At low frequencies (a relative term) the air in the pipe acts like a lumped mass and moves together as a rigid body with the back of the driver's cone. So the air motion in front of the driver's cone and at the open end of the pipe have the same magnitude but are exactly 180 degrees out a phase, a classic textbook dipole. As the frequency increases, the air in the pipe starts to "flex" due to the distributed mass and stiffness of the air column and a magnitude and phase difference starts to appear as rigid body motion transitions to wave motion eventually leading up to the fundamental resonance. But I think at some point the same phenomenon will occur for a bass reflex, a longer TL, and a back loaded horn but at much low frequencies. The only unique thing about the U or H frame is the higher frequencies that the rigid body motion of the air slug continues to occur due to the relatively short lengths involved. My MathCad models include both behaviors and the transition is accounted for in the simulation. I think this also explains the magnitude and phase curves in your plotted data.
Well that is the conventional way to look at it. At low frequency the air acts like a piston and you can ignore the compressibility effects, i.e rigid body motion. At higher frequencies you need to start considering them. You know, lumped parameter, distributed parameter, 1-D wave equ, graduating finally to the full blown 3-d wave eq. The assumptions are what approximations are applied and where are they valid. But they are approximations, not the true physics. After all, if the air really moves like a piston at low frequency (wave length much greater the U length), then why would damping the resonance change the phase a low frequency? We start with an undamped case where the phase of the front at the driver cone surface is basically identical (+180 degrees) to the phase at the rear exit plane, regardless of the length of the U, even though it takes an acoustic disturbance initiated at the rear of the driver L/c seconds to reach the exit plane. Now we damp the resonance and all of a sudden the phase at the rear exit plane shows that L/c delay. Is the air no longer moving like a piston at low frequency? W certainly can't have it both ways. Either it moves like a piston at low frequency all the time or it doesn't.
The convectional lumped parameter analysis of a U frame (Backman, 1999 AES presentation), for example, shows that the lumped parameter model says that at LOW frequency the volume velocity at the exit plane is Uexit = Udriver x 1/(1 +jwRCb) where Cb is the capacitance of the air in the U and R is the resistive damping. That is, Uexit is just a low pass filtered version of Udrive and the delay that comes about with the resistive damping is just the delay of the LP filter. Unfortunately, while this looks good on paper, it is only part of the story. First it contradicts the assumption of pistonic motion of the air in the duct which would require the Uexit always equal Udriver. Second, as I discussed on my U-frame web page, adding the resistive damping doesn't just LP filter Udriver. It also damps the resonance compounding the change in the delay.
That the lack of the delay in the rear response when the U is undamped is a result of the resonance can also be demonstrated in a fairly straight forward way. For an undamped U and measure the response at the front. Next, measure that at the rear. It's like the picture in my previous post. But now, instead of acoustically damping the response, electronically equalize the rear response so that its amplitude matches the front response measured without eq applies. You will now find that the equalized rear response has amplitude as the unequalized front response and the phase of the rear will be the phase of the front (inverted) plus a delay of L/C.
Another interesting way to show that the delay, L/c, is still present in the response, but countered by the GD associated with the duct resonance is by minimum phase analysis. Any causal response can be decomposed into the minimum phase response associated with the amplitude, and some form of all pass response. Measure the rear response and compute the minimum phase. You will find that it will be necessary to add a delay of L/C to the minimum phase to match the measured phase at the exit plane.
This is the problem with these simplified analysis. They may give reasonable engineedring results, but they often hide the true physical nature of the problem.
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John,
I understand a resonance causing phase shift, at least I think I do, however, playing content with frequencies only below that first resonant frequency there is no resonance, so what causes this constant negative GD? I want to understand the cause, so I can figure out alternatives to combat it.
Take another look at this picture. (http://www.musicanddesign.com/pubimages/neg_GD.gif)
The delay is just a result of the phase shift. Phase shifts and time dealys are really the same thing. Just like any filter has a phase shift, you don't have to excite the resonances at the resonant frequency for the phase shift to occur at lower frequency. In the region where the GD is constant it just implies that the phase is varying linear with frequency.
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I am sorry John K, I have no idea what you are talking about. I need things broken down into simple analogies for me to understand. That is my limitation. I thought what I wrote was fairly clear and an accurate way of modeling what you had shown in your plots.
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I am sorry John K, I have no idea what you are talking about.
Don't feel bad Martin. You're not the first to say that.
I'm just trying to make the point that the idea air in a duct moves pistonicly at low frequency (or rather when the wave length is large compared to the duct length) is an approximation that can lead to reasonable engineering results, but is never really the case. Let me try a couple of figures.
