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How to make your walls absorb energy...

post #1 of 62
1/25/12 at 3:37pm
Thread Starter 
Hi Folks,

Geddes, Toole, and others have suggested that one way to fix room modes is to just create "lossy" walls that absorb energy and in turn reduce room resonances.

But, how to make walls that absorb energy without major room construction?

A: Take a piece of sheet metal a couple of mm thick and glue it to a piece of foam that is an inch thick and attach it to the wall.

Since the absorber is in the pressure region of the sound wave it will vibrate turning the modal energy into heat in the foam. It doesn't actually make the foam hot, but that is the physics of where the energy goes.

This site:http://www.casakustik.de/forum/index...c=532.msg11459
is in German and describes the process (with pictures too) and then shows the change in frequency response and time domain (waterfalls). You should be able to get a good idea of what is going on just from the pictures.

It seems that the performance can be changed by varying the size of the panel, as well as the amount and density of the foam.

Other bass trap type solutions work by reducing air flow in the velocity zone of the bass wave. Since the velocity zone of the wave is 1/4 wavelength out from the wall, the absorber needs to be huge or place well off of the wall.

This site: http://www.pia-alfa.de/en/dat_vprbka.htm
has some information on panels and foam in their products.

EDIT: There is also a thread on this at gearslutz.
http://www.gearslutz.com/board/studi...-absorber.html
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post #2 of 62
1/25/12 at 4:02pm
I've thought about this before. I was thinking, in a basement I'd frame in the size of room I want, but just don't sheet it. I'd insulate and poly them, and possibly dress them with black fabric or something, but leave the sheet rock off.

Now, there's bound to be at least 1 concrete wall in a basement that the room would be against. But I'd guess one or two reflective walls with no other walls with any significant reflectability would provide a pretty smooth room. Perhaps put the sub(s) against the concrete wall or in the corner of the cocrete walls corner and then the sound waves would radiate into nearly free space (less the floor and ceiling of course).
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post #3 of 62
1/25/12 at 9:29pm
A mechanical form of a hemholtz resonator. many studios use plate absorbers operating in the pressure zone and these products are also commercially available. The big downside is that they typically rely on resonance and are only efficient absorbers at that frequency. This is also an upside in that you can target modal frequencies without overdamping the room... the downside is that it complicates diy efforts.

If you're really smart and dont just use foam for the energy conversion, you could create a panel absorber which was tunable within a frequency range with a simple turn of a knob, allowing for targeted absorption of trouble room modes after construction and installation. I haven't seen any commercial or diy attempts of such, aside from using subs passively with a varistor or similar to alter Fb. Not exactly the most cost or space effective solution.
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post #4 of 62
1/25/12 at 9:57pm
Thread Starter 
"The big downside is that they typically rely on resonance and are only efficient absorbers at that frequency."

that is not what we have here. http://www.pia-alfa.de/de/download/VPR-A2.pdf

absorption alpha is around 0.8 across most of the bass. it is not a frequency dependent device.
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post #5 of 62
1/25/12 at 10:44pm
Don't read German and can only guess at what is being shown. Looks like relatively wide band absorption, but what is the actual effectiveness? ie you can absorb an even 0.5dB across the audible bandwidth and be a terrible absorber. Just asking as I didn't see what looked to be that kind of info. I'll take a closer look with the assistance of online translators and read up a bit more. Looks interesting at least if it provides a reasonable amount of broadband absorption in a low profile panel.
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post #6 of 62
1/25/12 at 11:43pm
Here's BBC report of similar type absorber.
http://downloads.bbc.co.uk/rd/pubs/reports/1995-07.pdf

and bit thicker but cheaper hard board version.
https://docs.google.com/viewer?url=h...df&chrome=true
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post #7 of 62
1/26/12 at 1:27pm
Thread Starter 
"...but what is the actual effectiveness?"

in the before and after pictures there was about a 30db dip around 42hz that appears to have been completely eliminated.
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post #8 of 62
1/26/12 at 1:34pm
Quote:
Originally Posted by Ile View Post

Here's BBC report of similar type absorber.
http://downloads.bbc.co.uk/rd/pubs/reports/1995-07.pdf

and bit thicker but cheaper hard board version.
https://docs.google.com/viewer?url=h...df&chrome=true

I've read a lot of these BBC works, ....good stuff.

Ile, good to see you. I hope the IB is going well, and I hope the weather is tolerable.

All the best
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post #9 of 62
1/26/12 at 5:04pm
Quote:
Originally Posted by Bigus View Post

The big downside is that they typically rely on resonance and are only efficient absorbers at that frequency.


