7 Myths about Room Acoustics
(Part II)

by Gerry Lemay of Sencore Electronic Test Instruments
Courtesy of CE Pro

Myth 4: The center channel is only for
movie dialog.

t’s been years since Dolby Pro-logic introduced the consumer audio world to the center channel. Actually, it would be more accurate to say re-introduced, since Dr. Fletcher of Bell Labs first demonstrated “stereo” with a center channel back in 1933. That’s right, 1933.
It seems the exciting new technology called stereo only worked if the listener remained in a fixed position precisely between the right and left speakers. Dr. Fletcher corrected the problem by adding an identical center speaker designed to play the common signal from the right and left channels.

Stereo actually translates as “solid” from Latin (as in three-dimensional) not “two” as so many assume. Today’s center channel is a direct descendant of this earliest work and retains this critical role for the home theater. It is true the center channel also contains the dialog though. In fact, the center channel is the source for more than 95 percent of the dialogue in a typical movie. The center channel can be justifiably regarded as the most important speaker in a home-theater system.

Myth 5: The most important component in the system is the speaker.

In reality, the definition of a sound system must include the listening room as one of the components. While good sound is heavily dependent on good speakers and electronics, the weakest link in the system is usually the room. Over the years there have been many accounts of consumers who’ve become disillusioned with their “systems” because of poor sonic performance. The aimless cycle of switching out speakers, cables, and other components to solve what are in reality acoustical problems can destroy a client’s faith in the installer and potentially in the A/V industry. The consumer must understand the room is the trump-card component in the system.

Many rooms in today’s high-end homes are poorly designed from a sonic perspective. As a home theater owner, you must be educated about the role the room plays in the enjoyment of the other components you’ve purchased. Unfortunately, most consumers don’t have an intuitive feel for the qualities a good sound room should have.

Myth 6: Surround speakers are only for special effects and are otherwise silent.

This myth may be at the heart of one of the most common complaints we hear from consumers about the side or rear channels: “I can’t hear the rear surrounds.” It’s common for installers to either turn the speakers’ levels up, or explain that the surround speakers are only audible for special moments during the film, and are mostly silent otherwise.

The truth is that in a properly mixed movie or music recording, the surrounds are constantly relaying vital sonic information. A useful demonstration is to play a good multi-channel classical recording or a movie scene set in a cave or other reverberant space. During a quiet passage allow yourself to experience the enveloping sense of atmosphere of the recorded venue; then have someone abruptly turn the surrounds off. The resulting collapse of the surround soundstage should be quite noticeable.

Another thing to keep in mind is that the surrounds have a vital role in relaying localizable sound effects. Effects like a helicopter zooming around, or a dinosaur stomping on trees behind you are very important sequences in a movie.

If you take the easy road by turning the surrounds up louder, you are exposing yourself and your listeners to a worse experience: the often described “exit door effect”. This occurs when the listeners, especially those closest to the surround speakers are constantly reminded of the presence of the speaker beside or behind them. This ever present sonic tap on the shoulder snaps the listener out of the “suspension of disbelief” the movie makers worked hard to achieve.

Avoiding this dreaded problem requires precise calibration of all speaker levels. The usual cause of “silent surrounds” is excessive ambient noise. “Excessive” doesn’t necessarily mean loud. Disruptive levels of ambient noise can be quite innocuous and otherwise tolerable.

This is why the enveloping field of sound created by the surround speakers is often lost. Those rear-channel effects are being drowned out by the relatively low-level noise in the room. There is no substitute for quiet in a sound room. High-performance sound depends on it.

Myth 7: Acoustical treatment is ugly and always expensive.

This myth is perhaps the inspiration for some of the previously mentioned legends. Most folks are willing to ignore the room because acoustical treatment is prohibitively expensive and makes the room look like a recording studio. The truth is that acoustical treatments can be easily disguised.

There are three basic reasons for adding
acoustical treatment:
1. To reduce the time sound is allowed to bounce around (reverberate) in a room
2. To eliminate exceptionally strong reflections of sound common when speakers and listeners are positioned near walls in a small room
3.To control uneven bass response

While high performance depends on the expert placement of just the right kind and amount of absorption or diffusion, great things are still possible even using minimal treatment. Have you ever been in a room that sounds good despite a lack of fiberglass or sculptured foam on the walls? This is because the everyday furnishings of the room can operate as acoustical absorbers. Drapery, carpeting, cloth furniture, throw rugs, pillows, an eight foot stuffed grizzly bear...well, you get the idea.

Expert calculations of acoustical requirements are desired, but reducing the reverberation time to an acceptable level can be estimated with good results by an experienced and properly trained professional.

Other acoustical issues include avoiding too much absorption, which can cause the loss of the enveloping surround field, and those pesky reflections that were mentioned earlier need to be dealt with in order to preserve the clarity and focus of the performance. Flat reflective walls on either side of the room are especially bad, but treatment can be disguised as other architectural elements in a room. The good news is that a properly designed listening room can expose little evidence of the acoustical treatment. Dialing in the proper mix of absorption, diffusion, and bass treatment is the correct way to achieve high-end results. For the very best results, use a professional acoustical designer.

This article provided by Kevin Ray of Custom Audio Video, LLC. Member CEDIA (Custom Electronic Design & Installation Association)