First off, I would simply like to state that this was one of the most informative and easily comprehendible articles pertaining to this issue I have read. Bennett Prescott really knows his stuff but also has a way of explaining sometimes complex concepts in an easy to understand way.
Now to the nitty gritty. This topic has been one that has always intrigued me. Low end sound is crucial to any mix and getting it to sound its absolute best is often difficult and tedious. Luckily Bennett Prescott was kind enough to list off a few techniques and information that can help every system tech have their low end sounding its very best.
Prescott shows how a "small sound source", being an acoustic sound source where no dimension of the source is larger than one-quarter wavelength at the frequency of interest, demonstrates near perfect omnidirectional response in the free field he is testing. But when one subwoofer is set up next other subwoofers, the omnidirectional response demonstrated by the single sub has now been altered and begins to demonstrate what is called pattern narrowing, which occurs due to phasing. Prescott defines phase as, "the offset between two waves, measured in degrees". When two waves are perfectly in phase with one another, meaning a difference of 0 degrees, there is a boost in gain of 6 dB. However, two wave out of phase by 180 degrees will cancel each other out perfectly. Any other kind of phase relation will contribute anywhere between 6 dB gain or wave cancellation. Any sound sources outputting waves that are between 120 to 180 degrees out of phase with each other will begin resulting in sound reduction, which is what we as system techs do not want.
From there, Prescott demonstrates different arrays of subwoofers and the effects each one has on the sound they output by diagramming the noise boosting and canceling caused by phasing. Some of the most intriguing diagrams were the ones demonstrating a left and right stereo array causing what is called a "Power alley", where the sound from both subwoofers arrives at the center of where an audience would be located and would be boosted in gain, where as about a third of the sound that would reach the audience, roughly the far left and far right of the audience, is reduced or cancelled out.
The effects that physical and/or digital delay have on subwoofer arrays was very eye opening, in that simply moving one or two subwoofers a foot behind the others can have a dramatic effect on the way the sound is heard within the audience. I never thought about how a physical delay focuses sound back onto the stage while a digital delay affects the sound on both sides of the array equally.
One of the best tidbits of information I gained from this article is the mixing of both center subwoofer arrays and stereo loudspeakers to create the most even sound coverage possible. Prescott states that he uses the technique of using a left right center sub array in tandem with the stereo loudspeakers most often than any other, which is something I'll take with me and try applying when and where I can.
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