Community VERIS Series - Operation and Installation Manual - Page 25
subwoofer and several full-range enclosures in the same system, it will, of course, be easier
to reverse the subwoofer’s polarity to test the response. Ultimately, as we’ll see below, it’s
best to keep the subwoofer in a polarity-positive state.
If there is no discernable difference or only a very minimal difference in the measured or
audible response when the polarity is reversed, it indicates one of two things:
(1)
The full-range system that the subwoofer is being used with does not reproduce
enough low-frequency output to cause either cancellation or addition with the
subwoofer. This would be true if the full-range system is a very small loudspeaker, like
those that are used for front-fill and underbalcony fill.
(2)
The placement of the subwoofer in relation to the full-range loudspeaker is not
optimum. Little or no response variation will occur if the physical relationship results in
an approximate ¼ wavelength of offset at the center of the crossover frequency.
The solution to (1) is for both systems to remain in positive polarity. No harm will occur if
the full-range system simply does not reproduce enough low-frequency energy to either
add or cancel with the subwoofer’s output.
The solution to (2) is to either change the physical relationship of the two systems, or to
delay one of the two systems (whichever one is positioned closer to the listeners) with a
digital delay. A high-quality, high resolution measurement system that can read and depict
phase response or impulse response would be very useful in this situation. However,
without such a system, you can determine an effective delay time by trial and error.
Simply increment the delay time in small steps (1 ms), until the action of reversing the
polarity produces maximum cancellation in one orientation and maximum addition in the
opposite. By using a digital delay, you will have preserved the phase and impulse response
of the system and you can now filter out any objectionable mid-bass overlap with an
equalizer.
If a delay is not available, it is recommended that either the subwoofer or the full-range
loudspeaker be relocated closer together, so that reversing the polarity of either the
subwoofer or the full-range loudspeaker (but not both at once) will result in a distinct dip at
crossover as discussed above.
If this cannot be done due to physical restrictions, the subwoofer and the full-range
loudspeaker should be moved further apart, again until there is a distinct dip at the
crossover frequency in one position of polarity. It may take some trial and error to
determine the optimal physical relationships.
C-TIP:
It’s a good idea to experiment with different loudspeaker locations by conducting
listening tests before you finalize the locations (especially important in permanent
installations). Make sure to use live microphones and live instruments (if applicable), as
well as track playback. Choosing the physical location of the loudspeakers in the room is
always
the most important part of any successful system installation.
Note that in some acoustical environments, the system may sound better when the phase
relationship is non-optimum resulting in a dip at crossover, compared to optimum phase
where the crossover region is
accentuated
by the overlap of the subwoofer and the full-
range speaker(s). However, this is not the best way to achieve the sound that you’re
seeking. The proper course of action is to equalize (EQ) the peak at crossover with a
parametric equalizer until the response is flat, or until you’ve achieved the tonal response
you desire (we’ll explain why below).
Alternatively, you might insert a high-pass filter in the full-range system (typically at 80 –
100 Hz with a 12dB/octave slope), so that the overlap with the subwoofer is reduced in
magnitude.
A third technique is to increase the slope of the crossover to 24 dB per octave or 48 dB per
octave, if the crossover has such capability, thereby reducing the bandwidth of the
spectrum in which the two sources overlap.
There’s an important reason for taking one or more of the measures discussed above. If
that nice-sounding response dip at crossover is in fact due to phase cancellation, it means
that the drivers and amplifiers will be working harder than they should to produce less
sound pressure level than they are capable of, due to the acoustic cancellation taking place.
All that cancelled energy uses power unnecessarily!