Downdrift

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In phonetics, downdrift is the cumulative lowering of pitch over time due to interactions among tones, called downstep, in a tonal language. It is distinct from the general lowering of the pitch during prosodic contours of a tonal or non-tonal language.

Two basic types of downdrift are found. In one, called discrete downdrift, when downstep occurs all tones shift downward, so that their relative difference in pitch remains constant; in the other, called tone terracing, the pitch of the low tone remains at the lower end of the speaker's vocal range, while the other tones shift downward, so that their difference in pitch narrows.

It is very common for only the low tone to cause a downstep in pitch, whether the result is that all tones, including subsequent low tones, are downstepped, or whether only non-low tones are.

Pitch reset is required in the first instance because the tones approach the lower end of the speaker's comfort range, and in the second because the tonal distinctions of the language start being lost.