Low end problems

            My second article was about low-end problems with subwoofer arrays.  The author begins by showing a visual depiction of a typical subwoofer’s omnidirectional pattern.  By itself, a single subwoofer unit has a perfectly omnidirectional pattern that is very easy to predict.  Instead, the problem occurs when subwoofers are paired with one or more other units that then begin interfering with the array.  The issue in dubbed “pattern narrowing” and causes the pattern to become more oval-shaped.  As more units are added there is a noticeable dent in the center of the oval.

            The problem is simple: phase.  The author shows some pictures of how subwoofers react with each other at different frequencies, with higher frequencies resulting in large lobes between the two and null areas coming out diagonally.  The higher the frequencies go, the more null areas are created because the energy is arriving out of phase in those particular areas, none of which can be fixed by amplitude or equalization.

            One solution is to add more subwoofers closer together to eliminate the null areas.  While this does create a much more consistent pattern, it does narrow the area that the subwoofers cover.  By physically moving the subwoofers into an arc pattern, the delay can cause the sound to even out a bit.  The problem can also be solved through digital delay but is a little less likely due to resources.

            Where it gets complicated is when the subwoofers are integrated into the full sound system.  Because of the crossover between the low and high frequencies, we once again have the same interference patterns as before with nodes.  In narrow areas, longer arrays can work well because of the large coverage in the center but this problem is difficult to compensate for. 

http://www.prosoundweb.com/article/optimizing_the_low_end_run_gun_subwoofer_arraying_techniques/P4/