[/quote]
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post #10 of 62
1/26/12 at 5:14pm
Quote:
Originally Posted by LTD02 View Post

Hi Folks,

Geddes, Toole, and others have suggested that one way to fix room modes is to just create "lossy" walls that absorb energy and in turn reduce room resonances.

this is a critical part of many studio designs (acoustic models) that has been known and used for decades --- a LPF inner shell that is placed asymetrically within the outer shell such that the listening position is outside of the center null, while the inner shell is designed to redirect early specular wavelets to the rear of the room (managing the specular energy via geometry/splayed walls).
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post #11 of 62
1/26/12 at 5:36pm
Thread Starter 
rockwool is a typical absorber and is good for wideband, but as your chart shows, like most all such products, it doesn't really do much in the region under 80hz.

bigus was thinking that this was a helmholtz resonator, which as he pointed out, are narrow band absorbers.

i'm not really clear what you are saying in post 10.
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post #12 of 62
1/26/12 at 6:07pm
Get flimsy walls. Use sheetrock (1/2" or 5/8") with metal studs with internal roxul. The walls will pump the energy. You can add resilient furr to the inside walls.

The idea is to absorb energy...

My 2 cents
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post #13 of 62
1/26/12 at 6:30pm
Thread Starter 
"Get flimsy walls."

the proposed solution was directed at folks who already have a room built and want a solution that is not invasive.

if one is starting from scratch, lossy walls 'may' be a better solution as you propose...though i'm not sure what you mean by, "The walls will pump the energy."
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post #14 of 62
1/26/12 at 7:07pm
Quote:
Originally Posted by LTD02 View Post

rockwool is a typical absorber and is good for wideband, but as your chart shows, like most all such products, it doesn't really do much in the region under 80hz.

porous absorption is functional below 80hz, granted that you are using thick traps of low gas-flow-resistivity (eg, pink fluffy attic insulation - loosely filed). higher density/GFR will perform worse at LF absorption.



Quote:
Originally Posted by LTD02 View Post

bigus was thinking that this was a helmholtz resonator, which as he pointed out, are narrow band absorbers.

false: see post 9

you may be interested in: http://www.amazon.com/Acoustic-Absor...dp/0415471745/
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post #15 of 62
1/26/12 at 7:25pm
Thread Starter 
"false: see post 9"

?

that post doesn't have anything to do with a helmholtz resonator. it is a wideband absorber.

a helmhotz resonator is very frequency specific. just like a ported cab, which is also a helmholz resonator, most of the boost is in a very narrow frequency region.

"porous absorption is functional below 80hz, granted that you are using thick traps of low gas-flow-resistivity (eg, pink fluffy attic insulation - loosely filed)."

for them to be very effective, you are looking at 1/4 wavelength of depth. you have to get into the velocity zone for such a device to be effective. of course, the lower you go in frequency response, the larger such a device must be.

for the device that i posted, which is pressure-based, it can be quite small because all bass no matter what the frequency has maximum pressure right on the boundary.
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post #16 of 62
1/26/12 at 8:29pm
Quote:
Originally Posted by LTD02 View Post

"false: see post 9"

?

that post doesn't have anything to do with a helmholtz resonator. it is a wideband absorber.

a helmhotz resonator is very frequency specific. just like a ported cab, which is also a helmholz resonator, most of the boost is in a very narrow frequency region.

you're mistaken.
maybe you should look at the graph again.
pressure based absorbers do not have to have a very narrow notch - in fact, i would consider this a flawed design when most small acoustical spaces need broadband LF absorption.


Quote:
Originally Posted by LTD02 View Post

for them to be very effective, you are looking at 1/4 wavelength of depth. you have to get into the velocity zone for such a device to be effective. of course, the lower you go in frequency response, the larger such a device must be.

wrong - this shows a clear lack of understanding of porous (velocity-based) absorbers, and the inherent myths within.

Quote:
Originally Posted by LTD02 View Post

for the device that i posted, which is pressure-based, it can be quite small because all bass no matter what the frequency has maximum pressure right on the boundary.

i think you mean relatively "thin", not "quite-small". big difference.
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post #17 of 62
1/26/12 at 9:00pm
Thread Starter 
"wrong - this shows a clear lack of understanding of porous (velocity-based) absorbers, and the inherent myths within."

how do you mean? putting a porous absorber on a wall does little to nothing in the bass. that is a fact.

you have to get it out into the room at 1/8 wavelength for it do anything significant. that is a fact.

