On why Japanese /r/ sounds are difficult for children to acquire

Many studies have pointed out that the /r/ sounds in Japanese tend to be difficult for native children of Japanese to acquire. To verify this, we first investigated Japanese /r/ sounds uttered by two-year-old twins as a case study. The acoustic analysis of the recordings, which included several words with various /r/ sounds, revealed that certain /r/ sounds are difficult to produce and are often produced with speech errors. We also analyzed a set of utterances of Japanese /r/ spoken in a variety of phones pronounced by an adult male speaker. Then, for comparison, we synthesized Japanese /r/ sounds using four parameters. We conducted two perceptual experiments: one for the natural speech by the male speaker of Japanese, and another for the synthesized speech sounds based on the four parameters. The results showed that variation in pronunciation in adults was widely distributed. We discussed the reasons that it takes time for children to acquire /r/ sounds, and we concluded that it is possibly due to the combination of two factors: 1) some /r/ sounds themselves are difficult to produce, and 2) there is a wide distribution of pronunciation variation in adult speakers.