Monophonic sound

Monaural or monophonic sound reproduction (often shortened to mono) is intended to be heard as if it were a single channel of sound perceived as coming from one position (unlike stereo, which uses two channels to convey the impression of sound coming from different places from left, middle, and right). In mono, only one loudspeaker is necessary, but, when played through multiple loudspeakers or headphones, identical signals are fed through each of the wires into each speaker, resulting in the perception of a one-channel sound, which "images" in one sonic space between the speakers (provided that the speakers are set up in a proper symmetrical critical-listening placement). Monaural recordings, like stereo, customarily use multiple microphones, fed into multiple channels on a recording console, but each channel is "panned" to be in the center. In the final stage, the various center-panned signal paths are usually mixed down to two identical tracks, which because they are identical, are perceived upon playback as representing a single unified signal in a single place in the soundstage. In some cases the multitrack source is mixed down to a one track tape becoming one signal. In the mastering stage, particularly in the days of mono records, the one-track or two-track mono master tape was then transferred to a one-track lathe intended to be used in the pressing of a monophonic record. However, today monaural recordings are usually mastered to be played on stereo and multi-track formats, yet retain their center-panned mono soundstage characteristics when played back.

Monaural sound has been replaced by stereo sound in most entertainment applications. However, it remains the standard for radiotelephone communications, telephone networks, and audio induction loops for use with hearing aids. A few FM radio stations, particularly talk radio shows, choose to broadcast in monaural, as a monaural signal has a slight advantage in signal strength over a stereophonic signal of the same power.

While some experiments were made with stereophonic recording and reproduction from the early days of the phonograph in the late-19th century, until the second half of the 20th century, monaural was the rule for almost all audio recording.

...
Wikipedia