control circuits are common in subwoofer amplifiers, and may be found in crossovers and as do-it-yourself electronics projects. Phase controls allow the listener to change the arrival time of the subwoofer sound waves relative to the same frequencies from the main speakers (i.e., at and around the crossover point to the subwoofer). A similar effect can be achieved with the delay control on many home theater receivers. The subwoofer phase control found on many subwoofer amplifiers is actually a polarity inversion switch. It allows users to reverse the polarity of the subwoofer relative to the audio signal it is being given. This type of control allows the subwoofer to either be in phase with the source signal, or 180 degrees out of phase. The subwoofer phase can still be changed by moving the subwoofer closer to or further from the listening position, however this may not be always practical. Servo subwoofers Some active subwoofers use a servo feedback mechanism based on cone movement that modifies the signal sent to the voice coil. The servo feedback signal is derived from a comparison of the input signal to the amplifier versus the actual motion of the cone. The usual source of the feedback signal is a few turns of voice coil attached to the cone or a microchip-based accelerometer placed on the cone itself. An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible. The primary disadvantages are cost and complexity. Servo-controlled subwoofers are not the same as Tom Danley's ServoDrive subwoofers, whose primary mechanism of sound reproduction avoids the normal voice coil and magnet combination in favor of a high-speed belt-driven servomotor. The ServoDrive design increases output power, reduces harmonic distortion and virtually eliminates power compression, the loss of loudspeaker output that results from an increase in voice coil impedance due to overheating of the voice coil. This feature allows high-power operation for extended periods of time. Intersonics was nominated for a TEC Award for its ServoDrive Loudspeaker (SDL) design in 1986 and for the Bass Tech 7 model in 1990. Applications Home audio The use of a subwoofer augments the bass capability of the main speakers, and allows them to be smaller without sacrificing low-frequency capability. A subwoofer does not necessarily provide superior bass performance in comparison to large conventional loudspeakers on ordinary music recordings due to the typical lack of very low frequency