Corner bass traps using OC703, revisited - AVS Forum
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post #1 of 19 Old 06-09-2012, 05:55 AM - Thread Starter
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My plan is to finally get two corner traps in the back of my video room, my front has two very large tube trap devices in each corner.

With my corners being a perfect 90° angle, my thought is to make a very sturdy triangle shelf with support, and stack on that shelf 2" OC703 about six feet in height. I can get four triangles out of one 2'x4' panels......

The face would be 34" wide and the depth would be about 18" from face to the inside corner...and of course about 6' tall.....

Your thoughts
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post #2 of 19 Old 06-09-2012, 06:44 AM
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Quote:
Originally Posted by kgveteran View Post

My plan is to finally get two corner traps in the back of my video room, my front has two very large tube trap devices in each corner.
With my corners being a perfect 90° angle, my thought is to make a very sturdy triangle shelf with support, and stack on that shelf 2" OC703 about six feet in height. I can get four triangles out of one 2'x4' panels......
The face would be 34" wide and the depth would be about 18" from face to the inside corner...and of course about 6' tall.....
Your thoughts

Why put on a shelf? If you need to use a shelf, consider making it open enough so air can move through - might as well let as much sound into your trap as possible.

Do you have space for 2' x 2' squares instead of triangles?

Do you have vertical modes you also want to damp?

Everything I say here is my opinion. It is not my employers opinion, it is not my wife's opinion, it is not my neighbors opinion, it is My Opinion.
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post #3 of 19 Old 06-09-2012, 07:27 AM
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Quote:
Originally Posted by kgveteran View Post

My plan is to finally get two corner traps in the back of my video room, my front has two very large tube trap devices in each corner.
With my corners being a perfect 90° angle, my thought is to make a very sturdy triangle shelf with support, and stack on that shelf 2" OC703 about six feet in height. I can get four triangles out of one 2'x4' panels......
The face would be 34" wide and the depth would be about 18" from face to the inside corner...and of course about 6' tall.....
Your thoughts

Last time I worked with Christ Whealy's Porous Absorber spreadsheet http://www.whealy.com/acoustics/Porous.html I came up with a design that used fiberglass that seemed less dense than 703 and had a sizeable air space behind it.
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post #4 of 19 Old 06-09-2012, 07:47 AM
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I used Roxul AFB for my superchunk traps - cheaper than OC 703, similar density (2.5 vs. 3 lb/cf) and absorption, and I recall bpape spoke of its use favorably for superchunks.
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post #5 of 19 Old 06-09-2012, 07:59 AM
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porous (velocity-based) absorbers for LF/modal absorption need to be sufficiently thick such that they are placed (spaced away from rigid boundary) into areas of high particle velocity for the lower (longer) wavelengths. as you continue to build thicker and thicker absorbers (a requirement for velocity-based LF absorption), you will want to use a porous material with lower gas-flow-resistivity (GFR).
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post #6 of 19 Old 06-09-2012, 08:53 AM
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Quote:
Originally Posted by Brad Horstkotte View Post

I used Roxul AFB for my superchunk traps - cheaper than OC 703, similar density (2.5 vs. 3 lb/cf) and absorption, and I recall bpape spoke of its use favorably for superchunks.

If we put the design of suberchunk bass traps through Chris Wealey's spread sheet, we come up with a number of issues stemming from the fact that the trap is chock full of a thick layer of highly absorptive material that runs right up to the wall.

(1) The trap would work at least as well if half of the space between its front and the wall were empty - namely the space that is closest to the wall. This seems counter-intuitive because I'm basically saying that nothing is about as good if not better than something.

(2) The trap would work better if the remaining sound absorbing material were less absorptive. This seems counter-intuitive because I'm basically saying that less absorption is better than more absorption.

I found this interesting article that may relate:

http://forum.recordingreview.com/content/why-your-bass-traps-don-t-work-156/

"
@32:31 He starts to talk about the difference between using fiber-based absorption vs membrane based absorption. It turns out that 703 or Rockwool place right up against the wall does almost nothing for the low end.

DanTheMan brought up this very same concept in a really awesome thread on optimizing bass traps here.

@35:20 He says that he wouldn't recommend working with less than 4-6” thickness of Roxul/703. That's not a breakthrough or anything, but I see a lot of people dabbling with 2” 703 when it's clear that greater thicknesses are necessary.

@36:30 Dave Pensado asks what's the lowest frequency a frame with 4-6” thick of rockwool/703 spaced 4-6” off the wall will absorb. It's important to note that this is accepted as a bass trap in home recording land. Most people don't even go this far with depth or air space from the wall.

