12
lengths that the waves take before arriving at the listening position. Although the first arriving bass
wave will have flat frequency response, fractions of a second later the same waves have bounced off
of the front wall, rear wall, side walls, and the floor. These reflections both reinforce and cancel the
original direct wave, depending on the wave length vs. the path length. If the path length is an even
multiple of the direct wave at any one given frequency, the reflection will reinforce the first wave
since it will be mathematically additive and will cause a peak in the response at this frequency.
However, if the path length is such that the reflected wave is out of phase with the main wave, there
will be a cancellation in the frequency response. Researchers have found that the dips and peaks in the
response can be very powerful, with 18dB variances across the bass range! Note that this effect occurs
with ANY speaker design:
no
speaker system can overcome the law of physics.
When playing a recording with plenty of deep bass, a listener can walk around the room, finding that
every foot or two, there is either flat response, a peak, or a dip. By walking around the room, you can
“map” the response, finding the places where the pressure alternates from high to low. These
alternating pressure zones are a natural part of room acoustics and must be dealt with by correct
speaker placement.
THEORY OF RECIPROCITY REGARDING BASS RESPONSE:
Since standing waves are not easily amenable to correction by equalizing the speaker passively, it is
necessary to place both the speakers and listener/s in neutral zones. According to the mathematics of
the pressure zones, the flattest response occurs when both the speakers and listener are at opposing
ends of the room, directly against the wall surfaces. Since against-the- wall placement is not a prime
location for soundstage “openness” and imaging focus, we recommend placing the speakers out into
the room, at least 12" from the back wall to start. Then, playing a repeating bass line, walk around the
room in the general vicinity of the intended listening position. If you find that the smoothest bass
response does not occur where you would like to place the listening seat, you can use the Law of
Reciprocity to find the optimum spot.
The physics of bass response are the same for the listener or the speaker when interchanged. That
means that you can place one speaker where you wish to sit, then play the repeating bass line, and
then walk around on the other side of the room where you want to place the speakers. When you find
a neutral zone, where the bass is not too loud nor too soft, that’s where you place that particular
speaker. Next, do the other side in the same manner. Make sure you use a variety of recordings, since
it is possible to optimize the placement accidentally for only one frequency range!
TOO MUCH BASS OVERALL:
If you have optimized the placement, yet find that the bass is too powerful due to a very small room
and/or mandatory corner placement due to the nature of the furniture arrangement, you can “tune” the
bass balance. If the entire bass range is too loud, purchase a 2 lb bag of Dacron Polyfil from the local
five and dime store or sewing shop. (Dacron is a synthetic cotton and is used for pillow stuffing).
There are two different ways to use the Dacron: a). filling the woofer cavity with more stuffing to
reduce bass power, or b). partially blocking the vent to reduce the 20-30Hz tuning frequency range.
A). To reduce the overall bass power, insert a ball of Dacron approximately the size of a large
