[Davey]

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.

[JohninCR]

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.

[JohninCR]

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.

[JohnR]

Interesting thread, but a lot of speculation and subjective-only comment. 

Could you post a link to your measurements please? Thanks

[Davey]

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.

[Davey]

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:



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.



Cheers,

Davey.

[JohninCR]

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.

[Davey]

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.

[scorpion]

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

[JohninCR]

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.

[JohninCR]

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.

[johnk...]

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

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...

[JohninCR]

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.

[scorpion]

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. 

/Erling

[JohninCR]

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.

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.   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.

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?

This stuff is infinitely more interesting than boxes.

[johnk...]

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

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.

[JohninCR]

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?

[johnk...]

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. 

Later....

[JohninCR]

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.

[Russell Dawkins]

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. 

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.