An individual attempting to sustain /f/ is shown in a midsagittal view. The most important reason why this speaker will not produce an acceptable /f/ is that

• A. the upper central incisors are tipped too far labially
• B. the lower central incisors tipped too far labially
• C. intraoral air pressure will be insufficient
• D. lip placement is inadequate

Answer: (C)

Producing a voiceless labiodental fricative relies on a steady airstream pushed through a narrow gap between the lower lip and the upper teeth, creating turbulent air that we hear as the fricative. The amount of intraoral pressure behind that constriction is what powers the turbulence and sustains the sound. If the speaker cannot build enough intraoral pressure, the airflow won’t become sufficiently turbulent, so you won’t get an audible, acceptable /f/.

The position of the teeth and lips shapes the constriction, and those details can affect how crisp or stable the sound is, but they can’t compensate for a lack of pressure. Inadequate lip placement or incisor tipping might degrade quality, yet the fundamental obstacle to producing the sound is insufficient intraoral pressure behind the constriction.