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In describing sound changes, sound patterns and alternations in languages one clearly needs to refer to a natural class of r sounds, or rhotics. This phonological class encompasses sounds with a wide variety of manners and places of articulation, and it has been suggested that its “phonetic correlate” is acoustic in nature, namely a lowered F3. Acoustic properties of phonetically different /r/s were investigated in languages like American English, Yoruba, French, Southern Swedish, Hausa, and Edo, using several speakers of each language. The results do not show any single acoustic parameter underlying the phonological class of +rhotic. The third formant does not lower for all types of /r/s. There is often a decreased intensity associated with the rhotic, but this is by no means consistent. The natural class of +rhotic sounds is a phonologically convenient class, but it is not controlled by any single articulatory or acoustic correlate. Instead, this phonological class is associated with complex combinations of both articulatory and acoustic parameters. Work supported by NSF.
Mona Lindau (Tue,) studied this question.