The reproduction of sound information from a source of audiofrequency electrical signals. A complete sound reproduction system contains the original source of audio information, preamplifier and control circuits, audiofrequency power amplifier(s), and loudspeaker(s). The source of sound may be a compact disc, gramophone record, magnetic tape, a broadcast transmission, or a sound-on-film recording.

Monophonic sound reproduction uses only a single audiofrequency channel. One or more loudspeakers may be used in parallel at the output. Stereophonic sound reproduction uses two channels to carry the audio information and at least two loudspeakers. Stereophony utilizes the human binaural (two-ear) processing ability to detect the direction of sounds by assessing the difference in time taken by the sound to reach each ear and the difference in volume caused by the screening effect of the head.

Stereophonic signals require at least two microphones. A coincident pair of matched directional microphones may be used at the same location or a pair of separated microphones may be used. In the latter case the outputs are divided in the correct proportion between the two channels using a suitable potential divider – a panoramic potentiometer. In order to provide a signal that is compatible with monophonic sound reproduction the stereophonic signals are combined into sum and difference signals. If the two channels are A and B, the sum (A  + B) and the difference (A −B) of the signals are used. A monophonic system produces an output using only the (A  + B) signal. The stereo system combines them to produce two signals that correspond to the original A and B information. In stereophonic broadcast transmission the sum signal is used to modulate the main carrier and the difference signal is used to modulate a subcarrier, separated from the main carrier frequency.

A sound-reproduction system that is composed of high-quality expensive parts and reproduces the original audio information faithfully and with very low noise levels is referred to as a high-fidelity (hi-fi) system. In such a system several different inputs may be available, which share the use of the power amplifiers and loudspeakers. Domestic systems usually have facilities for reproducing compact discs, tape recordings, gramophone records, and broadcast radio transmissions. Suitable impedance matching circuits are required for each input. The system may be modular, i.e. each unit is separately boxed and interconnections made between units by means of wires and plugs; alternatively all the units may be combined in a single housing. Sound can be reproduced accurately in two and three dimensions using, for example, ambisonics or HRTF-based techniques to create a horizontal surround sound, a vertical surround sound, or periphonic surround sound.