R2SUBZ and R2SUBDF - Installation Guide - Page 7
One polarity (either normal or reversed) should result in a discernable cancellation through the crossover region, while the
opposite polarity should result in either a flat response or a peak through the crossover region.
Note: When experimenting to determine the proper polarity, reverse only the full-range loudspeaker(s) or the subwoofer,
never both at the same time. If there is no discernable difference, or a very minimal difference in the measured or audible
response when the polarity is reversed, it indicates one of two things:
(a)
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, or;
(b)
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 (a) 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 compete (either positively or negatively) with the subwoofer’s output.
The solution to (b) 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 good quality 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 optimum impulse response and phase response, 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 re-located so that
they are closer together and 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 is impossible 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 relationship.
Note that in some acoustical situations, the system may sound better when the phase relationship is non-optimum
(resulting in a dip at crossover), than when 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 a good sounding system. The proper remedy 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). Another valid approach is to high-pass the full-range system (typically at 80 – 100 Hz) so that the
overlap with the subwoofer is reduced in magnitude. A third way 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 is an important reason for this: if the response dip at crossover is due to phase cancellation, the drivers and
amplifiers will be working harder than they should to produce less sound pressure level than they are capable of, because of
the cancellation taking place. All that cancelled energy uses power unnecessarily!
Conversely, if you attenuate the peak at crossover with equalization
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, you are reducing the power that’s required to obtain a
given sound pressure level. The result is more available power, more headroom, and far less stress on the drivers.
Once you have either delayed or re-positioned the speakers, you can permanently wire the system in the phase relationship
that produces maximum acoustical addition at crossover, but first read the section below on “Absolute Phase.”
Absolute Phase
Quite a bit has been written about absolute phase, particularly in regard to studio recording and hi-fi sound reproduction.
The subject is, however, often ignored in the field of sound reinforcement. Essentially, positive absolute phase refers to
configuring the system so that the driver(s) moves forward toward the listener, thereby producing a positive wavefront
upon the first cycle of excitation by the source material. For example, at the instant of impact, when the head of the kick
drum moves outward towards the microphone, the speaker cones also move outward.
Obviously, the phase integrity of the entire signal processing chain must be maintained for this to occur. Is absolute phase
audible? Should you be concerned? Yes, it is audible and though subtle, it makes a big enough difference to warrant taking
the time needed to insure that the signal chain is phase-positive throughout. You’ll hear an improvement in sonic impact,
especially in the lower frequencies.
We recommend that absolute phase be kept positive in all low-frequency devices whenever possible. This can be checked
with a small handheld style phase response test unit, available from numerous manufacturers.
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The three corrective methods referred to in the text (equalizing, high-passing, and increasing the slope of the crossover) are all various
implementations of equalization.
