VERIS 2 – Operation Manual - Page 25
POLARITY
Unless the full-range loudspeaker(s) are stacked directly on top of the subwoofer(s) with cone
drivers physically aligned with the subwoofer cone drivers, it’s likely that the phase
relationship of the two systems may not be optimal. This can be tested by reversing the
polarity of one system relative to the other, as described below.
First, however, it’s important to understand that the correct polarity of the full-range system
relative to the subwoofer is a function of their physical placement in relation to one another.
This is known as the Phase Relationship of the two systems, though Absolute Polarity plays a
role as well, which will be discussed later.
Depending on the placement of the subwoofer in relation to the full-range loudspeaker(s), as
well as the selected crossover point, the optimal response of the system might be obtained by
reversing the polarity of the full-range loudspeaker(s). The easiest way to determine the
proper polarity is to excite the system with a test signal (such as pink noise) and to view the
resultant response on an audio spectrum analyzer. If such equipment is not available, it is also
possible to determine the best polarity relationship by careful listening.
One orientation of polarity, either normal or reversed, should result in a discernable dip
through the crossover region due to acoustic cancellation. The opposite polarity should result
in either a flat response or a peak through the crossover region, due to acoustic addition.
Note:
When experimenting to determine the proper polarity, you can reverse the full-range
loudspeaker(s) or the subwoofer, but never both at the same time (reversing both at the same
time will not alter the phase relationship of the two systems). If there is one 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 made up of very small loudspeakers, 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 inherent harm will
occur if the full-range system simply does not reproduce enough low-frequency energy to
either add to, or cancel, the subwoofer’s output - though the use of a high-pas filter or
external crossover is always recommended to avoid sending low frequency energy to any
small full-range loudspeaker, in order to keep it from being damaged by trying to reproduce
frequencies that are below its intended operating range.
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 listener) 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 or less), 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 again, not both at once!) will result in a distinct
dip or peak 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 until there is a distinct dip at the crossover
frequency in one position of polarity. It will take some trial and error to determine the optimal
physical relationships.
