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  1. The property of a material that changes acoustic energy into (usually) heat energy. A material or surface that absorbs sound waves does not reflect them. Absorption of a given material is frequency dependent as well as being affected by the size, shape, location, and mounting method used. Source: Church Audio & Acoustics Glossary
  2. In acoustics, the changing of sound energy to heat. Source: http://www.owenscorning.com/around/sound/glossary.asp