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  1. The distance from a sound source at which the reverberant sound is equal to the direct sound. A very reverberant room has a short critical distance while an acoustically dead room can have a much longer critical distance. Room treatment types and speaker choice affect critical distance. Source: Church Audio & Acoustics Glossary
  2. The distance from a sound source at which direct sound and reverberant sound are at the same level. Source: http://www.owenscorning.com/around/sound/glossary.asp