Thomas Jouanjean says that the ¼ wavelength thing applies.

The home run came at TIME where Thomas Jouanjean said, “These systems based on rockwool or owens-courning [703] do not work so well under 100Hz”.

Going back to this ¼ wavelength busies. I'm not sure where you measure from. I'm assuming when dealing with a 4” sheet of rockwool that is 4” from the wall means we can call that 8 inches. If that's the case we need to find a wave with a wavelength of 32 inches and that's what we are optimally absorbing. I just came up with about 423Hz. YUCK!!! That's not a bass trap. That's a boxiness trap (not that we don't need those, too).

Assuming 6” of 703 and 6” of air space from the wall, we are looking at frequency of 282Hz. YUCK!! This is when sacrificing 12” of space! Who can do that in their bedroom studio?

To get down to 100Hz we need more like 3 feet of space from the wall. That's not gonna happen in any bedroom converted to a studio.
"

Now, I can spot a number of possible errors and exaggerations in the information above, but there are also a number of pieces of truth, one of which being that trapping low frequencies requires such large thicknesses of absorbtive material that excessively absorbtive material can actually decrease the effectiveness of the absorber. Even 703 or products like it can be too much.
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post #7 of 19 Old 06-09-2012, 09:07 AM
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lol all of this discussion like there is anything new here to present regarding porous (velocity-based) absorbers. it has all been well known for some time as are the limitations.
and then it must be a wonder why professional studios have utilized pressure-based LF absorbers now for how many decades?

anyone particularly interested would be wise in obtaining a copy of Acoustic Absorbers & Diffusers (Cox/D'Antonio) - http://www.amazon.com/Acoustic-Absorbers-Diffusers-Theory-Application/dp/0415471745/
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post #8 of 19 Old 06-09-2012, 09:36 AM
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OP here is one recent related thread

http://www.avsforum.com/t/1401657/first-run-with-rew-advice-and-tips-please/120

There are many others here. Some more useful than others.

I think you'll find that there really is no real agreement on how to best to treat a room. And I think that is because no room/speaker arrangement is the same.

The theory is pretty easy if you treat a whole wall (floor or ceiling), and assume all sound approaches that boundary perpendicularly. But that does not happen
in most real spaces.

Air gaps can be useful, but they essentially make a tuned cavity - better absorption at some frequencies, less at others. That may or may not be a good solution.

If upstate NY is near Albany drop me a PM. It might be fun to get together and talk/listen.

Everything I say here is my opinion. It is not my employers opinion, it is not my wife's opinion, it is not my neighbors opinion, it is My Opinion.
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post #9 of 19 Old 06-09-2012, 10:02 AM
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Quote:
Originally Posted by arnyk View Post

Now, I can spot a number of possible errors and exaggerations in the information above, but there are also a number of pieces of truth, one of which being that trapping low frequencies requires such large thicknesses of absorbtive material that excessively absorbtive material can actually decrease the effectiveness of the absorber. Even 703 or products like it can be too much.

There's more outright error than exaggeration, including the last part. If having "too much" material really absorbed less, anechoic chambers wouldn't be lined completely on all surfaces with material that's 3 feet deep. For listening rooms, even rigid fiberglass that's "only" six inches can absorb well to below 40 Hz without straddling corners if you have enough of it. Proof in the image linked below.

--Ethan

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post #10 of 19 Old 06-09-2012, 10:32 AM
 
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Quote:
Originally Posted by dknightd View Post

OP here is one recent related thread
http://www.avsforum.com/t/1401657/first-run-with-rew-advice-and-tips-please/120
There are many others here. Some more useful than others.

I think you'll find that there really is no real agreement on how to best to treat a room. And I think that is because no room/speaker arrangement is the same.

The theory is pretty easy if you treat a whole wall (floor or ceiling), and assume all sound approaches that boundary perpendicularly. But that does not happen
in most real spaces.

Air gaps can be useful, but they essentially make a tuned cavity - better absorption at some frequencies, less at others. That may or may not be a good solution.
If upstate NY is near Albany drop me a PM. It might be fun to get together and talk/listen.

The most significant problem is that there is less agreement in the specific acoustic response one wants to achieve!
Once well defined, achieving a desired specified response is not that difficult!

I am not sure to what theory you refer regarding treating the "whole wall". Nor is it clear that any theory becomes "easier" as a result.

