Sencore News: Myths about Room Acoustics Nov 04 courtesy of Gerry Lemay

Room Acoustics

Finding good sound often requires the rearrangement of seating and speakers. Room-mode EQ equipment is a “band-aid” solution. Boomy bass is not necessarily good bass. We’ve all heard urban myths. There’s the one about “Mikey” from the Life cereal commercials. According to the myth, he died after eating the candy “Pop Rocks” while drinking a Coke. Of course, it never happened and “Mikey” is still alive. But it sounded pretty wild to kids who bought the candy back in the late 1970s. .. The custom-installation business has its own urban myths, and while they aren’t as sensational as the legends surrounding celebrities and food, they can be just as destructive to the quality of a company’s installations. Often times, the custom field’s myths or inaccuracies center around the topic of room acoustics.

Myth 1 It doesn’t matter where you place the subwoofer. This is a story retold everyday by enthusiastic A/V salesmen eager to please appearanceconscious customers. The sub is discretely hidden behind the TV in the corner, behind the couch or even built into the floor. In many cases, an otherwise successful installation comes up short because of poor bass quality. Small rooms do strange things to long wavelength bass frequencies. In fact, the typical room becomes a virtual topographic map of mountainous bass peaks and hollow bass nulls. And the smaller the room, the worse the variation. Most installers don’t always have the option of placing any speaker much less a bulky subwoofer in the perfect sonic position. It is worth the effort, though, to choose the speaker location wisely rather than assuming it doesn’t matter. It doesn’t make sense to spend big bucks on a top of the line subwoofer then leave its calibration to chance. Calibration of a sub means an alignment of 5 basic elements: .. The sub’s relative volume compared to the main speakers .. The phase relationship between the sub and the main speakers at the crossover frequency .. The position of the listener’s chairs in the room .. The position of the sub based upon the chairs .. The parametric equalization of any remaining response anomalies An additional consideration is the production of artifacts by the sub’s woofer or port. These noises are not in the soundtrack and are unwanted. They are not always obvious but they create an annoying side effect. While low frequency sounds are almost impossible for humans to localize (that’s where this myth originated by the way), these higher frequency artifacts are easily localized and often draw attention to the sub’s location. The first step in stopping this is usually to turn the subwoofer by pointing the port or subwoofer away from the listener until the sub’s bass sounds fully integrated with the main speaker’s sound. The important thing to remember is that a sub can be behind, beside or in front of the listener and remain undetected. But the location and position needs to be scientifically chosen and the sub properly calibrated. Room modeling software can help an integrator determine where a room’s peak is and where null modes are located.

Myth 2 Equalization can fix all acoustical problems. Equalization is the process of electronically correcting the frequency response of a sound system. Recently, there has been a sudden increase in the number of manufacturers with parametric equalization built into their processors and subwoofers. This is a good thing. It demonstrates to consumers the need to calibrate home-theater systems in order to produce the best performance. The proper use of equalization can provide an outstanding improvement in sonic fidelity if the EQ is low noise and low distortion. The acoustical distortion of bass in a small room can be greatly reduced by a thoughtfully set up parametric equalizer that minimizes room-mode peaks and smoothens frequency response. Unfortunately, the impression some have is that this fix results in a cure for all acoustical ills. This is not true. Some response distortions cannot be equalized away. Frequency response dips due to bass mode nodes or boundary interference will not be changed by EQ. In fact, a poorly set-up equalizer can make matters worse if the operator does not understand the nature of these cancellations. Many response distortions are only solved by movement of the speaker and/or listener as a fix. Some problems are too severe for an equalizer to repair. Other acoustical problems are not directly related to frequency response. Excessive reverberation in a room, for example, can only be solved by physically changing the interior surfaces of a room, making it more or less reverberant. An improperly placed speaker can cause a disjointed and in-cohesive soundstage, or a lack of surround envelopment, bad imaging and acoustically mono presentation among other things. None of these problems can be cured by EQ. Equalization, particularly with a high quality DSP parametric equalizer is a powerful and necessary tool in the hands of a competent calibrator, but it is only one tool among many that are needed to calibrate a sound system.

Myth 3 Sound-system calibration is only for “tweaky” audiophiles. Calibration is a relatively new term for the consumer-audio industry. Most people are aware of the “tweaking” of a system to bring out the best sound quality. In the past, “tweaking” often involved hours ruminating over the relative strengths of one speaker position over another. Most consumers had little patience for such an evolution, especially given that the best sounding speaker position always seemed to be in the middle of the room. More often than not, the speakers were relegated to inconspicuous positions behind the couch and ended up becoming just a glorified background-music system. Today, however, with the advent of home theater, consumers are making the “theater” the centerpiece of their home, and the movie event has become a key source of family home entertainment. In turn, this phenomenon has spurred on the growth of theater budgets dramatically. In that light, tweaking a system to achieve the best results makes even more sense to quality conscious consumers. The techniques those tweaky audiophiles employed have been improved by scientific calibration processes to dial in the system and hone the acoustical environment of the theater. Advancements such as time delay, real-time analysis, parametric equalization, acoustical treatment and bass management have made speaker positioning more flexible and easier for good looks and good sound to coexist. Sencore’s SP295C real time analyzer can provide integrators with a number of test functions, including octave and 1/3 octave analysis, sound-pressure level readings, reverb decay time functions and energy-time graph readings.

Myth 4 The center channel is only for movie dialog. It’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. Can you imagine if the only way to hear the 3-D audio soundstage was to sit in the precise middle of the room? The poor placement of the center channel in some home theaters results in many off-axis listeners unable to hear the threedimensional imaging. 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 justifiable 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 our industry. The consumer must understand the room is the trump-card component in the system. Many rooms in today’s high-end homes are so poorly designed from a sonic perspective; it’s hard enough to hold an intelligible conversation, much less listen to a high-resolution sound system in them. A home theater client must be educated about the role the room plays in the enjoyment of the other components they’ve purchased. Unfortunately, most consumers don’t have an intuitive feel for the qualities a good sound room should have. Hard surfaces, such as a wood floor and sheetrock walls, can have a negative influence on the sound of a home-theater system.

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.” The installer either turns the speaker’s level up or explains that the surround speakers are only audible for special moments during the film and 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 multichannel classical recording or a movie scene set in a cave or other reverberant space. During a quiet passage, allow the listeners to experience the enveloping sense of atmosphere of the recorded venue; then abruptly turn the surrounds off. The resulting collapse of the surround soundstage should be enough to make your point. The best way to determine room modes is through the use of a real-time analyzer. 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 the listener or a dinosaur stomping on trees behind you are very important sequences in a movie. If we take the easy road by turning the surrounds up louder we are exposing the listener 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 to 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: .. To reduce the time sound is allowed to bounce around (reverberate) in a room .. To eliminate exceptionally strong reflections of sound common when speakers and listeners are positioned near walls in a small room .. 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 wainscoting, bookcases or other architectural elements in a room. The use of common household items, like pillows and throw rugs, can help tame problematic room modes. 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 HAA trained acoustical designer.