The absorption coefficient α is a property of a material. It efines the extent to which a material absorbs energy, for example that of sound waves or electromagnetic radiation. Wallace Sabine was a pioneer of this concept in acoustics and defined the unit of the Sabine. A Sabine is defined as a fraction of acoustic power absorbed by a 1m square of open window. In SI units, absorption coefficient is measured in inverse metres, and is represented by the Greek letter α.