The behavior of absorption, reflection and diffusion is well understood and predictable at all angles of incidence, so I am not sure where the notion that normal incidence is anything special - except to say that the degree will vary from that oft cited reference. That is simply a factor that one observes in the process of being aware of the full range of behavior in real rooms, and a reason that measurements in situ are so valuable, especially for those for whole calculations are a challenge.

Ar gaps with porous absorption are seldom tuned cavities unless specifically designed, and thus far no one has proposed tuned resonant treatments which do utilize tuned cavities. A gapped absorber is NOT a tuned trap. It simply takes advantage of the optimal 1/4 wavelength spacing whereby the particle velocity is maximum at the 1/4 or 3/4 wavelength points, eschewing the placement of porous material in the region of the boundary 0/4 or 1/2 wavelength points where velocity goes to zero and the material losses efficiency and the placement of material reaches a point of diminishing returns.

All in all, I am not sure what the big issue is here.
The behavior of porous materials is rather well understood, and with the advent of a few additional predictive modelling tools based on a plethora of very accurate mathematical material models, our understanding of the various materials now extends outside of the simple cursory measurements of a few commercially available products. And those tools have led to the modification of a few older, but oft cited treatment models.

Two such examples of heretofore commonly accepted 'rules' that have been modified, but which are routinely debated and of which many are still unaware, are in the constitution of the 'Superchunk' style corner traps and in the advantage of unframed absorptive panels over framed. The first being modified to account for the superior low frequency behavior of lower gas flow resistance(GFR) fill and the later due to the recent investigations in to the effects of edge diffraction, whose loss contribution is roughly equal to that of the small amount of additional porous surface area, rendering the choice pretty much as a 'wash'.

But the fact remains that while porous materials are relatively cheap and easily worked, that they are not optimal for the control of low frequencies if efficient space utilization is a prime concern. For that purpose you turn to tuned resonant traps - another area that has been well understood for a long time as well - although the calculations and the subsequent in situ adjustments may not be to the liking of someone not familiar with the math. measurement and iterative adjustments necessary for optimal implementation.

But as local has suggested, such information for the most part is presented in Acoustic Absorbers and Diffusors, or in the associated reference materials if one wants to read the details of each specific mechanical behavioral model.

So if one wants to assert that they are not intimate with all of the acoustical models, or that they they are not intimate or comfortable with such mechanical material information and the associated design processes, that's fine.

But it seems to me that a much larger issue that continues to exist is that most are not aware of the various acoustical response models in order to decide exactly what response they desire, and then they continue to look for a one size fits all treatment that is supposed to magically 'correct' any space regardless of topology simply by following a cookie cutter design placed in a standard one size fits all location in a room whose topology and the equipment and listening position within vary individually!

The fact is, while there are a few standardized treatment designs that will accommodate quite a few applications, for all the rest a few preliminary decisions must be made first regarding the desired acoustical response, and then a few simple measurements will need to be made in order to refine those 'generalized' specifications into actionable and optimal room specific treatments and their positioning to achieve the desired response.

The failure of most to do the necessary per-requisite work does not lead one to the proper conclusion that the 'what' to do and 'how' to achieve the response is neither agreed upon nor understood.

Nor does the fact that many are want to use porous materials and yet are unwilling to accept the design parameters that go along with both the material and spacing limitations of a velocity based treatment.
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post #11 of 19 Old 01-21-2013, 02:34 PM
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I've read through many threads and watched a couple of videos and feel like I have a better understanding but is this an ok generalization? Good option is to space some 703 (or similar but since I have 703 that's my example) off the wall in the corners and frame it so that behind it is air. Better is to stack wedges of it from floor to ceiling. What about maybe stacking 2 feet of wedges on the floor and then another 2 feet at the ceiling and facing out a 2x4 sheet in the middle with air behind it. What I'm trying to avoid is another order of 703 so if this an affective approach to tackle all four corners I have enough for this and the panels I'm doing on the side. However, I'll make the trip for the stuff if this is not a good idea. Thought this was superchunk at first but not sure if superchunk refers to floor to ceiling wedges. What happened was a changed my mind on how I was doing my front wall and I know have the space for floor to ceiling but did not calculate that into my purchase since at the time I was not going to have space for it.
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post #12 of 19 Old 01-21-2013, 05:33 PM
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re-post from here http://www.avsforum.com/t/255432/acoustical-treatments-master-thread/9270#post_22762342
Quote:
Quote:
Originally Posted by Mr Rogers AK View Post