(http://www.musicanddesign.com/pubimages/mjkdealy.gif)
Starting at the lower left I show the front and rear SPL and phase of an undamped U frame. At low frequency the phase is basically identical. (I have inverted the rear phase for comparison). Thus we would expect to see dipole behavior at low frequency. We agree on that. However, both of these response curves were obtained from the impulse responses shown above at the upper left. The upper impulse is that obtained at the rear opening of the U frame; the lower at the front, driver surface. We see that the impulse for the rear is delayed 1.177 msec = (L/C) relative to the front. But in both cases the SPL data was obtained using an FFt window for the impulse that started at t = 0.0. Thus the phase of the rear response includes excess phase associated with the 1.177 msec propagation time. Yet the phase at low frequency is the same. So the question is why? How is that excess phase at low frequency removed from the rear response? To the right I show the front response and the front response with a 1.177 msec delay added. And here I show the front with delay added overlaid with the rear response.
(http://www.musicanddesign.com/pubimages/mjkdealy1.gif)
What I hope this shows is that below the resonance peak it is apparent that the rear phase shift is less (negative) than what the front is if it were delayed by the 1.177msec propagation time for the duct. At the same time, above the resonance peak it is greater than would be attributed to the propagation delay. At the resonance peak it is the same. Now if you look at the phase response of the Q boost filter in my previous post you will see that if you add the phase of the Q boost filter to the phase of the delayed front response it starts to look very much like that of the rear response. The Q boost resonant response turns the phase up below the resonance peak which is equivalent to removing delay. Conversely, if you remove the resonance from the rear response at low frequency the phase will turn back down and the excess phase associated with the propagation delay will become apparent again.
So it is not because the air moves pistonicly at low frequency that the front and rear phase of an undamped U look the same. It is because of the establishment of the resonance which adds and removed enegry from the system resulting in pistonic like motion at low frequency. Remove the ressonance and you removed the apperance of pistonic motion.
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I modeled the U frame you described in my MathCad TL worksheet and got a similar pair of time responses with the relative delay you have shown. I think my model and your measurements are showing essentially equivalent results. How one visualizes it and explains it simply so others understand appears to be causing the confusion, I think we are starting to beat nits to death. I need to move on to some other work.
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JohnK,
First, I want to thank both of you and Martin for your perseverance on this topic, even if I'm the only one taking so long to get it.
What I like about Martin's explanation is that it also explains the cause of the resonance to me. Before I just drop the matter, I have a question. At what frequency is the impulse response measured?
It seems to me that if you oscillate a piston in the end of a relatively short pipe very slowly (say once per minute), that the air in the pipe will obviously behave as a lumped mass and move out of the other end at the same time you push it at the other. Obviously, at high frequency oscillation this won't happen because the air is too compliant (is that the accurate term). At some point a transition between the two behaviors must occur. If I understand correctly this transition occurs around the fundamental 1/4 wave resonance of the pipe. I don't think this contradicts anything either of you guys have said, as long as below resonance it's not really an acoustic wave inside the pipe, just an oscillating lumped mass.
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An impulse is an electrical pulse in the time domain. In theory it is infinitely short with a magnitude of infinity. In practice it is short enough and produces an audible click. It excites all frequencies with approximately the same magnitude.
The transition from the lumped mass behavior to the flexible column of air starts at very low frequencies and becomes more pronounced as frequency increases culminating with a very high amplitude response at resonance. In reality there is always some very small flexibility, and some small amount of phase difference, even at the lowest frequencies. There is no such thing as a completely rigid lumped mass response for the short column of air. But at the lowest frequencies, it is not a bad analogy that is easily understood and a reasonably accurate assumption.
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JohnK,
First, I want to thank both of you and Martin for your perseverance on this topic, even if I'm the only one taking so long to get it.
What I like about Martin's explanation is that it also explains the cause of the resonance to me. Before I just drop the matter, I have a question. At what frequency is the impulse response measured?
It seems to me that if you oscillate a piston in the end of a relatively short pipe very slowly (say once per minute), that the air in the pipe will obviously behave as a lumped mass and move out of the other end at the same time you push it at the other. Obviously, at high frequency oscillation this won't happen because the air is too compliant (is that the accurate term). At some point a transition between the two behaviors must occur. If I understand correctly this transition occurs around the fundamental 1/4 wave resonance of the pipe. I don't think this contradicts anything either of you guys have said, as long as below resonance it's not really an acoustic wave inside the pipe, just an oscillating lumped mass.
Well I don't want to beat this to death any more that I have so I will make one last post and more on as well. I thin Martin answered you impulse question. So all I'll add is that any system of the type we are looking at here has a 1 to 1 correspondence between the impulse and frequency response thorough a mathematical transformation which is the Fourier Transformation. If you don't know what that is that's ok. But many acoustic measurements are made today but some means of generatining an impulse and then transforming the impulse response to a frequency response.
