Room Acoustics

Just like light bounces off a mirror, sound waves travel around the room before reaching your ears (See figure 5). Some sound frequencies get reinforced, while others get suppressed, the result of which causes alteration in the sound's overall character. Any boundary surface (back wall, side wall, ceiling, etc.) can cause problematic reflections, so you should think about the reflective properties of the room when setting up your monitors. You can greatly improve the accuracy of your monitoring system by neutralizing the strongest reflections in a room.

Most people use a spare bedroom or other rectangular space for their project studio. Unfortunately, rooms with parallel walls often give rise to a phenomenon known as "standing waves" (also known as room modes). These occur when sound reflects off of parallel surfaces and travels back upon its own path, causing phase cancellations that interfere with a room's amplitude response. Although this type of setup can pose difficulties, it is possible to turn a rectangular room into an accurate monitoring space. You can minimize reflections and improve the listening experience by analyzing the sonic characteristics of the room and making adjustments.

Tip: Everything we hear is a combination of direct sound and reflected sound. In the studio, sound wave reflections can boost certain frequencies while cutting others. By sitting closer to the speakers, you will hear more of the direct sound and fewer reflections. (Remember to maintain the equilateral triangle arrangement when changing your listening position.)

Using Acoustical Treatment

Every room needs some sort of acoustical treatment. Sometimes this can be accomplished by carefully placing couches, bookshelves and other items in the room. In most cases however, it will be necessary to use sound absorption and diffusion materials. Appropriate acoustical treatment can reduce frequency reflections and smooth out the reverberation characteristics of a room—preventing insidious reflections from reaching the listening position.

Absorption

The primary goal of absorption is not to soundproof a room, but to neutralize the most troublesome reflections. Dense porous materials—such as heavy cloth, fiberglass, and acoustical foam—absorb mid‑ and high‑frequency sound. It's easy to attach these materials to walls and ceilings to tame multiple room reflections and/or dampen reflections. Generally speaking, two inches of acoustical foam is effective for absorbing frequencies above 500 Hz—while four‑inch foam will work all the way down to 250 Hz.

First, examine the potential reflection points in your monitoring environment. Are there any nearby surfaces that could reflect sound back towards the listening position? The most troublesome surfaces are usually the side walls, rear wall, and wall behind the speakers (see figure 6). These areas should be treated with absorptive materials.

Tip: You can construct portable absorption surfaces by mounting acoustical foam onto plywood boards. These can be placed 4‑6 inches from the walls—improving your studio acoustics without permanently altering the walls. You can even rearrange the boards to form a temporary isolation booth for recording vocals.

Stay tuned for the next article in our series, where we'll explain how to deal with bass frequencies. Learn more about M-Audio monitors.