Does anyone have any suggestions on how I can treat (bass trap) the corner by the white doors. I don’t think my wife will go for not being able to open both doors.

sure, here you go:
I'd suggest you make movable non-wall attaching corner bass traps.
Eric took my concept and applied it with pink fluffy, so this is my recommendation to you, a win-win with your wife.
He gives step-by-step instructions for you to follow.
http://www.avsforum.com/t/1312693/diy-construction-methods-of-hang-able-acoustic-panels-not-fixed-frames/120#post_22131618
Quote:
Originally Posted by Eric2000 View Post

I followed a similar construction technique that Mike posted for his corner bass traps, but made mine from pink fluffy R-19 instead and wrapped the fabric around the outside of the threaded rod. Here is the photo journey:

My traps are 24x24x34 inch triangles, are about 3.5 feet tall, and probably weigh less than 10lbs each. Since I need access to one of the corners for a doorway, I made them stackable and moveable. I started by cutting triangles, drilling holes for the threaded rod, and inserting T-Nuts for the sides that stack on one another:
287

Then I cut 23" squares of insulation, cut them diagonally for triangles, and clipped off the corners so they fit snugly between the threaded rods:
390

A wire mesh made from separated Cat5 wire goes between each layer to keep the insulation from sagging over time. The numbered arrows indicate the direction of winding the wire to support the insulation. A bead of solder keeps the wire from unwinding. Each layer of insulation is about 5" thick.
400

Here is the first one all stacked up. You can see the supportive wire mesh wrapped around the threaded rod on each side:
516

And then with the Kraft paper glued to the front with spray adhesive:
529

Then, turn the trap upside down to affix the cloth wrapping - a two-pack of curtains from the giant W for $15. Each pack is enough for two traps.
400

The cloth is stapled to the underside of the top plate to prevent sagging over time:
291

When the fabric is fully secured to the top panel, turn the trap right side up again and pull the fabric around to the back. Trim off the excess and then just pull it tight and use a desk stapler to hold the fabric together in the back:
550

The staples produce a few ripples in the sides, but you won't see those once you put it in the corner. The front looks nice and clean:
528

Here is a closer shot of two of them stacked together:
600

And finally, the entire back half of the theater. With a few bean bag chairs up front, we can comfortably seat 10-12 people. The colors are a little off from a combination of CFL and flash lighting:
326

Below are the before and after REW plots. The purple trace is the original measurement with no traps and no EQ. The yellow trace is the difference made by the bass traps alone. The traps took 5dB off of the room-induced peak at 45Hz without sacrificing anything else in the audible range:
329

This is the original waterfall plot made by REW - no traps, no other corrections - just a mess with room modes at 45Hz and 90Hz:
443

And here is the waterfall plot after ONLY the traps are put in place. I was surprised by how much of a difference the traps made in the decay:
447

After several days of tweaking the parametric equalizer with the traps in place and a first order high-pass filter in place, here is my "final" room response curve - flat from 7Hz to 100Hz, plus/minus 3dB :'( The peak at 105Hz won't ever really happen because the preamp crosses the LFE channel at 60Hz.
449

And the "final" waterfall plot - nice and smooth. If I adjust the waterfall graphing limits in REW, the entire response curve is down by 20dB within the first 100-120ms with the exception of a 2-3dB narrow bump at about 23Hz. I am really pleased with this result:
443

It sounds great! The EQ and high-pass filter reduce (but don't eliminate) the incidence of amp clipping, but still provide enough punch to cause visitors to literally jump up from the couch :P

All of my other projects are on my web page, which I think is linked in my signature.

Eric
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post #13 of 19 Old 01-22-2013, 11:30 AM
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Quote:
Originally Posted by ArthurVandelay View Post

I've read through many threads and watched a couple of videos and feel like I have a better understanding but is this an ok generalization? Good option is to space some 703 (or similar but since I have 703 that's my example) off the wall in the corners and frame it so that behind it is air. Better is to stack wedges of it from floor to ceiling. What about maybe stacking 2 feet of wedges on the floor and then another 2 feet at the ceiling and facing out a 2x4 sheet in the middle with air behind it. What I'm trying to avoid is another order of 703 so if this an affective approach to tackle all four corners I have enough for this and the panels I'm doing on the side. However, I'll make the trip for the stuff if this is not a good idea. Thought this was superchunk at first but not sure if superchunk refers to floor to ceiling wedges. What happened was a changed my mind on how I was doing my front wall and I know have the space for floor to ceiling but did not calculate that into my purchase since at the time I was not going to have space for it.

