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Sound Absorbers’ Dirty Little Secret

February 8, 2013

It is common thinking, even among some seasoned sound professionals, that sound absorbers reduce the overall level of sound energy equally across the entire audio spectrum.

This idea has led many well-intended – but uninformed – people down the path of making mistakes when trying to improve the “sound” in an acoustical environment. If they’re lucky, they’ve stumbled on something that works, but not because it was planned that way. If they’re not lucky, they’ve actually degraded the room’s sound.

The importance of good acoustics is simple. Whether you’re cutting effects or dialog for television, recording and mixing your own music in a home studio, or mixing feature films at a major facility, you’re making level, equalization, and other adjustments based on what you’re hearing. You need to be confident that your room acoustics aren’t skewing your judgment by coloring what you’re hearing.

Every track is going to sound a little different when it’s played in different rooms. But if you’ve worked on your tracks in an acoustically neutral space, you maximize the chance that other people will hear your work in their rooms as you intended it to sound. This is particularly important when working on highly collaborative projects like feature films or TV shows.

It Was The Best of Treatments, It Was The Worst of Treatments

Sound absorption is the best known acoustical treatment. In fact, for many people it is the only acoustical treatment they know of. This makes it the most used, and, alas, most misused acoustical treatment.

There are a number of attributes that sound absorbers have which few people understand. The attribute that is the least understood and the most likely to damage a room’s sound is what I call a sound absorber’s dirty little secret.

This dirty little secret is that they don’t absorb all frequencies equally. And that means they’re not performing the way we think they are and may be working against the neutral acoustics we’re looking for, unless they are properly selected and installed.

This isn’t to say sound absorption is bad, it is in fact a crucial part of treating a room. But it needs to be done right, and to use it right, you need to understand it.

So Just What’s Happening?

For any given absorber, there is a point somewhere in the low end of the audio spectrum where their absorption rolls off. To look at it another way, as the frequency drops, so does the amount of absorption. So while absorbers are commonly thought of as level controls or attenuators, they are actually low pass filters.

The frequency at which this happens is determined by the thickness of the absorber – the thicker it is, the lower that frequency is. (Now that you know that, it makes you wonder about the performance of sculptured foam absorbers. The thin portion of the foam isn’t absorbing as low frequencies as the thicker sections.)

This means when we add absorption to a room, it may sound “deader” or less reverberant, but it also is duller.

So this is the “dirty little secret” of sound absorbers – they don’t absorb all frequencies equally. Armed with this knowledge, we can now use the absorbers to our best advantage.

My favorite sound absorber is 2 inch thick Owens-Corning 703 semi rigid fiberglass board. In its published performance specifications, it is nearly 100% absorbent from roughly 300 Hertz up. So if we need to take energy out of the room, perhaps because it is highly reverberant, we can do so with some 2 inch 703.

But much of the coloration and early reflection problems occur below 300 Hertz. Since our favorite 2 inch 703 isn’t helping us in that region, we need another tool that works there. By using a second device, we can better tailor the entire frequency spectrum of the room.

That is the Helmholtz Resonator (to be covered in another entry). By using a judicious combination of 703 and Helmholtz resonators, we can start approaching the overall “level reduction” we’re looking for.

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