# Do panel absorbers work



## MarkusBonk (Jun 5, 2010)

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. 
How? you may ask. 
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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## Chriswil (Jun 14, 2010)

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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## bpape (Sep 14, 2006)

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


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## MarkusBonk (Jun 5, 2010)

bpape said:


> 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/forums/home-audio-acoustics/30640-whats-frequency.html and telling me.


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## Armand (Jul 11, 2010)

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/Helmholtzabs...71&w=59.62&pw=238.46&area1=1.42#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..


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## bpape (Sep 14, 2006)

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


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## Armand (Jul 11, 2010)

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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## MarkusBonk (Jun 5, 2010)

bpape said:


> 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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## MarkusBonk (Jun 5, 2010)

Armand said:


> 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.


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## Chriswil (Jun 14, 2010)

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."



MarkusBonk said:


> 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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## bpape (Sep 14, 2006)

MarkusBonk said:


> 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.


Believe what you will. 

The very fact that tapping it produces one result and the measured result from in-room and outside sound wave excitation is something else is pretty hard to argue with. It's right in front of you.

Bryan


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## Armand (Jul 11, 2010)

OK, I have built one of my planned three panel absorbers.
My goal is to damp a measured standing wave of 42 Hz in my room. The calculator i referred to earlier and also an article on soundonsound.com predicts that the tuned frequency will be lowered as much as up to 50% when the absorber is filled with mineral wool. I therefore tuned the absorber to 51 Hz and planned to fill it with wool untill it came down to 42Hz.
The panel is 118 cm wide and 129 cm high. Depth is 17 cm.
The panel is made of MDF, its 12mm thick and weigh 8,8 kg/m^2. The back cavity is made of fibre board stiffened up with a framework of 3,5x7cm wood. The air cavity is carefully sealed with lots of glue and acryl. I have just completed the first tests without anything inside besides air...
The measurements are done with a ACH-01 accelerometer glued to the panel surface. This is hooked up to a sound measuement system (Sample Champion) where I can do spectrum analysis.

And now for the results:

I measure several frequencies. 35, 44 and 55 Hz. All of them with almost equal amlitude and well over the noise floor. It does not make sense...

If I move the accelerometer to another location i loose the 44Hz but keep the two others.

Measured in the centre of the panel I get 26Hz and 70Hz if I thump near the middle. But if I thump far from the centre, get som 46Hz as well...

All this leads me to think that the panel is in fact tuned to around 45Hz which isn't that far from the theoretical 51Hz.

I'll do some more testing with speakers and a signal generator later. I'll see if I can post some pictures too. It's bad I can't post pictures here..


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## Armand (Jul 11, 2010)

Just found out how to post pics...
This is from the middle of the absorber when "thumped" in the upper corner.


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## Armand (Jul 11, 2010)

Here's a quick sweep I did with the bass just in front of the panel. The accelerometer mounted also in the middle.
I have compensated the curve for the falling response of my (small) bass.
Again we can se the 45Hz response which is pretty close to the theorethical 51Hz, but look at that 70Hz peak!!
Can it be because of the speaker being really close..??
Well, I have to sleep now. 3AM i Norway now and my mother in law is sleeping in the next room (or trying to.  )
More later...


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## Armand (Jul 11, 2010)

I am finished measuring the panel. I will start with the conclusion. Panel traps DO work.

To induce vibration in the panel I used a small subwoofer 65 cm up from the floor on the 234 cm high and 118 cm wide panel. Or about a 1/4 of the height. It was placed 10 cm from the panel.
To measure the panel I used a calibrated AHC-01-03 accelerometer connected to a Behringer microphone preamplifier which again fed my M-Audio Audiophile soundcard.
I used the HOLMImpulse software (free) to play and record sine sweeps simultaneously.
I did 11 measurements across the panel in a kind of random pattern and then summed all measurements in the HOLMImpulse software to get the average response. Note that I have also corrected for the nonlinear response of my woofer.
 

As you can see, there is a great deal of absorption around 45 Hz which is good and fairly close to the theorethical 51 Hz. With a little mineral wool I will surely get it down to my goal of around 42Hz.
However, there is more going on... look at the peaks at 27, 77 and 95 Hz!!
These peaks are without doubt contributing factors. And I can assure you that these peaks are not artifacts from knocking or other artifacts. (The only artifact I know about can barey be seen as a peak at 50Hz as my accelerometer picked up a little bit from the 50Hz powergrid we use in Norway)

It surprises me how little scientific information and seriously controlled tests there is on the internet about how these panel traps work.
IMHO I think that the tests I have done on my panel are quite good but if someone has any comments or see any faults I would be glad if you could point them out for me.

I will post more measurements after I have filled some minral wool in the panel.
I also have two other panels almost ready that I will tune to higher frequencies.


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## Armand (Jul 11, 2010)

Here are my measurements of the sound in the ROOM before and after installation of panel trap #1.
Note that the location of both the sub and the mic are within 1 cm before and after so there should be a minimum of error here. (There might be slightly more empty soda and beer cans scattered around on the latter measurement..  )
It's easy to see that the peak I had at 45 Hz is now gone and also some of the peak at 55Hz. (yellow to yellow-greenish). We can also see that this frequency range seems to die faster.
The peak at 70Hz is the same, and the peak at 90 seems in fact to be a bit higher.. :foottap:

Panel traps DO work. So does the online calculators it seems..


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## bpape (Sep 14, 2006)

Amazing 

Seriously, congrats. I know it's difficult to just put your faith in something. However, you're now seeing the proof of functionality as well as measuring the results that matter - how they impact the room.

Bryan


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