Quadraphonic sound comes from four speakers at the same time — usually two in front of you and two behind you. In the 1970s, shops sold it as the next step after stereo (stereo uses two speakers). You will also see the spelling quadrophonic. The idea was exciting, but it never quite settled in ordinary homes: extra speakers cost money, and different companies used different systems. Later, “surround sound” for films took over the same job. Today the word mostly belongs to old hi-fi, record notes, and music history. People sometimes use it loosely for any four-speaker setup.