Acoustic Treatment

There are four primary goals of acoustic treatment:

1) To prevent standing waves and acoustic interference from affecting the frequency response of recording studios and listening rooms.

2) to reduce modal ringing in small rooms and lower the reverb time in larger studios, churches, and auditoriums.

3) to absorb or diffuse sound in the room to avoid ringing and flutter echoes, and improve stereo imaging.

4) to keep sound from leaking into or out of a room. That is, to prevent your music from disturbing the neighbors.

Proper acoustic treatment can transform a muddy sounding room, having poor midrange definition and erratic bass response, into one that sounds clear and tight, and is a pleasure to work and listen in. In a home theater, poor acoustics can make the sound less clear, harder to localize, and with an uneven frequency response. Even if you spent many thousands of dollars on the most accurate loudspeakers and other equipment available, the frequency response you actually realize in an untreated room is likely to vary by 30 dB or even more.

There are two basic types of acoustic treatment - absorbers and diffusors. There are also two types of absorbers. One type controls midrange and high frequency reflections; the other, a bass trap, is mainly for low frequencies. All three types of treatment are usually required before a room is suitable for serious listening.

DIFFUSORS AND ABSORBERS

Diffusors are used to reduce or eliminate repetitive echoes that occur in rooms having parallel walls and a flat ceiling. Although there are different philosophies about how much natural reverberation recording studios and listening rooms should have, all professional studio designers agree that periodic reflections caused by parallel walls are best avoided. Therefore, diffusion is often used in addition to absorption to tame these reflections. Such treatment is universally accepted as better than making the room completely dead by covering all of the walls with absorbent material. The ideal listening room has a mix of reflective and absorptive surfaces, with no one large area all live or all dead sounding. Understand that "live" and "dead" as described here concern only the mid and upper frequencies. Low frequency treatment is another matter entirely, and will be described separately.

Fast repetitive echoes - also called flutter echoes - can color the sound in the room and cause an emphasis at frequencies whose wavelengths correspond to the distance between the walls, and between the floor and ceiling. Flutter echoes are often identified as a "boing" sound that has a specific pitch. If you clap your hands in a live room or an empty stairwell or tunnel, you can easily hear the tone. If the room is large, you'll probably notice more of a rapid echo rat-a-tat-tat effect - the "flutter." Smaller rooms resonate at higher frequencies, so there you are more likely to hear a specific tone that continues even after the original sound has stopped. This effect is called ringing. Besides the obvious ill effects caused by the echoes, ringing creates an unpleasant sonic signature that can negatively affect the sound of everything played through loudspeakers in that room.

Note that echo, flutter echo, and ringing are intimately related, so the delay time and pitch always depends on the distances between opposing surfaces. With small spacings the flutter echo's pitch is directly related to the distance. I have a long stairwell in my home with a spacing of 36.5 inches between walls. When I clap my hands loudly I hear a distinct tone at the F# whose pitch is about 186 Hz, and the half wavelength for 186 Hz is 36.5 inches. But with larger distances you may hear a higher frequency than the spacing would indicate, depending on what sound source excites the echoes. For example, when you clap your hands or otherwise excite a room with only midrange frequencies, the only resonances that can respond are also at mid/high frequencies. So if the distance between parallel walls fosters a resonance at, say, 50 Hz, you might hear 200 Hz, or 350 Hz, when you clap your hands.

Bass Traps

Low frequency absorbers - bass traps - can be used to reduce the low frequency reverb time in a large space, but they are more commonly used in recording studios and listening rooms to reduce modal ringing and flatten the frequency response in the bass range. This is especially true in smaller rooms where a poor low frequency response is the main problem. In fact, small rooms don't really have reverb at all at low frequencies. Rather, ringing at the room's individual mode frequencies dominates. But in large recording studios, churches, and auditoriums, reducing low frequency reverb is an important reason for adding bass traps.