George Crumb (born October 24, 1929) is an American composer of  avant-garde music. He is noted as an explorer of unusual timbres, alternative forms of notation, and extended instrumental and vocal techniques. Examples include seagull effect for the cello (e.g. Vox Balaenae), metallic vibrato for the piano (e.g. Five Pieces for Piano), and using a mallet to play the strings of a contrabass (e.g. Madrigals, Book I), among numerous others. He is not an electronic music composer; however, many works call for amplification of instruments, such as Black Angels (string quartet) or Ancient Voices of Children (mixed ensemble). Crumb defines music as "a system of proportions in the service of spiritual impulse."