Community VERIS Series - Operation and Installation Manual - Page 24
When choosing subwoofer location(s), be careful, however, not to sacrifice sonic quality
for sheer power. If the available wall or corner location results in the subwoofer being
located behind, or too close, to one or more open microphones, early feedback is likely
to occur. If the wall or corner location is too far away from the full-range
loudspeaker(s), such a location may result in the subwoofer being drastically out of
time sync with one or more of the full-range speakers.
Sometimes the sound quality of a wall or corner placement is not desirable, simply due
to the room’s acoustical properties. Keep in mind that when wall and corner locations
are
appropriate for use, they’ll provide a tremendous increase in power output, but
they may not always be the best choice for sound
quality
.
(2)
Keeping the subwoofer(s) as close as possible to the mid and high loudspeaker(s) will
decrease phase irregularities and
time smear
. If the subwoofer(s) is placed too far
away from the mid/high loudspeaker(s), the listener will experience a disjointed
character to the program material, causing the musicality of the system to suffer.
(3)
Although the subwoofer is not highly directional, still its acoustical output follows the
inverse square law. That is, every time the distance from the subwoofer to the listener
is doubled, the output level will decrease by 6dB. When covering a large space with
multiple subwoofers, it may be of benefit to space them some distance apart from one
another to even out the levels throughout the space. Typically, this would only be done
if the mid/high loudspeakers are also spaced apart from one another, such as in a
distributed system in a sports venue. Although this will help maintain an even level
throughout the listening space, there may be some areas that lie between two or more
subwoofers that experience a certain amount of power subtraction caused by
destructive interference
. Destructive interference occurs when waveforms meet and
are partially or wholly out of phase with each other, due to unequal path lengths.
Conversely, if multiple subwoofers are located directly adjacent to one another, their
power output will add together almost seamlessly. This is known as
constructive
acoustic addition
. However, this may produce an undesirable hot-spot of low-frequency
energy that might possibly be too close to a seating area.
It’s always a good idea to experiment with trial locations, before finalizing your
installation plan. Even a very experienced sound system designer cannot predict the
precise effect that one location may have over that of another, if he/she is not
intimately familiar with the room acoustics. When planning loudspeaker locations, it’s
wise to discuss the matter with someone who may have prior experience operating
sound systems in that particular room, such as the resident mixing engineer, if the
building has previously had a sound system installed in it.
POLARITY
Unless the full-range loudspeaker(s) is stacked directly on top of the subwoofer(s) with its
cone drivers 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