Monaural

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

Monaural sound is normal on:

• Phonograph cylinders
• Gramophone records made before 1958, such as those made for playing at 78 rpm and earlier ​16 ²⁄₃, ​33 ¹⁄₃ and 45 rpm microgroove records
• Some FM radio stations that broadcast only spoken-word or talk-radio content (in order to maximize their coverage area)
• Subcarrier signals for FM radio, which carry leased content such as background music for businesses or a radio reading service
• Background music services such as Seeburg 1000; satellite broadcasts by Muzak, and some public address systems

Incompatible standards exist for:

• Later vinyl records (although monophonic records—which had almost disappeared in the United States by the end of 1967—could be played with a stereo cartridge)
• Reel-to-reel audio tape recording (depending on track alignment)

Compatible monaural and stereophonic standards exist for:

• MiniDisc
• Compact audio cassette
• FM (and, in rare circumstances, AM) radio broadcasting
• VCR formats
• TV
• Digital audio files on many computers in many formats (WAV, MP3, etc.)

No native monaural standards exist for:

• 8-track tape
• Compact disc

In those formats, the mono-source material is presented as two identical channels, thus being technically stereo.

At various times artists have preferred to work in mono, either in recognition of the technical limitations of the equipment of the era or because of simple preference (this can be seen as analogous to filmmakers working in black and white). An example is John Mellencamp's 2010 album No Better Than This, recorded in mono to emulate mid-20th century blues and folk records. Some early recordings such as The Beatles' first four albums (Please Please Me, With the Beatles, A Hard Day's Night, Beatles for Sale) were re-released in the CD era as monophonic in recognition of the fact that the source tapes for the earliest recordings were two-track, with vocals on one track and instruments on the other (even though this was only true on the first two albums, while the latter two had been recorded on four-track). This was actually intended to provide flexibility in producing a final mono mix, not to provide a stereo recording, although because of demand this was done anyway, and the early material was available on vinyl in both mono and stereo formats. In the 1960s and 1970s, it was common in the pop world for stereophonic versions of mono tracks to be generated electronically using filtering techniques to attempt to pick out various instruments and vocals; but these were often considered unsatisfactory, owing to the artifacts of the conversion process.

Directors Stanley Kubrick and Woody Allen, among others, preferred to record their films' sound tracks in mono.

Monaural LP records were eventually phased out and no longer manufactured after the early 1970s, with a few exceptions. For example, Decca UK had a few double issues until the end of 1970 – the last one being Tom Jones's "I Who Have Nothing"; in Brazil records were released in both mono and stereo as late as 1972. During the 1960s it was common for albums to be released as both mono and stereo LPs, occasionally with slight differences between the two (again, detailed information of The Beatles' recordings provides a good example of the differences). This was because many people owned mono record players that were incapable of playing stereo records, as well as the prevalence of AM radio. Because of the limited quantities pressed and alternative mixes of several tracks, the monaural versions of these albums are often valued more highly than their stereo LP counterparts in record-collecting circles today.

On 9 September 2009, The Beatles re-released a remastered box set of their mono output spanning the Please Please Me album to The Beatles (commonly called "The White Album"). The set, simply called The Beatles in Mono, also includes a two-disc summary of the mono singles, B-sides and EP tracks released throughout their career. Also included were five tracks originally mixed for an unissued mono Yellow Submarine EP. Bob Dylan followed suit on 19 October 2010 with The Original Mono Recordings, a box set featuring the mono releases from Bob Dylan (1962) to John Wesley Harding (1967). On 21 November 2011, The Kinks' mono recordings were issued as The Kinks in Mono box set, featuring the releases of the band's albums from Kinks (1964) to Arthur (1969), with three additional CDs of non-album tracks that appeared as singles or EP tracks. When the initial run of the box set sold out, no more were pressed, unlike the Beatles and Dylan sets.

Sometimes mono sound or monaural can simply refer to a merged pair of stereo channels - also known as "collapsed stereo" or "folded-down stereo". Over time some devices have used mono sound amplification circuitry with two or more speakers since it can cut the cost of the hardware. Some consumer electronics with stereo RCA outputs have a microswitch in the red RCA output (i.e., the right stereo channel) that disables merging of stereo sound into the white (left stereo channel) RCA output. Common devices with this are VCRs, DVD/Blu-ray players, information appliances, set-top boxes, and the like. Video game consoles sometimes have male RCA ends of cables with a proprietary multi-A/V plug on the other end, which prevents automatic stereo merging unless adapters are used.

Disadvantages of merged stereo involve phase cancellations that may have the effect of muffling the final sound output. If channels are merged after being sent through a power amplifier but before being connected to a loudspeaker, it places more stress on the loudspeaker. It has usually been the practice in recording studios to make separate mixes for mono recordings (rather than folded-down stereo-to-mono), so that the final mono master will avoid the pitfalls of collapsed stereo. In video games, merging stereo to mono sound prevents player from discerning what direction distant SFX are coming from, and reverse stereo has a similar setback too. Having an array of loudspeakers connected to their own amplifier outputs can mitigate issues with the electrical load for a single loudspeaker coil and allow the listener to perceive an "image" of sound in the free space between the speakers.

• Stereophonic sound
• Binaural recording