Downdrift

In phonetics, downdrift (also known as 'automatic downstep') is the cumulative lowering of pitch in the course of a sentence due to interactions among tones in a tonal language. The usual cause of downdrift is when the tones in three successive syllables are H L H. In this case the second high tone tends to be lower than the first. The effect can accumulate so that with each low tone, the pitch of the high tones becomes slightly lower, until the end of the intonational phrase, when the pitch is 'reset'.