A technology used on speakers (mostly subwoofers and notably on some Velodyne subwoofer lines) to correct driver movement for decreased distortion. In normal operation, a speaker’s drivers (the cones or domes or other structures which move to create vibrations and thus sound) encounter distortion, an instance where the speaker’s output (the sound being emitted) does not exactly follow the signal. In a perfect world, a speaker driver would move in exact and perfect relation to the signal being applied to it, but in real life the driver’s mass, the air pressure on it, the driver’s speed and other factors make this impossible. Thus the driver does not exactly follow the input signal and distortion is created.

In order to tame this distortion, a servo loop may be engaged. An accelerometer is attached to the speaker driver itself (usually to a large woofer cone). The accelerometer is an electronic device which measures the movement (acceleration of the driver in one direction or another) of the speaker driver. The accelerometer then sends a signal into a servo loop showing what the driver is doing (what signal it is putting forth).

The accelerometer signal is compared against the input signal (the signal the driver is supposed to be following) by a microchip in the servo loop. If the signal does not match the information from the accelerometer, there is distortion. The microchip servo controller then sends a correction to the driver to bring it back in line with the signal. This process happens hundreds or thousands of times per second, constantly correcting the speaker driver to bring it into alignment with the input signal.

Using the servo technique, speaker manufacturers are able to achieve very low distortion (however, amplifier power is required to correct the driver motion and bring it back into alignment with the input signal meaning that amplifier power is not fully utilized to output the maximum sound level – this is a trade off between higher sound levels and more accurate sound). Accelerometer servo technology can only be used with self-powered or active speakers (speakers containing their own amplifiers) due to the signal manipulation required. This is one reason servo technology is used primarily with subwoofers (most subwoofers today are powered by their own internal amplifier, although few make use of servo technology which adds cost and complexity and can be difficult to properly implement).