All of that sounds fine. Chunks are a little better than 4" thick panels, but they require much more material. The quote below is from my Audio Expert book.

--Ethan

Quote:
Originally Posted by The Audio Expert 
Filling a corner fully with rigid fiberglass is only a little better than using a four-inch-thick panel straddling the corner. If you can afford only a limited amount of material, it’s better to have more panels straddling additional corners, rather than fewer corners filled solid. Since the material in the deepest part of a corner is near to the wall boundaries, there’s less wave velocity for the material to act on. But when performance matters more than cost, filling a corner fully does maximize absorption. A good compromise is to place rigid fiberglass panels four inches thick straddling each corner, with the cavity behind each panel filled with less expensive fluffy fiberglass.

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post #14 of 19 Old 01-28-2013, 11:12 AM
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Quote:
Originally Posted by Ethan Winer View Post

All of that sounds fine. Chunks are a little better than 4" thick panels, but they require much more material.

Thank you Ethan! I took care of my front corners now on to the back. I don't have the space for 24 inch wide chunks that would require 17 inches of wall space on both sides. Only have about 14 inches between back wall and door so I can cut these guys up into chunks that are 17 wide that only extend the walls 12 inches. Think I know the answer to this, which would be this is better than nothing at all but before making a mess, figure I'd check to see if it's a waste of time. I bought enough 703 to cover wall to wall before I decided to go with batting on the top panels and realized I actually have enough to do this floor to ceiling. Good stuff.
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post #15 of 19 Old 01-28-2013, 01:12 PM
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If you have a window or door near the corner on one wall but not the other, you can angle chunks (or panels) to extend more along one wall. There's no need for symmetry in this situation.

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post #16 of 19 Old 01-30-2013, 06:31 AM
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So Dragonfly, what you are saying is that there are not any 'one size fits all' bass traps? I was a little lost on your post as most of I am still trying to grasp, but what I am trying to decipher is how to best go about improving the sound quality and quantity of bass in my room. I am ordering a UMM-6 mic tomorrow to start with measuring my room and will most certainly post both the frequency responce graphs, and the waterfall graphs that I remember you recommended.

Now can you, Dragonfly, explain how different types of potential bass traps work and how are the best ways to construct them? You mentioned about 'tuned responce traps' but I am not sure what a tune is, what it does, and how it is made?
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post #17 of 19 Old 01-30-2013, 07:00 AM
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An absorptive trap is broad band down to whatever frequency ir ceases to be effective enough. They absorb sell and less as frequency goes down. I read about a NYC recording studio that had 18to Feet of absorption at the back wall.

Other bass traps work by resonating at a set of frequencies. Limp membrane and Helmholtz resonators spring to mind. They are not broad band but only affect a relatively narrow range of frequencies so you need multiple differently tuned resonators to fix a wider range of freqs.
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post #18 of 19 Old 01-30-2013, 01:49 PM
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Carefull... tuned resonators have a Q value that determines if you need multiples at different tunings, or 1 will do. A Helmholtz absorber will have a very high Q, while a diaphragmatic absorber will have much lower Q, allowing it to absorb over a "wider" range. It's still not anywhere close to the range of a broadband. absorber, but it can be designed to be effective at frequencies well below the broadband absorber's cut-off frequency. Fig 12-23 on page 203 in this link shows some representative curves, albeit tuned for midrange absorbance.
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

I'm seeing ~2 octave spread at 1/2 absorption for the more effective design (resistive absorber in the cavity behind the panel), so if one were designing for a 30Hz peak, one would expect significant absorption from 15-60Hz. Not "broadband" but also not "High Q."

Have fun,
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post #19 of 19 Old 01-30-2013, 02:57 PM
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Quote:
Originally Posted by fbov View Post

Carefull... tuned resonators have a Q value that determines if you need multiples at different tunings, or 1 will do. A Helmholtz absorber will have a very high Q, while a diaphragmatic absorber will have much lower Q, allowing it to absorb over a "wider" range. It's still not anywhere close to the range of a broadband. absorber, but it can be designed to be effective at frequencies well below the broadband absorber's cut-off frequency. Fig 12-23 on page 203 in this link shows some representative curves, albeit tuned for midrange absorbance.
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

I'm seeing ~2 octave spread at 1/2 absorption for the more effective design (resistive absorber in the cavity behind the panel), so if one were designing for a 30Hz peak, one would expect significant absorption from 15-60Hz. Not "broadband" but also not "High Q."

Have fun,
Frank

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