Compromised cabinets

While at RMAF one of my readers approached me asking what the ultimate subwoofer cabinet might be like. I replied that one made of concrete or stone would be best: one that did not move, did not flex, did not impart any noise or signature of its own. That wish list is a tall order for mere mortals with hi fi systems to bring home. Can you imagine a one-ton cement woofer cabinet in your living room? Or the struggle to ship it and get it there? Whatever cabinet your subwoofer is in probably won't weigh a ton. In fact, it'll weigh far less, won't be made from stone or concrete. Or, put another way, your cabinet will be a practical compromise - an anathema in hi fi parlance. But fear not, there are many fine subwoofers available in cabinets we humans can lift into place. They work well because of some great engineering techniques that can be applied. For example, many cabinets are internally braced such that they exhibit very little external vibrations. But my favorite is what Martin Logan did in their descent servo controlled subwoofer series (among others). They use multiple opposing woofers to balance out the vibrations of the box. I remember, with fondness, one of the ads Gayle Sanders (former owner of ML) and company had for the Descent. The ad featured a glass of wine on top of the woofer cabinet. The idea was the liquid in the glass showed no signs of ripple when the woofer played (a tall order indeed). Contrast that with other designs, some of which literally 'walk' across the room when producing bass, and you'll appreciate the benefits of good subwoofer enclosure design. Here's a picture of the sub from ML showing the three opposing woofers. As one pushed the cabinet one way, the other pushed the other way and between the three, they did a nice balancing act producing nearly zero cabinet vibrations. The job of the enclosure is to make sure the front firing woofer produces all the sound, the cabinet as close to the idea block of non-moving concrete as possible.