Within the last few years, car audio has seen a few subwoofers using non-circular shapes from manufacturers such as Kicker, Sony, Bazooka, X-Tant, etc. There is no audible advantage to these shapes, and they may carry some distortion penalties. They can, in some situations (limited mounting space, for instance) give a slightly greater cone area and so slightly higher output at low frequencies. The intent of non-round subwoofers is to increase the surface area of the cone. In enclosures of a particular size, more subwoofer cone area is possible with some of these shapes, although the difference may be trivial. The increased cone area moves more air, producing higher sound pressure levels. However, these drivers are not optimized for maximum output level, so the advantage is largely theoretical. The effect of corners in their surrounds is to increase distortion.
An important factor in the "square sub vs round sub" argument is the effects of the enclosure used. In a sealed enclosure, all displacement is determined by
where Vd stands for volume of displacement, xmax to the amount of linear excursion the sub is mechanically capable of (in mm), and Sd to the cone area of the subwoofer (in cm2). These are some of the Thiele/Small parameters which can often be found in the owner's manual or driver specifications. Because of the multiplication, excursion is as important as cone area to generate SPL. However, the importance of these parameters shifts if the subwoofer is used in a ported enclosure. When a subwoofer produces frequencies near the tuning frequency of the ported enclosure, excursion decreases and cone area becomes more important.
