Do panel absorbers work - Home Theater Forum and Systems - HomeTheaterShack.com

Old 07-10-10, 10:21 AM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Do panel absorbers work

Hi All,

I seriously doubt that panel absorbers really work. There are formula with which you may calculate the resonant frequency of a panel absorber, but they basically assume that the membrane moves like a piston - which it obviously doesn't because it is glued/nailed along the edges - so that the resulting change in volume increase simply the area of the membrane multiplied by the displacement. The formula also totally ignores the returning force resulting from bending the panel. In other words the model used to formulate the equation has nothing to do with reality.

There appears only one way to determine the resonant frequency of a membrane resonator and that is to measure it.
Extremely good question.

Anyway, anyone who builds a panel absorber sticks it in the corner and sees something get better and is happy. My claim is that all he has done is change the geometry of the room, as a resonator, whatever the keen audiophile built is failing miserably. I have built two panel absorbers, designed to combat a certain mode but you know who knows what they are in fact doing.

So to test the hypothesis that the main difference is due to the change in geometry I have done two measurements the second with a sheet of plywood leaning up against the corner

So the plywood is sorting out the 120Hz dip? Its a panel resonator?

Markus
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Old 07-10-10, 10:37 AM
Shackster
Chris

Join Date: Jun 2010
Location: Wil, Switzerland
Posts: 13
Re: Do panel absorbers work

Hi Markus

Maybe it is a panel absorber. To determine whether it is change in geometry it needs to be a non vibrating solid.
Can it be that like a speaker without a baffle the air is rushing around to the front and canceling itself out?

rgds

Chris
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Old 07-10-10, 03:02 PM
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Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: Do panel absorbers work

A hard, sealed, membrane absorber is very easy to measure. Simply attach a contact mic to the center of the front membrane surface and play a sweep tone. It will be very clear where it's tuned.

I've used sealed membrane absorbers very successfully in several high end theater designs. They absolutely do work.

That said, yes, the SIZE of the panel can change the tuning. I usually recommend using the formula to equate to a 2'x4' absorber. Bigger or smaller and the tuning can change somewhat.

Bryan

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-11-10, 03:15 AM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: Do panel absorbers work

Quote:
bpape wrote: View Post
A hard, sealed, membrane absorber is very easy to measure. Simply attach a contact mic to the center of the front membrane surface and play a sweep tone. It will be very clear where it's tuned.
I am so glad you said that. Then you could try looking at http://www.hometheatershack.com/foru...frequency.html and telling me.
MarkusBonk is offline
Old 07-11-10, 04:36 AM

Join Date: Jul 2010
Location: Kongsberg, Norway
Posts: 14
Re: Do panel absorbers work

I am redecorating my living room and have ripped off all the old plates from the walls. I have already bought the new plates. They are made of MDF and have a painted nice finish.
I am very tempted to use this opportunity to use these plates as panel absorbers.
They have a weight of 8.7 kg/m^2 (1.8Lb/Ft^2).

According to this online calculator: http://www.mh-audio.nl/Helmholtzabso...#PanelAbsorber

With a depth of 23 cm (9 inches) the damping frequency vill be 42 Hz which is the frequency of my first node. The distance from the front to back wall is 4.1 m (7.5Ft).
The problem is that the calculator recommends a surface area of 4m^2!! ( 43Ft^2 !!)

My question here is then:
1. Is it really necessary to have such a big area?
2. Will the center frequency be 42 Hz, or can I end up with 58?? Seems MarkusBonk have a problem with this..

PS. I have all the measurements gear..
Armand is offline
Old 07-11-10, 09:07 AM
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Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: Do panel absorbers work

Looks to me like it's tuned to 70Hz. The thumping you mentioned isn't going to measure the same as wavefronts hitting it since you're only 'thumping' at one spot on the absorber while air pressure hits the entire panel.

Amand, the lower the frequency, generally, the larger the area recommended. This is due to the added mass/stiffness of panels required to do lower frequencies resulting in lower efficiency and flex.

Bryan

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 07-11-10, 09:38 AM

Join Date: Jul 2010
Location: Kongsberg, Norway
Posts: 14
Re: Do panel absorbers work

Ok. I think I will go ahead and do this then. I will post my results here.
If I tune the traps to 42Hz will they also damp 84Hz?
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Old 07-11-10, 03:04 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: Do panel absorbers work

Quote:
bpape wrote: View Post
Looks to me like it's tuned to 70Hz. The thumping you mentioned isn't going to measure the same as wavefronts hitting it since you're only 'thumping' at one spot on the absorber while air pressure hits the entire panel.
Rubbish! The panel will swing at its resonant frequency if it is displaced. Thumping it assures that it it excited by a short impulse. Everyone who has studied control systems will know that the impulse response determines the how a system resonates.
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Old 07-11-10, 03:06 PM Thread Starter
Shackster

Join Date: Jun 2010
Location: Munich
Posts: 70
Re: Do panel absorbers work

Quote:
Armand wrote: View Post
2. Will the center frequency be 42 Hz, or can I end up with 58?? Seems MarkusBonk have a problem with this..
I will only be to happy for you to confirm that the calculators available on the web are rubbish.
MarkusBonk is offline
Old 07-12-10, 03:32 AM
Shackster
Chris

Join Date: Jun 2010
Location: Wil, Switzerland
Posts: 13
Re: Do panel absorbers work

bpape wrote:
"Looks to me like it's tuned to 70Hz. The thumping you mentioned isn't going to measure the same as wavefronts hitting it since you're only 'thumping' at one spot on the absorber while air pressure hits the entire panel."

Quote:
MarkusBonk wrote: View Post
Rubbish! The panel will swing at its resonant frequency if it is displaced. Thumping it assures that it it excited by a short impulse. Everyone who has studied control systems will know that the impulse response determines the how a system resonates.
I think it depends on where you thump it. If you equate it to a string, plucking in the middle will maximize the fundamental, picking it towards the ends will emphasize the partials, but you will still get the fundamental it's just more difficult to get maximum amplitude. In general I must agree with Markus and I presume that he will be thumping it in the middle.

Armand wrote:
"2. Will the center frequency be 42 Hz, or can I end up with 58?? Seems MarkusBonk have a problem with this..

I will only be to happy for you to confirm that the calculators available on the web are rubbish. "

Armand, I suspect the same. bpape mentions that the calculator works best with a size of 2 feet x 4 feet. I don't know if this is learnt through experience or if there is scientific foundation for this. But I suspect that this would be only for a small range of wood thickness. Again if we consider a string, the thicker it becomes the less flexible the edges are. It means that the length is effectively shorter than actual length. bpape might correctly comment that this is a string and not a board, but I do not see the board as anything other than many parallel strings joined together. The calculator only deals with an area of mass and volume of air behind it. There is no consideration for the actual surface area, or edge stiffness.

Markus mentions that the calculator may only be applicable if the resonator is acting as a piston, i.e. the edges are not restricted from moving (and so in effect is working like a speaker in reverse). If the absorber panel is fixed at the edges then it would appear to make sense to make it as large as you can to reduce the effect of the edge stiffness. Alternatively if you are making a small resonator then consider using a steel plate which has a more flexible edge.

It doesn't answer your question about the calculator accuracy though. But if the above theories are correct then the larger the panel the more accurate it should become.

If you get around to making one we would be really interested to see your measurements. So far I haven't see any positive proof that these things actually work. Please note the experience of Markus where simply placing a board in the corner reduced a null. i.e. it looks like the change in room shape caused the change and not the absorber.

Chris
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