Locating Your Room's Reflective Trouble Spots

Congratulations. If you've made it this far, you're past the most technical information. You've learned how room reflections affect the sound of your stereo or home theater system. And you've learned how absorptive and/or diffusive materials can help control those reflections. Now the fun begins. You're going to learn how to locate your room's reflective trouble spots.

At frequencies above 500Hz, sound waves bounce off reflective surfaces (like walls) as predictably as light bouncing off a mirror. In the above illustration, sound from a speaker is reflected off a side wall toward the listening position.

Mirror, mirror on the wall...
Along with standing waves, the reflections that distort sound the most are the loud reflections that bounce once off the side walls, ceiling and floor on the way to your listening/viewing spot. These strong reflections are called "early" reflections. Controlling the intensity of early reflections is crucial to achieving optimum sound.

So, how do you figure out exactly where these reflections are coming from?

An easy, accurate way to locate the precise points of sound reflectivity on your walls, ceiling and floor is to use a mirror. You'll need a friend or family member to act as an assistant. While seated in the listening position have your assistant slide a small mirror (8" x 10" works well) along the left wall at the height of the tweeter. Your assistant should start across from the left speaker and move slowly toward the listening position. As the mirror is moved toward you along the wall, you will at some point see a reflection of the left speaker in the mirror. Mark the spot on the wall where the tweeter reflection appears with a piece of tape.

As the mirror continues moving toward your listening position, you will next see a reflection of the right speaker. Mark the location of the right tweeter reflection spot on the wall with another piece of tape. Now repeat this procedure on the right wall to locate the corresponding two reflective positions there.

Early sound reflections from the points you located are adding significantly to the sound you hear at your listening position. They cause some sounds to be canceled out while others are amplified, resulting in smeared stereo images.

If you can see it, you can hear it. Wherever you see your speaker reflected in the mirror, that's a point of reflection that should receive absorptive, or in some cases, diffusive acoustic treatment.

The solution to this problem is to treat these points of reflectivity with some form of absorptive material. Panels made of 1" fiberglass or foam (polyurethane or melamine) installed on the walls do an excellent job of absorbing these reflections. Be sure to use enough absorptive material so that it extends at least 18" on either side of the marked locations. The material should also be at least as high as the tops of the speakers for best reflection control.

If you refer back to the first page of this article, you'll see that sound reflections from your room's ceiling and floor can also contribute to sonic problems. Repeat the wall procedure on the ceiling. Have your assistant move the mirror along the imaginary lines on your ceiling that would connect each of your speakers to your listening position. You should be able to locate one reflective point on each line about midway between the speakers and the listening position. Mark each of these spots with a piece of tape. Apply absorptive material to the ceiling extending at least 12" on either side of each marked location.

The floor between your listening spot and your speakers can also be a source of reflected sound. If your floor is carpeted, you needn't worry about floor reflections. But if the floor is a hard surface like hardwood, tile, or linoleum, use the mirror technique to find the points of reflectivity. Move the mirror along the imaginary lines connecting each speaker with your listening position. Again, you will be locating one point along each line roughly midway between the speaker and the listening position. An easy, good-looking way to treat floor reflections is to cover the points of reflectivity with a reasonably thick area rug.

It may seem strange, but reflections from the wall behind your speakers also contribute to the sound you hear at your listening/viewing position (mostly frequencies below 500 Hz). Use the mirror technique again to find the point of reflectivity for each speaker on this wall and treat it with absorptive material. By using thicker material, or providing for an air space between the material and the wall, you'll get improved low-frequency absorption.

The wall behind your listening area may require treatment also, though if it's several feet away, reflections probably aren't a serious problem. This rear wall is the surface that would benefit most from diffusion products or a bookshelf.

Most people are more comfortable thinking about music as notes or tones, rather than frequencies. Middle "C" on a piano is 262 Hz. Low "E" on a bass guitar is 41 Hz. Cymbals can go out to 15,000 Hz.

Summing up
The next time you sit down to watch or listen, think about the ways — good and bad — that your room may be affecting the overall sound of your system. Although some of the concepts discussed here may be difficult to wrap your brain around, a basic understanding of room acoustics (and speaker placement) can help you maximize the performance of any audio or home theater system.

Also be on the lookout for "acoustics-savvy" products such as powered speakers with built-in bass equalization, and receivers and processors with DSP room correction.

For more room acoustics information:
The Master Handbook of Acoustics, Fourth Edition, by F. Alton Everest. A wide-ranging look at room acoustics, both in the home and the studio. Includes detailed explanations and some math, yet is very accessible. Complete with essential information for building or remodeling a listening room. Highly recommended. 592 pages.

The Complete Guide to High-End Audio, Second Edition, by Robert Harley. 558 pages.

Special thanks to RPG for their assistance. Their website is www.rpginc.com