Which source(s) produce the fricative /z/?

The production of the fricative /z/ involves both the vibrating vocal folds and a constriction in the oral cavity. The fricative sound is characterized by the turbulent airflow created when air is forced through a narrow constriction, which, in this case, occurs in the oral cavity. For the /z/ sound, the vocal folds vibrate, which adds a voiced quality to the sound, while the constriction—typically created by the tongue approaching the alveolar ridge—creates the friction necessary for the fricative quality.

This dual contribution of voiced sound from the vocal folds and turbulence from the oral cavity constriction is essential for accurately producing the /z/ sound, distinguishing it from other speech sounds that may involve only one of these components.