Spontaneous Music Ensemble

The Spontaneous Music Ensemble (SME) was a loose collection of free improvising musicians convened beginning in the mid-1960s by the late South London-based jazz drummer/trumpeter John Stevens and alto and soprano saxophonist Trevor Watts. SME performances and recordings could range from Stevens–Watts duos to gatherings of more than a dozen players. 
Ethos.
As critic Brian Olewnick writes, the SME emphasised an "extremely open, leaderless aspect where a premium was placed on careful and considered listening on the part of the musicians. Saxophonist Evan Parker observed that Stevens had two basic rules: (1) If you can't hear another musician, you're playing too loud, and (2) if the music you're producing doesn't regularly relate to what you're hearing others create, why be in the group? This led to the development of what would jocularly become known as 'insect improv' – music that tended to be very quiet, very intense, arrhythmic, and by and large atonal."
History.
The SME began an intensive six nights per week residency at the Little Theatre Club in London in January 1966 and recorded their first album "Challenge" the following month. 
One can loosely divide the group's history into two periods: the more horn-oriented earlier ensembles (typically with some combination of Watts, saxophonist Evan Parker and trumpeter Kenny Wheeler), and the later string-based ensembles with guitarist Roger Smith (who became as central to the second edition of SME as Watts was to the first) and violinist Nigel Coombes. The transitional point is the quartet album "Biosystem" (Incus, 1977), which also featured cellist Colin Wood. 
Countless other musicians passed through the SME over the years, including Derek Bailey, Paul Rutherford, Maggie Nichols, Dave Holland, Barry Guy, Peter Kowald and Kent Carter. The final edition of the group was a trio of Stevens, Smith, and the saxophonist John Butcher, a configuration documented on "A New Distance" (1994). 
Inspired both by American free jazz and by the radical, abstract music of AMM, as well as influences as diverse as Anton Webern and Samuel Beckett (two Stevens touchstones), the SME kept at least a measure of jazz in their sound, though this became less audible in the later "string" ensembles. 
Stevens' death in 1994 brought an end to the SME.