Vocal tract length differs by sex and its resonances form speech formants.

Evidence

The vocal tract from the glottis to the lips is about 17 cm in length for an adult male (Rabiner and Schafer, 1978), and about 14.5 cm for females (Klatt and Klatt, 1990). In the middle of the vocal tract, the tip of the soft palate called the velum opens or closes the sideway through the nasal tract to the nostrils. In normal production of speech this is open only for nasalized sounds. The effect of these two tracts is to determine the acoustic transfer function in such a way that the spectral properties of the radiating voice can convey speech information through a rich set of distinct sounds. From a signal processing point of view, the vocal and nasal tracts act as filters with controllable resonances that emphasize specific frequencies. These resonances are called formants, which are one of the most important cues in speech. The tracts may also create antiformants, dips in the spectrum; in other words, zeros in the transfer function. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 556

Categorization

• DEFINE: Defines formants as controllable tract resonances and antiformants as spectral zeros, framing speech as a filter-bank signal-processing problem.
• XREF: The source-filter framing (glottal source + moving-tract filter) is the exact transfer-function architecture used in speech synthesis and analysis (LPC, vocoders). — connects to: Body size constrains the frequencies animals can vocalize — Almo Farina, Soundscape Ecology: Principles,…, 385