The lumped parameter analysis is fine for the basic understanding of the resonance. A TL is not unlike a vented box in that there is a capacitive element and an inductive element which lead to a LC resonance.
Your observation about low frequency is also fine. As you say, for a relatively short pipe if you push the air at one end very slowly you can expect the air to move as a solid body. That seems reasonable, IF there is no friction or resistance. But when there is friction things are different. Consider the slinky again. At low frequency it moves as one when there is no resistance to motion. But add resistance distributed in sections along the length. Before each section can move the resistance to motion must be cover come. Each section must start to compress a little, one after the other, before the next section can move. So it no longer moves as a solid body and the far end of the slinky doesn't move in phase with the end you are pushing on. This is the effect of damping restoring the delay at low frequency in a U - frame.
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JohnK,
It seems like we're all in sync now, as long the impulse response doesn't truly reflect the U's rear radiation below the fundamental resonance.
I want to apologize for understanding only small portions of your scientific explanations. I also want to thank you for sharing your discovery of U-baffle behavior, because I see it as the only way open baffle speakers can break into the consumer market. It's the only way to minimize size and maintain reasonable bass response without skyrocketing driver costs. While us OB lovers can appreciate the sonic difference of OB sound, that's probably only marketable to a minor segment of the market. What is VERY marketable to the consumer market is the same thing that we fight with in our designs, off axis bass cancellation. Keeping bass in-room is a tremendous and real advantage for anyone with children or close neighbors. Just look at what Bose does with only smoke and mirrors. Throw in better clarity for dialogue playback and it's an easy sale.
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I have been away a couple of days to marry my eldest son and this interesting discussion occurs. Well, I had to choose party.
My hope was indeed to engage MJK and johnk in the thread and I think that even we laymans are a bit wiser now. JohninCR served us all as a good clarifier and middleman. A lot of thanks to all of you.
Of course the discussion shall continue.
Some conclusions so far:
The U-baffle of johnk's design would be an obvious choice for the most compact and also very cost effective sub-woofer solution. I have experimented with two 12"ers in push-pull configuration on a baffle which I could turn into a kind of Ripol/W-baffle and a H-baffle inspired by the Linkwitz' designs. With the Ripol/W-baffle I had no difficulties in both measuring and hearing the fundamental resonance. The H-baffle I measured a much smaller resonance and it was not at all so obvious in listening if it could be identified at all. The sound was much more open and uncoloured. The U-baffle would promise the same response as the H at smaller dimensions.
I think that the lot of us having some kind of baffle with folded wings also should be encouraged by MJK's analysis. I have tried various chassis in wing baffles. The best have been the Ciare CH-250 and the B200 as fullrangers. I have not been able to measure or hear any resonances from these elements in the baffle. It probably will be a bit different with potent bass-elements. But it will also depend on dimensions.
I think that the easyness of identifying the resonances in my Ripol/W-baffle was the result of a small open surface i relation to the cavity volume which I think will affect the amplitude of the resonance. The topless U-baffle I think would be my second choice for bass-baffle also beeing able to go a bit higher in bass before crossing over. And that even with a choice of 90 degree wingattachment. But experiment, listen, measure and report ! :D
/Erling
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Scorpion,
Welcome back to your own thread. Something we should all keep in mind that JohnK had reminded us of on several occasions is that even if we are only using the bass section below resonance, there is a bass gain to be had of as much as +6db if we properly damp any rearward facing cavities of any folded design. This acoustic resistance prevents the lumped mass behavior, so you achieve the full propagation delay possible. This goes for dipole designs like H's as well, as long as you don't damp the frontside. The most compact designs will derive the most benefit, so if you have a very tall and wide H baffle in relation to your driver size the gain may be minimal.
I hope to come up with some non-standard shapes that have a similar effect as damping, but we'll see how that goes.
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With the Ripol/W-baffle I had no difficulties in both measuring and hearing the fundamental resonance.
Erling
Hi Erling
Nice test.
At what frequency did you get the resonance in the ripol?
Bjorn
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Hi Björn,
It was some time ago. I can do only discretionary measurements. The Ripol/W was like a N-baffle with two cheap Pro Audio SW12" elements in push/pull over each other on a baffle measuring 60 x 35 cms, openings were 60 x 10 cm, just to accomodate the drivers. This baffle measured 60 x 27 x 35 cms (hwd). As far as I remeber resonance started to build up about 150 Hz topping at about 250 Hz. It was easily recognised. But most annoying was the colouration starting to set in over 100 Hz. I don't think you can go higher than that with a Ripol or W-baffle.
In the H-baffle the sound was open and on music you would not be able to hear any resonance. The H was only 60 x 35 x 22 cms (hwd). Two of those elements was not quite enough to create satisfactory SPL in the bass.
/Erling