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Trevor Cox is fascinated by auditory illusions - sounds that trick the ear. He discovers how they can be used in music and challenges composer Sarah Angliss to write a piece of music commissioned specially for this programme, inspired by the illusions he finds for her.

Trevor talks to musicians - musicologist and former drummer Bill Bruford, baroque violinist Rachel Podger, and Dan Stowell who wrote a thesis about beatboxing. We hear from composers Steve Reich and Francis Shaw. And Diana Deutsch, Professor of Psychology at the University of California, introduces some of the illusions that she has spent a lifetime investigating.