Argument against panel absorbers

M

mgraffeo

New member
Ethan, John (and of course, anyone else who has an opinion ;-)

I seem to have read some conflicting views between you guys on your opinion of panel absorbers. Over on the Recording Studio Design forum (which is linked here often), I came across this post, challenging the effectiveness of panel absorbers:

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The data that never seems to get mentioned with respect to absorbers is their absorption coefficient (efficiency) in their respective bandwidth. Of course, you can build a panel absorber with whatever resonant frequency you like. 10Hz? No problem! But in order to do this you need a very massive panel. Massive panels have more inertia and are, therefore, more difficult to excite. More difficult to excite translates to less efficient. The only way to make a highly efficient panel absorber is to use a very light panel in conjunction with a very deep cavity. In other words you need a lot of space.

There's no getting around it. If you want to absorb low frequencies, you need space. Absorbent hangers just help maximize the efficiency of a given space
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Now, I'm no expert. In fact, I don't have 1% of the expertise of Ethan, John, or others on this board. So I've reserved judgement. I'm just curious to hear the opinion of those in the know on this board.

Thanks!

-mg
 
I don't think that's stating anything new. I've heard John mention several times that low frequencies pose the biggest problem in attenuation, and to do so effectively would require a large absorber set far away from the wall.
The problem has to do with the wave length of the low frequency. It's huge.

The half wave length of an A2 tone or note is around 5 feet. So, a 6" wide panel absorber set against a ceiling or wall, is going to have much of an effect on that frequency.

I believe the idea behind panel absorbers are to attenuate middle and middle high frequencies where the wave length is much shorter.

To absorb mid to mid lows, from what I understand, a helmhlotz resonator works well for that application.

Low frequency absorption requires a hanger filled cavity behind a cloth covered wall. Which would, in effect, be a massive panel.

Panel absorbers have their place, but its not the be-all-end-all in frequency attenuation.

Maybe have a look here again, if you haven't already.
 
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MG,

> The data that never seems to get mentioned with respect to absorbers is their absorption coefficient (efficiency) in their respective bandwidth. <

One of the problems is that panel absorbers are not usually portable, and so can't be brought to an acoustics lab for testing. But my RealTraps units are portable, and we paid a lot of money to have them tested. The results are on our web site. The short version is they can be very absorbent at their center frequency if designed properly. For example, our low-bass model measured just over 1.0 at 100 Hz. which is perfect absorption. The high-bass model measured slightly lower, but I'm 99.99% certain that's because the absorption peak falls between two standard third-octave measurement points.

There is no doubt in my mind that panel traps work extremely well. I've built plenty of them before I even started RealTraps, and in every case they changed a room that sucks into a room that's great. However, if they are not built properly I'm sure the efficiency could be compromised. For example, if the seams are not sealed air tight, the air inside will be less springy making the trap less efficient. And if too much or too little fiberglass is used, or it's the wrong density of fiberglass, that affects the Q. If the Q is too low the trap will be broader than necessary and the peak absorption lower than ideal. Likewise, if the Q is too high the trap will be great at the center frequency, but not so good even a few Hz. away.

Of course, I'm biased. :D

--Ethan
 
I would have to agree that the effectiveness of a panel absorber is directly related to its “excitability” – the correlation between the panel’s surface and the strength of the waves hitting the panel – in other words volume in a given room.
In some cases this can lead to a room changing its ‘sonic character’ when material is played at different levels, which is not exactly a desired effect.
I would therefore favour using panel absorbers in carefully chosen positions, to address a specific measured problem in a narrow frequency band, especially in a small to mid-size room.
 
sjoko,

> In some cases this can lead to a room changing its ‘sonic character’ when material is played at different levels <

That could happen only if the materials were nonlinear, and I don't see how that's possible unless you're talking about SPLs far beyond what any louspeaker could ever produce!

--Ethan
 
Hey Ethan,
The problems ZI've had with panels have all been in "surround rooms", rooms with subs, where there seemed to be a definate 'volume point' where panels became effective, this in turn leading to a situation where you had to maintain a certain volume throughout a mixing session.
(don't worry.......I'm no antipanellite, I've got a fair few of them, even in my tracking rooms);)
 
sjoko,

> there seemed to be a definate 'volume point' where panels became effective <

What brand of panels? Very few companies even make panel traps. But what you describe tells me either the traps were not built properly, or something else is going on. Wood panel bass traps like the kind I make are very linear, and they work very well at all volumes.

--Ethan
 
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