The Ins and Outs of PolarityMackie’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.
