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Finding phonetic correlates of rhotics as a natural class has been elusive, leading to the suggestion that any class-based relationship between different rhotic categories is purely phonological in nature. This paper examines native English speakers' perception of three different non-native rhotics (i.e., /r ɻ ʀ/) compared to non-native sounds from four other manners of articulation (stops, nasals, fricatives, and laterals). The results revealed that speakers cannot reliably discriminate between the rhotics examined here and that the perceptual distance between members of the class of rhotics is smaller than the other tested classes, aside from the comparison with laterals. The current results suggest that there is an acoustic-perceptual correlate to rhotics as a natural class and that their perception explains the relative rarity of large rhotic inventories cross-linguistically. The comparison with laterals also suggests why rhotics are often paired with laterals in inventories with two or more liquids.
Howson et al. (Thu,) studied this question.