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11
The Ins and Outs of Polarity
Mackie’s Thump TH-18s subwoofer includes a switch
that allows you to quickly invert the polarity of the sub-
woofer’s output relative to the input signal it is receiving
from the mixer or other sound source. But what exactly
does that mean? A subwoofer works by literally pumping
air as the woofer cone moves in and out with respect to
the cabinet in which it is housed. It does so according to
the low-frequency portion of the signal it receives from
the sound source.
The woofer cone is simply following the waveform
as seen in the sine wave in Figure 1. As the sine wave
rises, the woofer cone pushes out. Likewise, as the sine
wave falls, the woofer cone pulls into the cabinet. A
musical signal is much more complex, of course, but the
same principle applies. Movement of the woofer cone
causes air pressure changes that we perceive as sound.
When the Polarity Invert switch [6] is engaged, the
original waveform is simply reversed 180º (see Figure
2). Again, the subwoofer cone follows the waveform.
However, this time the woofer cone starts by pulling into
the cabinet followed by the woofer cone pushing out. If
you have ever experimented with a subwoofer polarity
switch, you may not have noticed any changes to the
sound regardless of its position, especially if you are
listening to just the subwoofer. This is normal, as our
ears perceive them both as the same.
The polarity invert switch comes into play when the
TH-18s subwoofer is paired with a loudspeaker. Ideally,
the woofer cones of the subwoofer and full-range
loudspeaker would work together by pushing and
pulling in unison. TH-18s subwoofers are designed to
be used in a broad range of applications, and with a
wide variety of full-range loudspeakers. The flexibility
provided by the polarity switch is necessary to ensure
that you are receiving the best possible sound from your
system, regardless of your setup.
Listed below are some recommended configurations
for pairing a Mackie TH-18s subwoofer (using the High-
Pass output) with a Mackie full-range loudspeaker.
These settings are a good start, but be sure to
experiment to find the best sound for your venue. The
loudspeakers may be stacked, pole-mounted, or flown.
•TH-15A full-range loudspeakers - Engage the
Polarity Invert switch on the TH-18s.
•SRM450v2 full-range loudspeakers - Disengage the
Polarity Invert switch on the TH-18s.
In alternate setups where the full-range loudspeakers
are not co-located with the subwoofers or are not
connected to the TH-18s subwoofer’s High-Pass outputs,
you will need to experiment with the Polarity Invert
switch to determine which setting gives you the desired
bass response for your application.
Figure 1 Figure 2
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