The domestic power amplifier, at its best, is designed for maximum fidelity in the true sense of that word. The most straightforward power amplifiers have input sockets and output terminals, and nothing else. Power amplifier specifications include sensitivity, maximum output power into a given load, power bandwidth, frequency response, slew rate, distortion, crosstalk between channels, signal-to-noise ratio, input impedance, output impedance, damping factor, phase response, and DC offset. A theoretically perfect amplifier should then double its output when the impedance it drives is halved. This is desirable as it gives a degree of protection for the amp and speakers against subsonic disturbances and RF interference. Signal-to-noise ratio is a measure of the output residual noise voltage expressed as a decibel ratio between that and the maximum output voltage, when the input is short-circuited.