What effect does rounding the lips have on the vocal tract?

Rounding the lips in speech production alters the shape and configuration of the vocal tract, which can have a significant impact on formant frequencies. Specifically, when the lips are rounded, it lengthens the effective vocal tract. This change in length affects the resonant frequencies of the vocal tract, resulting in a lowering of the first two formant frequencies, F1 and F2.

The reason for this is related to the acoustics of the vocal tract. A longer vocal tract generally lowers the frequencies of the resonances produced within it. By rounding the lips, the oral cavity is effectively lengthened, due to the additional distance that sound waves travel before exiting the mouth. This contributes to a unique acoustic quality that characterizes rounded vowels and affects the perception of different vowel sounds.

In summary, the interaction between lip rounding and vocal tract length is an important aspect of speech science and phonetics, specifically concerning how it alterations formant frequencies, particularly lowering F1 and F2.