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Acoustical Building Block #1 – Absorption

December 21, 2010

Absorption graphic The first acoustical building block is absorption. This is the best known, and hence the most misapplied, acoustical tool. “Misapplied”, because there is a widespread misconception that the use of a lot of absorption will make a room sound “good”, and keep the sound from intruding into another room. This situation usually starts with someone spending a lot of money to corner the fiberglass market, and ends up with a dull sounding room. (And very little, if any, sound isolation to show for it, by the way.) Nope, wrong.

What absorption does is reduce acoustical energy in a space; it does not spread sound around, a major component for obtaining a “good” sound in a room. That’s done by diffusion.

And absorption does not block sound, which would be acoustical isolation. Absorption just turns down the spigot. But the acoustical spigot acts more like a leaky faucet. You can reduce the drip by cranking down harder on the handle, but you can’t completely stop the flow. And as you crank down harder and harder, there is less and less reduction of the leak; more work brings smaller benefits. The law of diminishing returns applies here.

This is why absorption is also not the right tool for reducing sound intrusion into or out of a room. It may reduce the problem, but it’s not a solution. And while you’re sucking all this energy out of the air, it’s getting harder to hear your speakers. So you turn things up, replacing the energy you just removed with more. Does this sound like you’re spinning your wheels?

And while absorption will reduce the amplitude of those pesky early reflections that damage sonic quality and localization, it doesn’t break them up. That’s another job for diffusion, which is much more adept at curing the problems that early reflections bring.

So among the many types of absorbers, the most practical are resistive and resonant. And one obscure fact about all absorbers is that they’re frequency dependent, which makes them trickier to use than one may think.

For resistive absorbers (like fiberglass and foam products), the thickness of the absorber determines the lowest frequency it will absorb, acting just like a low pass filter. The thickness is considered to be ¼ wavelength of the lowest frequency absorbed. You can use a thicker absorber or space it out from the wall to lower the roll off frequency, but at some lower frequency it will still stop absorbing energy. This also means for very low frequencies you need A LOT of depth. But physics comes to the rescue with another type of absorber.

Resonant absorbers are also frequency dependent, acting on a specific frequency band rather than as a low pass filter. And they accommodate low frequencies without the depth you’d need in a resistive absorber. You can tune a resonant absorber to a particular frequency by changing any of the various dimensions (width, length, depth, port size, port depth, etc.). Helmholtz resonators and panel resonators are examples of this kind of absorber.

I use resonant absorbers to control low frequencies. A typical application would be to tame an extended low frequency decay, which can turn up around the various resonant frequencies of the room.

But this frequency dependency also makes absorbers a flexible tool as well. You can easily tailor these absorbers to specifically address the particular problems in a room, like the long low frequency decays above, rather than applying it as a general treatment. You can also combine resistive absorbers with resonant absorbers to reduce the RT60 (the time it takes sound to decay 60 dB) of a room across the entire frequency spectrum with a minimum amount of coloration. Very handy and useful, once you know about it.

So you can now see why absorption is not the cure-all that it’s commonly thought to be, but a tool to help you get improved acoustic performance from your room. Use it in conjunction with the other tools to attain the best acoustic performance in a sonically sensitive space. Use it wisely and prosper.

Next tool – acoustical isolation! (cue the Supremes – “STOP! In the name of sound, before I break your neck”).

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