January:
Acoustical Design Element 19: Front LCR speakers should not be placed inside encasements or enclosures or steps have been taken to limit resonance and diffraction.
Any cubby holes or enclosures present in a sound room create opportunities for resonance. Installing a speaker inside such a cavity simply accentuates the problem. Speakers have finely tuned response curves that become heavily distorted when emitting sound from within such an encasement. Deadening the air space with sound absorbing material or isolating the front drivers from the encasement by an air tight baffle will significantly reduce unwanted resonance. In addition, at no time is it appropriate to use rear ported or rear firing speakers in such an enclosure.
This flaw is particularly common because of the popularity of entertainment center cabinets. Most of these furniture structures are designed by cabinet makers not acoustical engineers; that would be an interesting selling feature if they had been acoustically designed... hmmmm. My approach when discussing this issue with a client is to point out the sonic difference as I speak normally versus speaking through cupped hands. The nasally distorted sound through my hand-megaphone is very similar to the effect an encasement has on your speakers sound.
In extreme cases, for customers "requiring an intervention":). I will tote along a pair of speaker stands to their home and demonstrate the sonic difference with their own speakers; first placed as normal inside the cabinet then we listen on the stands in open air. This approach, while highly effective, does not always lead to the tossing of an expensive entertainment center on the scrap heap though. I'm sensitive to the need for good sound and good looks to get along; I remember as a kid ogling the beautiful entertainment center my neighbor had purchased. What a concept, deep rich bass and sparkling highs heard through maple louvered doors (tongue-in-cheek). An entertainment center can be designed to sound pretty good though and, in many cases for those existing cabinets, even modified to solve many sonic failures.
The first step is to soak up the sound energy resonating inside the cavity. The best sound absorber is a dense fiberglass, but I do live in the real world and fiberglass is itchy if not upholstered or treated. A better approach is to use an acoustical foam but if you are in a bind cost wise, consider upholsterers foam. I buy scrap from a local shop and stuff it inside the offending encasement. The immediate sonic benefit is more than startling. Of course this approach is not going to fly if the foam is visible; the hard part is concealing the foam behind an acoustically transparent cloth. This is tough because many cabinets do not have grilles to conceal the speakers and even if they do, the grilles contribute to the next flaw in many of these cabinets; diffraction.
Diffraction is the result of sound encountering a surface or edge of a surface as it propagates. When this occurs near the speaker, depending on the frequency (or more appropriately the wavelength) of sound, the result can create a ripple in the clean sound wave. This "ripple" is an acoustical distortion that make imaging less focused and can affect the frequency response if conditions are right. This is one of those innocuous distortions that doesn't poke you in the eye but does strip a soundstage of much of its spacious and enveloping imaging a.k.a realism.
To recap, you can eliminate the problem by eliminating the cabinet encasement. Short of that try stuffing the speaker cubbie with foam to reduce resonance and then pull the speaker as far forward out of the hole as is stable. You'll have made a big difference in your sound and perhaps began to ponder other subtle changes that could reinvent your home theater experience.
I am occasionally confronted in well meaning discussions about the relevance of one acoustical flaw or another. Does it really matter that much? At the end of the day, what is missing due to such tiny misalignments? Perhaps the answer is only fathomable from the perspective of the music lover teleported in time and space by the majesty of an amazing performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among many subtle acoustical distortions has robbed you of the experience? If you have not been surprised recently by your sound system perhaps Element 19 could be a contributing factor, of course, don't forget about the other 34 elements in the mix. Next month Element 20.
Gerry Lemay (Gerry is the Director of the HAA and writes the Home Theater Rx column for Home Theater Magazine)