The zero-crossing rate (ZCR) is the rate at which a signal changes from positive to zero to negative or from negative to zero to positive.
[1] Its value has been widely used in both speech recognition and music information retrieval, being a key feature to classify percussive sounds.
In some cases only the "positive-going" or "negative-going" crossings are counted, rather than all the crossings, since between a pair of adjacent positive zero-crossings there must be a single negative zero-crossing.
For monophonic tonal signals, the zero-crossing rate can be used as a primitive pitch detection algorithm.
Zero crossing rates are also used for Voice activity detection (VAD), which determines whether human speech is present in an audio segment or not.