7 Myths about Room Acoustics (Part I)
e’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 revolve 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 appearance-conscious 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 calibrated 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 readings, reverb decay time treatment, and bass management have made speaker functions and energy-time positioning more flexible and easier for good looks and good sound to coexist.
- Courtesy of CE Pro, Gerry Lemay of Sencore Electronic Test Instruments. Provided by Custom Audio Video
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