Well-designed studio monitors are engineered to reproduce an input signal with extreme fidelity, maintaining near-flat frequency and phase response at all levels up to the threshold of clipping. However, the performance of any monitor will be influenced by the acoustics of the space in which they operate. Difficult room acoustics, even with proper speaker placement, can interfere with achieving the highest fidelity your monitoring system can provide.
For example, if your room exaggerates high-frequencies, your mixes will lack life and feel flat elsewhere. If your room exaggerates low-end, that thundering tom roll will lose all its thunder when you hand off your mix to your drummer to take a listen.
In most home-studio environments, the room is not designed to maximize the listening experience, so it’s necessary to recognize and correct what that space does to the sound in order to optimize the monitor system’s performance.
In general, the following physical features of a room can affect a sound system’s performance:
Size. The size of the room directly impacts how well certain frequencies will be reproduced. For example, if you measure a room diagonally, you will discover how well that room will be able to sustain low frequencies. This may seem odd until you think about the physical length of audio waves at various frequencies. For example, a 50 Hz wave is about 22.6 feet long. (To calculate how long an audio wave is, divide the speed of sound—1,130 ft./second—by the frequency. For a 50 Hz wave, 1,130/50 = 22.6 ft.) Because of this, a room that is 45 feet on the diagonal is going to regenerate low frequencies more effectively than a room that is 15 feet on the diagonal.
When a room’s width or length correlates directly to the length of a waveform at a specific frequency, a standing wave can occur where the initial sound and the reflected sound begin to reinforce each other. Let’s say we have a long, narrow room where the distance from one side to the other is 22.6 feet. When a 50 Hz wave bounces off the wall, the reflective wave travels right back along the same path and bounces off the other wall, and the cycle repeats. In a room such as this, 50 Hz reproduces very well—maybe too well. So when you’re listening to your mix in that room, it will appear to have a heavy low end because the low frequencies are being exaggerated by the room acoustics. Since you hear exaggerated lows, you’re likely to compensate for them, and when the mix is played elsewhere, it will lack low end.
Construction. Low-frequency waves are powerful enough to cause the walls, ceiling, and even the floor to flex and move. This is called “diaphragmatic action,” and it dissipates energy and strips away the low-end definition. So if your room’s walls and floor are made of solid brick and concrete that don’t vibrate much, the bass response is going to be much more powerful than if you’re in a room where the walls are normal sheet rock construction and the floors are hardwood.
Reflectivity. Another way a room interacts with sound waves is through reflectivity. Like most room anomalies, reflections can be good and bad. Consider the effect of a cathedral’s reflections on a choir or a piano. This type of reverberation (reverb) is quite desirable for recording but not so much for mixing. If a speaker is placed near a reflective surface (such as a brick wall or window), the direct sound coming from the speaker and the reflected sound coming from the wall can arrive at the listener’s ears out of phase with each other, causing cancellation and/or reinforcement. If they’re 180 degrees out of phase with respect to each other, they will cancel each other out.
If you are setting up your mix environment in a reverberant space, position your speakers so that as much sound as possible is focused on middle of the room and steered away from reflective surfaces. You should also install acoustic treatment to lessen the impact of reflections at your listening position.
Mitigating Room Problems
Even the best room can use some acoustic treatment. This can be as simple as strategically positioning couches, bookcases, and rugs. In general, acoustic treatment falls into two categories: materials that absorb sound and materials that diffuse sound. Placing the correct type of acoustic treatment will reduce unwanted reflections that can impair the quality of your listening position.
Materials that absorb sound will help to reduce reflections. The best types of absorptive materials are generally dense and porous. Heavy cloth, acoustic foam, pillows, and fiberglass insulation are all examples of absorptive materials. This type of acoustic treatment is best for reducing the mid- and high-frequency energy bouncing around your room.
An easy trick to figure out whether and where you need absorption in your room is to sit down at your mix position and clap loudly. If you hear your clap reverberating around your room, you’ll benefit from some acoustic absorption. Take a look around your room and look for potential culprits that could be reflecting the sound back at you. The usual suspects are the walls around you, including the wall behind your studio monitors and the ceiling above your head.
Acoustic foam is relatively inexpensive, can be purchased at your favorite music-equipment retailer, and comes in different thicknesses. Generally speaking, 2-inch foam will best reduce frequencies above 500 Hz; 4-inch foam can reduce frequencies all the way down to 250 Hz. Whatever the thickness, acoustic foam is easy to attach to the wall using spray adhesive or tack nails.
If you are in a space where you are worried about damaging your walls, you can purchase a shadow box at your local arts and crafts store and attach the foam to that instead. Once the foam is attached to the frame, you can hang it on your wall just like a picture.
With your high and mid frequencies tamed, you’ll need to tackle low-frequency buildup. Uneven bass response is a notorious problem in project studios. Bass energy can build up in corners and other boundary points and make the bass response in your mix position muddy and ill defined.
Low-frequency energy behaves very differently than high and mid frequencies. Materials that absorb high and mid frequencies may not effectively absorb low-frequency energy and vice versa. To add to the problem, bass energy is not directional, so it can build up pretty much anywhere.
Bass traps are a special type of acoustic absorption material that reduces low-frequency resonance. Placing bass traps in your room’s corners and other boundaries (like where the wall meets the floor or ceiling) is usually adequate, but if you’re handy and don’t want to damage your walls, you can construct a mobile bass trap by wrapping layers of carpet and carpet padding around a large frame or bookcase and mounting castors on the bottom. This will allow you to create a custom mixing environment without permanently altering your room. The Internet is a great resource for other creative ideas to build custom acoustic treatment.
In general, rooms that are 10 x 10 feet or less will get great results with some combination of absorption material. If you are mixing in a larger space, you might want to add some diffusers to deaden it. Sound-wave diffusers are designed to break up standing waves by reflecting the waves at different angles. These panels can be mounted to the wall or ceiling as necessary and can get rid of “flutter echo.”
With two parallel reflective surfaces in a room, such as opposing walls or the floor and ceiling, there is always the possibility of successive, repetitive reflections that are equally spaced in time. Known as “flutter echoes,” these reflections can produce a perceived pitch or timbre that colors what you’re mixing. This can also reduce intelligibility. Diffusors can break up flutter echoes by reflecting the sound waves in different directions so that the repetitive reflections are eliminated.
Bookworms have a bit of an advantage with this type of acoustic treatment, as bookcases filled with books provide multiple absorbent, different-shaped objects that break up standing waves and flutter echoes. So if the problem is two parallel walls, putting your library on one or both walls may eliminate the need for additional acoustic treatment.