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 Jan 20, 2005 - 10:00 AM - by Michael
* Ecoustics: Why wall speakers suck

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PC Games/Hardware/Microsoft
Ecoustics has a rant on people who hide their speakers in wall enclosures and muffle the sound.

The reason that a well-designed loudspeaker sounds neutral, natural and "musical" when it's heard in an ideal setup away from room corners, ultimately depends on the speaker's "linearity," the technical term for the smoothness of the frequency responses that are radiated directly forward and at increasing angles to each side. The "off-axis" radiation at angles to each side is important because these are the sounds that are reflected laterally from the walls. Dozens of measurements of a speaker's output into the area of space in front of the speaker (the front "hemisphere") show that if the frequency response of the speaker remains smooth and consistent over a broad angle, the speaker will be musically accurate and free of tonal "colorations" and distortions that lessen the realism of the reproduced instrumental sound.

If we listen to instruments facing toward us in a live setting (think of horns, reeds, and woodwinds), the complex wavefront that represents the "sound" or timbral signature of a particular instrument is composed of fundamental musical tones plus harmonics that are generated by the resonances within the pipe or tube of the instrument. This combination of tones and overtones create the sonic signature for every instrument, and even establishes the unique character to our own voice. This complex mix of tones and overtones is radiated instantly and simultaneously, reaching our ears intact. If the instruments face away from us, they sound different, because the makeup of the instrument's set of fundamentals and overtones has changed.

Instruments have far-field reflections, not near-field reflections, so if you put a musical instrument inside something, you change the radiation characteristics. You create near-field reflections that happen too soon, so the frequency response and the spectrum gets distorted. (For exactly the same reason, you don't play your guitar facing the wall.)

Loudspeakers work the same way. When the designer knows the way the loudspeaker responds over a broad set of angles in a test setting (he has examined a "family of curves" ? the speaker's frequency responses into a broad space in front of it), he knows that when the speaker is placed in a room it will continue to disseminate its energy in the same manner, so there is a high degree of probability that the loudspeaker will continue to deliver its wavefronts accurately, and it will be heard that way by listeners in most conventional domestic rooms. In the development of a loudspeaker prototype, great care is taken to investigate and correct any deviations from smoothness or linearity. Even the destructive interference and resulting frequency response glitches caused by edge diffraction from the physical edges of the speaker's front baffle or the speaker grille frame and grille material can be documented and fixed.

Knowing this, it's not difficult to understand what happens when you put a tower speaker (or a bookshelf model) inside a wall unit, credenza, or entertainment center. The surfaces in the wall unit's cavity at each side and above and below it, create a set of near-field reflections that don't exist when the speaker operates normally unencumbered by cabinetry. These cancel and reinforce at various frequencies, affecting the linearity of the system and producing all kinds of nasty anomalies and unmusical colorations.
Basic wavefront physics, but something most people never realize because they can't SEE it.

Hopefully, a few clueless owners who don't realize why their sound doesn't work right will pick up on this one.

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