In speech, phonetic

Phonetics

Phonetics is a branch of linguistics that comprises the study of the sounds of human speech, or—in the case of sign languages—the equivalent aspects of sign. It is concerned with the physical properties of speech sounds or signs : their physiological production, acoustic properties, auditory...

 pitch reset occurs at the boundaries (pausa

Pausa

In linguistics, pausa is the end of a prosodic unit, such as an utterance. Some sound laws specifically operate in pausa only; for example, certain phonemes may be pronounced differently at the end of a word, when no other word follows within the same prosodic unit, such as in citation form...

) between prosodic unit

Prosodic unit

In linguistics, a prosodic unit, often called an intonation unit or intonational phrase, is a segment of speech that occurs with a single prosodic contour...

s.

Over the course of such units, the median pitch of the voice declines from its initial value, sometimes reaching the lower end of the speaker's vocal range. At this point it must reset to a higher level if the person is to continue speaking. In non-tonal languages, this sudden increase in pitch is one of the principal auditory cues to the start of a new prosodic unit.

In register tone languages which experience discrete downdrift

Downdrift

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....

, pitch reset is required as the tones approach the lower end of the speaker's comfort range, and in those languages which experience tone terracing

Tone terracing

Tone terracing is a type of phonetic downdrift, where the high or mid tones, but not the low tone, shift downward in pitch after certain other tones...

, it is in addition required in order to maintain the tonal distinctions of the language.