Rădulescu's fourth quartet is one of the more radical spectral pieces; it's for nine string quartets, one of which plays live, with the remaining quartets played back from recordings from different locations in the hall. The idea is to explore the harmonic spectrum, and to allow the audience to experience new spectra as the sounds criss-cross, interact and interfere.
The extent to which this is achieved is debatable, but a performance is undoubtedly an enthralling experience. A recording can only hint at this, but here is a recording from Colognes' Acht Brücken festival by the Asasello Quartet, from earlier this month.
infinite to be cannot be infinite, infinite anti-be could be infinite (infinit a fi nu poate fi infinit) op. 33 (1976; rev. 1987) - fourth string quartet for nine string quartets
Trinitatiskirche, Cologne; 3rd May 2016