Wiktionary
n. (context phonetics English) A lowering (over time) of the pitch of the tones of a tonal language.
Wikipedia
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'.
Two basic types of downdrift are found. In one, called discrete downdrift, in which 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.
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.
Usage examples of "downdrift".
Soon, very soon, the next sister downdrift followed suit, and the next.
Silence fell, or speechlessness, as the breezes swept the odors emitted by the Eldest and her sisters downdrift from the gallery, through the rest of the building, and back, softer echoes of their words, and away again, ever diluted, ever more blended with background scents, never completely vanished.
The ventilator fans forced the air to move in its accustomed direction, from the Eldest of the amaryllis sisterhood downdrift, and acquire messages in order of dominance within the hierarchy.