/ra/-/la/ continuum

It is widely known that perception of the  /r/-/l/ contrast is  difficult for native speakers of Japanese (Miyawaki et al., 1975). However, for native speakers of English, of course, the difference between the two phonemes is perceptually very clear. Now, what is the acoustical differemce between /r/ and /l/? According to the former studies (Dalston, 1975; Polka and W. Strange, 1985; among others), 1) the transitional trajectory of the third formant (F3) and 2) temporal variation of the first formant (F1) are the essential cues for native speakers of English to differentiate /r/ from /l/. This page focuses on these cues, and demonstrates that the perception gradually changes from /ra/ to /la/ as acoustic characteristics of F1 and F3 change.

Figure: illustration of F1 and F3 change in the ra-la continuum
(The figure is taken from Tomaru & Arai, 2014)

Sounds below are synthesized syllables that continuously change from /ra/ (Step 1) to /la/ (Step 10) used in the study by Tomaru and Arai  (in press). Try finding out /ra/ is heard for which steps, and /ra/ is heard for which steps.
Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10
F3s/
F3i
 (Hz)
1609/
1717
1756/
1846
1885/
1971
2013/
2085
2150/
2211
2292/
2325
2428/
2465
2557/
2573
2688/
2707
2827/
2827
F1s (Hz) 376 376 376 376 376 376 376 376 376 376
F1 transition starts at... (ms)
*counts from 0 ms in the figure
110
115
120
125
130
135
140
145
150
155

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References:

Dalston, R.,“Acoustic characteristics of English /w,r,l/ spoken correctly by young children and adults,” J. Acoust. Soc. Am., 57, 462-469, 1975.
MacKain, K. S., Best, C. T., and Strange, W.,“Categorical perception of English /r/ and /l/ by Japanese bilinguals,” Appl. Pshycholing., 2, 369-390, 1981.
Miyawaki, K., Strange, W., Verbrugge, R., Liberman, A. M., Jenkins, J. , and Fujimura, O.,“An effect of linguistic experience: The discrimination of [r] and [l] by native speakers of Japanese and English,” Percept. Psychopys., 18, 331-340, 1975.
Polka, L., and Strange, W.,“Perceptual equivalence of acoustic cues that differentiate /r/ and /l/,” J. Acoust. Soc. Am.,78, 1187-1197, 1985.
Tomaru, K., & Arai, T. (2014). “Perception of multiple series of English /ra/-/la/ continuum having different end frequencies of formant transitions,” Acoust. Sci. and Tech., 35(3), 166-169.