Quadraphonic sound – equivalent to what is now called 4.0 surround sound – uses four audio channels in which speakers are positioned at the four corners of a listening space. The system allows for the reproduction of sound signals that are independent of one another.
Sansui QS sound decoder
An RCA Quadradisc record
A four-channel reel-to-reel tape unit from the 1970s, one of the few ways to achieve discrete four-channel sound at home
Ambisonic mixing equipment
Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration of two loudspeakers in such a way as to create the impression of sound heard from various directions, as in natural hearing.
Two microphones set up to record a piano simultaneously, which creates a stereo sound
Diagram of Clément Ader's théatrophone prototype at the Opera during the World Exhibition in Paris (1881)
Bell Labs binaural demonstration at World's Fair
Domestic stereo system, having two speakers