This word was first used by ancient Greeks and describes an audio-only presentation of sounds (free from the distraction of association with the sounds Sound Sources). When we listen to music on the radio or through an mp3 player we are listening in an acousmatic way.
The term acousmatic is said to come from the ancient Greek philosopher Pythagoras who, according to tale, offered lectures to his followers from behind a curtain. Pythagoras claimed that his followers would listen more closely to the sounds of his voice if they could not see him (see also Heightened listening). The followers were unable to see that which they heard and were left imagining the unseen visual elements of what they were hearing. Acousmatic music presents sounds without any visual accompanyment, so as to encourage heightened listening and to focus attention upon the qualities of the abstract sounds themselves.