Sensing the right thing

A servo system is a generic name that has many meanings depending on who is using it. To me, a servo woofer system involves a motional feedback element capable of measuring the woofer's acceleration and position. To quite a number of others, a servo woofer can be as simple as another coil of wire on the woofer. Both definitions are correct but the results of each are radically different. Of the few servo woofer systems available today most rely upon a second woofer voice coil. The first voice coil acts as the magnetic motor that drives the woofer's cone back and forth in response to the amplifier's output. The second voice coil generates a small voltage in proportion to the woofer's movement. If the sensing voltage is compared to the driving voltage an approximate difference signal can be derived that can be used to correct for the box enclosure's restrictions. What you get is a flatter output from the woofer, thus we have a servo system. In the servo systems I prefer, an accelerometer or positional sensor is mounted to the woofer that can not only measure the woofer's movement but additionally offers precise information about its rate of acceleration and exact position. When this signal is compared to the original amplifier's input far more information is available. Because we can now trace the exact position of the woofer cone in real time our difference signal can be used for more benefits than a simple second coil. An accelerometer-based system flattens frequency response but then goes beyond what a second coil offers: lower distortion, reduced overhang, improved transient response. And here's where you can tell if a system is really giving you what you assume to be advanced technology. Can it be achieved by other, simpler methods? The simple servo can be replicated by EQing a particular woofer to a specific box—but it's far sexier to call it a servo system. The next time you hear a fancy technology label applied to a product, take a peek under the covers to discover what the claimed benefits of that system are.