Unvoiced speech arises from turbulence at constrictions or sudden pressure releases

Evidence

In addition to voiced sounds, speech signals contain unvoiced speech and mixed sounds with both voiced and unvoiced components. One type of unvoiced sound is created by frication, where the vocal tract has a constriction, a nearly closed point, where the airflow velocity is forced to become very high and turbulent, resulting in noise signal generation. Another case is the generation of explosive transient-like sounds where the vocal tract is suddenly opened with a sudden release of pressure. These sounds are called plosives, which may be either unvoiced or voiced; consider the difference between /p/ and /b/. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 560

Categorization

• DEFINE: Distinguishes fricatives from plosives by their acoustic generation mechanism — a clear definitional anchor for speech acoustics.
• XREF: connects to: Turbulent airflow produces the nonpitched noise sounds of speech like sh and ss. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1140