Editing Subwoofer (section)

===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.<ref name=ServoSubs /> 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.<ref name=ServoSubChip /><ref name=Doucet /> An advantage of a well-implemented servo subwoofer design is reduced distortion making smaller enclosure sizes possible.<ref name=Calabria2004 /> The primary disadvantages are cost and complexity.<ref name=DeutschDD12 />

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]].<ref name=TECAwards /> 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.<ref name=Danley1983 /><ref name=ServoDrive /><ref name=Danley1986 /> Intersonics was nominated for a [[TEC Award]] for its ServoDrive Loudspeaker (SDL) design in 1986 and for the Bass Tech 7 model in 1990.<ref name=1986TEC /><ref name=1990TEC />