Represented by the Greek letter Alpha, a measure of the relative amount of energy that will be absorbed when a sound hits a surface. Absorption coefficients are always a value ranging from 0 to 1 that when multiplied by the surface area in question yield a percentage of sound that will be absorbed by that surface. This percentage is in units known as Sabins, after the Harvard professor and acoustician, Wallace Sabine. An absorption coefficient of 1 means that all acoustic energy striking the surface will be absorbed and none reflected. A coefficient of 0 means that all the energy is reflected. The latter condition is virtually impossible and the former condition is rare. The absorption coefficient varies by frequency because most materials have different absorption characteristics at different frequencies. Acousticians use absorption coefficients to help determine the RT-60 or reverberation time of rooms, and as such many common building materials have been measured and results published for their absorption coefficients.