[scorpion]

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

[JohninCR]

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.

[AJinFLA]

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

[Bob in St. Louis]

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

[JohninCR]

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?

[Brad]

Given John's amazing productivity in building baffles, I'm starting to become concerned with the deforestation issues in Costa Rica 

Cool stuff, John/Russell/JohnK

[JohninCR]

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)

[AJinFLA]

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


You had great output down into the 50's, but disappointed with the extension of the B200 version? 

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

  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.

[JohninCR]

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)

[AJinFLA]

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
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

[JohninCR]

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?

[scorpion]

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

[johnk...]

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.

 

[JohninCR]

Hi John,

Martin has a fixed topless U on the backside.  In addition he has the folding wings.

[scorpion]

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

[JohninCR]

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.

[johnk...]

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.

[JohninCR]

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.

[MJK]

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.

[johnk...]

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.