February:
Acoustical Design Element 20: Front LCR speakers are set away from the rooms walls or response is optimized for near wall/ceiling installation internally or by equalization.
The phenomenon known as "Boundary Gain" results from placing a low frequency speaker near a solid wall. It produces an accentuation of low frequency sound energy. In-wall speakers, box speakers built into walls and speakers pushed against walls are subject to the effect. In many cases, speaker manufacturers design response compensation into the speaker's crossover. In other cases, the speaker must be pulled away from the wall or electronically equalized to correct this response distortion.
This sonic curiosity was one of the first acoustical phenomena I became aware of as a budding young audiophile. I could maximize the bass output of my homemade speakers by pushing them close to the wall. This trick became the mainstay of my adolescent calibration career as I found that most of my friends really liked the extra bass produced. It was made even more gratifying when I discovered the still greater low-end boost provided by pushing the speaker into the corner. It wasn't until I was enlightened that some musical instruments really didn't have fat bloated bass that the concept of tuning the sound by moving the speaker incrementally away from the wall became obvious.
Today, most well designed speakers are built to sound best at some general distance from the rooms boundaries. This might be a provocative statement given that many (not all) speaker companies will tell you their speakers sound best at any distance near or far from the wall including up against it. While it's true the speaker will likely sound good over a narrow range of near-wall distances, it is reasonable to assume there is a "best" distance. Determining that best distance is a calibration procedure requiring an experienced ear or a good Real Time Analyzer. Some speakers employ special filtering called boundary gain compensation (BGC) in the crossover to allow for near wall placement and some allow the calibrator to switch it on or off. The degree to which boundary gain is compensated for varies with the manufacturer so it remains that most speakers truly sound best well away from any boundary (there are other reasons too).
Speakers built into the wall encounter the strongest boost in the low end and therefore must employ BGC. Quality in-wall speakers often have two options selectable by the installer for boundary gain compensation. The first normal setting reduces the bass gain for the typical in-wall or ceiling installation. Alternatively, sometimes a speaker is installed near not one but two boundaries, as an in-ceiling speaker may be near a side wall. The second setting increases compensation for the additional boost the second reflecting boundary creates. The calibrator will usually choose which compensation is appropriate based upon the shape of the response plot for the speaker as measured by a Real Time Analyzer. The diagram (above) shows the hinged low end boost (upper plot) created by pushing a speaker against the wall. Obviously, accurate reproduction requires the smoother (lower plot) obtained by pulling speaker away from the wall or employing crossover BGC.
For the acoustical physics enthusiasts in the our readership it would be good to dedicate a sentence or two explaining boundary gain. The effect is due to the reflection of sound radiated toward the wall adding to the sound already radiated toward the listener; the acoustical output of the speaker is effectively doubled. Why is only bass boosted? This is because of the omni-directional nature of the lowest frequencies of sound; high frequencies simply do not radiate behind most speakers and thus are not part of the reflected sound. Additionally, the long wavelengths associated with the lowest frequencies allow for an in-phase addition of the output. Since we typically measure sound pressure level (SPL) rather than sound power level this additive phase relationship results in up to a 6 dB boost in SPL. If a second boundary is nearby, like near a wall-ceiling intersection, we now have a 12 dB boost. If the speaker is near three boundaries; a corner, you guessed it, we get an 18 dB gain.
A final tool used to control the response distortion caused by boundary gain is equalization. A calibrator can align the EQ to compensate for the boundary gain, shaping the final response of the speaker to be correct for the listening position. This is a powerful tool since it allows for more flexibility in speaker placement by making the BGC infinitely adjustable.
Things have come a long way since I was impressing my friends and family with the powerful booms of my Radio Shack 12" woofers. I have found that as much as home theater sound seems to be ruled by the thunderous sound of bass, a truly successful theater is one that demonstrates subtlety as well. I'm a sound system fanatic so it troubles me when I see systems seemingly adjusted for maximum output rather than dialed in for accuracy. I recommend when you start moving speakers closer to the wall, think of right-sizing the sounds you hear. Think of listener fatigue and how you'll be enjoying the bass after a two hour movie or after 10 movies. I love lots of bass; you should have no doubt of it's copious presence on your movie or music soundtracks and that it doesn't need to be exaggerated to be thunderously good.
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 20 could be a contributing factor, of course, don't forget about the other 34 elements in the mix. Next month Element 33.
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)