Audio signal processing always produces distortion. When distortion goes up to high level, we cannot listen the sound clearly. Understanding about distortion in audio signal is the way to choose the best amplifier we purchase.

An amplifier that has a distortion rating of 10% at full output is unlistenable. A amplifier that has a .01% distortion level at full output level would be excellent at all listening levels. Distortion specs are expressed by the term THD (Total Harmonic Distortion).

Signal-To-Noise Ratio (S/N)

Another factor is Signal-To-Noise Ratio (S/N), which is ratio of sound to background noise. The larger the ratio, the more the desirable sounds (music, voice, effect) are separated from acoustical effects and background noise. In amplifier specifications S/N ratios are expressed in decibels. A S/N ratio of 70db is much more desirable that a S/N ratio of 50db.

Continuous Power (RMS)

An additional factor is the ability of an amplifier to output its full power continuously. In other words, just because your receiver/amplifier may be listed as being able to output 100WPC, doesn’t mean it can do so for any significant length of time. Always make sure that, when you check for Specifications, that the WPC output is measure in RMS terms. This means that the listed power output is sustained output at a specific volume level.

Dynamic Headroom

Another factor is the ability of an amplifier to output power at a significantly higher level for short periods to accommodate musical peaks or extreme sound effects in films. This spec is important in home theater, where extreme changes in volume occur during the course of a film.
Dynamic Headroom is measured in Decibels. If a receiver/amplifier has the ability to double is power output for a brief period to accommodate the conditions described above, it would have a Dynamic Headroom of 3db.