Cowell, who did not consider himself to be homosexual, was charged with and confessed to homosexual conduct involving a 17 year-old male in 1936. In San Quentin State Prison he taught fellow inmates, directed the prison band, and continued to write music at his customary prolific pace, producing around sixty compositions including two major pieces for percussion ensemble.
One of Cowell’s most famous early compositional innovations was the tone cluster (thick chords made up of major and minor seconds) which he played all over the piano with his forearms and fists. Béla Bartók later wrote to ask Cowell’s permission to compose with them. Cowell invented a variety of other groundbreaking techniques for stroking, strumming and plucking inside the piano, directly on the strings, which he dubbed the string piano.
After his release Cowell tried to appear conventional: he married in 1941, toned down his radical politics and gave up some of his avant-garde musical ideas.