May:
Each month we look at a different acoustical design or calibration element. Taken from the HAA Acoustic Design and Calibration Review checklists, these elements detail the many attributes that define high end home theater sound. An HAA Calibrator can perform an analysis of your system by reviewing each of the over 70 elements. This month we look at ADR Element #22.
Acoustical Design Element 22: Subwoofer(s) placement allows for positional equalization techniques and/or parametric equalization is employed to smooth response.
Uneven bass response means too much or too little bass at various frequencies and often simultaneously different responses for each listener in the room. This distortion results in a lack of articulation and clarity and at worst, the result is often fatiguing. In either event, it ruins the performance potential for a high end sound system. Optimizing the location of the speaker and listener in the room and the correct use of parametric equalization is the key to solving the problem. Interior Designers should take heart; good sounding subwoofer placement can still be highly flexible for modern interiors.
I wrote an article a few months ago about seven acoustical myths. In it I point out the issue I have with the oft stated belief that a subwoofer can be placed anywhere because humans can’t discern the source direction of deep bass. I also point out that while it is a powerful and necessary tool, equalization cannot solve all acoustical ills. I won’t rewrite the entire article here, you can find it in the November 2004 issue of CE-Pro magazine, but I will restate the basic premise; the position of a subwoofer is a calibration element in small rooms. I add the “small room” caveat because first of all, that where most of us keep our subwoofers and also because the behavior of low frequencies in a small room predisposes terrible frequency response. Allow me to insert a short excerpts of the article of which I spoke:
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 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 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 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 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.
Here’s the link to the whole article: CE-Pro Article 7 Myths
I am sometimes confronted, during friendly discussions with clients, about the relative importance of one or another "minor" acoustic or setup flaws in their home theater. Does proper setup and calibration really matter that much balanced against other priorities? In the final analysis, just what is missing due to any of these seemingly innocuous misalignments? I think the answer is best explained from my perspective as a music lover relating how I am occasionally teleported in time and space by the majesty of an amazing recorded performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among these few subtle acoustical flaws has robbed you of the experience? If you have not been surprised recently by your sound system perhaps Element 22 could be a contributing factor, of course, don't forget about the many other Design and Calibration elements in the mix. Next month another Element.
Gerry Lemay (Gerry is the Director of the HAA, President of Quest Convergence Systems, and writes the Home Theater Rx column for Home Theater Magazine)