"i think you mean relatively "thin", not "quite-small". big difference."

yes good catch, thin, as small intrusion into the room which is what most everyone cares about. a 1" x 36" x 48" is only 1 cubic foot, so "quite small" probably applies too.

despite local, maybe we can get back on track....
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post #18 of 62
1/27/12 at 5:45am
Quote:
Originally Posted by LTD02 View Post

how do you mean? putting a porous absorber on a wall does little to nothing in the bass. that is a fact.

you have to get it out into the room at 1/8 wavelength for it do anything significant. that is a fact.

you stated: "for them to be very effective, you are looking at 1/4 wavelength of depth. " - this is not correct, and is a myth as i stated.

you may wish to look at post #14 again. just because everyone on this
forum assumes high density (high GFR) porous material is "best" for LF porous absorption does not make it true. too many misconceptions (as is the whole "1/4wavelength" myth) - if you're interested, you may wish to read through this thread: http://www.gearslutz.com/board/studi...q-4-avare.html


Quote:
Originally Posted by LTD02 View Post

yes good catch, thin, as small intrusion into the room which is what most everyone cares about. a 1" x 36" x 48" is only 1 cubic foot, so "quite small" probably applies too.

surface area is still generally a requirement, even with pressure-based (resonate) absorbers. that was the point of my statement. the traps will be relatively "thin" due to the luxury of being placed in areas of high pressure, but that does not mean they will not still be "large" in the surface-area context.

Quote:
Originally Posted by LTD02 View Post

despite local, maybe we can get back on track....

despite me? i just called you and Bigus out for assuming that a helmholtz can only be designed a very-narrow "notch" frequency -

i cant believe you still do not understand what is in post #9. are you misinterpreting that as if it is a wideband porous, velocity-based absorber ???

may i suggest you read chapter 6 of D'Antonio/Cox "Acoustic Absorbers and Diffusers"?

maybe also investigate AFMG SoundFlow (http://soundflow.afmg.eu/index.php/sf-features-en.html) and Porous Absorber Calculator (http://www.whealy.com/acoustics/Porous.html)
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post #19 of 62
1/27/12 at 7:30am
edit, waste of time.
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post #20 of 62
1/27/12 at 7:41am
If I understand local's screen shot (not familar with the tool, so I'm guessing) shows a perforated panel over fiberglas over airspace - not just the latter two elements. I'm not familar with that design, but I assume the perforated panel is what makes the difference.

That said - it doesn't appear to have much effect in the bass region (according to the chart)?
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post #21 of 62
1/27/12 at 7:46am
Quote:
Originally Posted by Brad Horstkotte View Post

That said - it doesn't appear to have much effect in the bass region (according to the chart)?

the illustration of that particular design wasn't to showcase attenuation of a specific low frequency, but to detail the assumption that "The big downside is that they typically rely on resonance and are only efficient absorbers at that frequency" is not necessarily true.
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post #22 of 62
1/27/12 at 7:55am
I was working on an idea earlier in the year that I think would be perfect. There's a company in Indiana that works with a bunch of different types of foam. Open cell, closed cell, etc. They throw away a bunch of thin junk strips about 1" wide that I use for packaging material. I get it for free. I sent some to a couple forum members and they liked the stuff.

But I asked them if they could cut us strips of foam about 3" wide and 1" thick. They said yes. Some of their closed cell foam is very strong stuff, but still good at stopping vibrations. I've got some if anyone wanted to experiment with it.

My idea was to glue these strips to the 2x4 stud, then mount the drywall like you normally would. The drywall screws go right through the foam without grabbing it and twisting it up.....I tried it. The foam is just the perfect density to hold the drywall snug without creating waves or dips, but still gives a nice thud when you tap on it. It can also be used under the floor. I've seen where they do that in sound studios.

I also thought of having them cut some for subwoofer risers as well. But I think open cell works better for that.

Here's a photos of some of the foam strips. They actually have about 10 different kinds, but can get pretty much anything we'd want.

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post #23 of 62
1/27/12 at 2:30pm
Quote:
Originally Posted by localhost127 View Post

maybe also investigate AFMG SoundFlow (http://soundflow.afmg.eu/index.php/sf-features-en.html) and Porous Absorber Calculator (http://www.whealy.com/acoustics/Porous.html)

Great links, Thanks!

JSS
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post #24 of 62
1/27/12 at 3:52pm
Thread Starter 
"you stated: "for them to be very effective, you are looking at 1/4 wavelength of depth. " - this is not correct, and is a myth as i stated."

technically you are right. absorption follows the entire half wave. however it is maximized at 1/4 wavelength, which is the point of highest air velocity. if you want to use a thin material to aborb bass frequencies it has to be moved off the wall and out into the room.

"surface area is still generally a requirement, even with pressure-based (resonate) absorbers. that was the point of my statement. the traps will be relatively "thin" due to the luxury of being placed in areas of high pressure, but that does not mean they will not still be "large" in the surface-area context."

correct.

"despite me? i just called you and Bigus out for assuming that a helmholtz can only be designed a very-narrow "notch" frequency"

by definition, a helmholtz resonator has a resonant frequency. it is not wideband. if what you have is wideband, it is not helmholtz resonator. we are probably just talking about two different things.
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post #25 of 62
1/27/12 at 3:54pm
Thread Starter 
erich, i'm not sure that would work. the wall panel has to be able to vibrate for it to absorb energy. perhaps if the fasteners has large rubber washers or something that allowed the panel to vibrate.
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post #26 of 62
1/27/12 at 4:07pm
Thread Starter 
just got into the excel spreadsheet freeware. it is really slick. thanks for posting it.
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post #27 of 62
1/27/12 at 4:16pm
Quote:
Originally Posted by LTD02 View Post

technically you are right. absorption follows the entire half wave. however it is maximized at 1/4 wavelength, which is the point of highest air velocity. if you want to use a thin material to aborb bass frequencies it has to be moved off the wall and out into the room.

standing modes, Vmax is always 1/4wavelength from boundary.
other frequencies, superposition of ingress/egress signals determines phase.

did you see post #14? 300mm pink fluffy attic insulation (~5000rayls/m gas flow resistivity) + 300mm air-gap = ~24inches.

did you read through this thread? http://www.gearslutz.com/board/studi...q-4-avare.html

Quote:
Originally Posted by LTD02 View Post

by definition, a helmholtz resonator has a resonant frequency. it is not wideband. if what you have is wideband, it is not helmholtz resonator. we are probably just talking about two different things.

i think you may want to revisit the screenshot in post #9 as well as investigate a little further resonate absorbers (eg, chapter 6 AA&D as listed above).
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post #28 of 62
1/27/12 at 4:44pm
Thread Starter 
"standing modes, Vmax is always 1/4wavelength from boundary.
other frequencies, superposition of ingress/egress signals determines phase.
did you see post #14? 300mm pink fluffy attic insulation (~5000rayls/m gas flow resistivity) + 300mm air-gap = ~24inches."

sounds like we were pretty much on the same page...

"i think you may want to revisit the screenshot in post #9 as well as investigate a little further resonate absorbers (eg, chapter 6 AA&D as listed above)."

you are clearly talking about different things on this one than bigus and i. a helmholtz resonator has one frequency. it is not a wideband device. a box with a port = helmholtz resonator.

post #9 shows absorption of rockwool. that is an acoustic absorber that is unrelated in function to a helmholtz resonator.

the point that i was making to bigus was that the device posted in the op seems to function off a property different from a helmholtz resonantor and it provides wideband absorption in the bass region while being minimally invasive in the room.
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post #29 of 62
1/27/12 at 5:04pm
Quote:
Originally Posted by LTD02 View Post

sounds like we were pretty much on the same page...

same page? you insisted: "for them to be very effective, you are looking at 1/4 wavelength of depth."

Quote:
Originally Posted by LTD02 View Post

you are clearly talking about different things on this one than bigus and i. a helmholtz resonator has one frequency. it is not a wideband device. a box with a port = helmholtz resonator.

post #9 shows absorption of rockwool. that is an acoustic absorber that is unrelated in function to a helmholtz resonator.

are you insisting there shall be no porous material in a helmholtz resonator?

may i suggest you read Brad's post#20 (as he saw the minor detail that is evident within the screenshot in post #9).

Quote:
Originally Posted by LTD02 View Post

the point that i was making to bigus was that the device posted in the op seems to function off a property different from a helmholtz resonantor and it provides wideband absorption in the bass region while being minimally invasive in the room.

im not discussing VPR's - i was merely refuting the common misconception that was presented within this thread that resonator traps "are only efficient absorbers at that frequency."

hopefully this will put us back on the same page -
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post #30 of 62
1/27/12 at 5:06pm
Quote:
Originally Posted by Ile View Post

Here's BBC report of similar type absorber.
http://downloads.bbc.co.uk/rd/pubs/reports/1995-07.pdf

+
http://downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdf
http://downloads.bbc.co.uk/rd/pubs/reports/1992-11.pdf
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