Acoustic cues for /θ/ in American English

Distinguishing between the voiceless fricatives /f/ and /θ/ is a difficult problem in natural and synthetic speech. In a previous experiment using natural stimuli [K. Hata etal., Proc. ICSLP 327–330 (1994)], it was found that adding vowel transitions increased identification for /f/ at least 15% in comparison with frication-only stimuli. However, with vowel transitions, the identification of /θ/ failed to show significant improvement. The purpose of the current study was to investigate, with an improved procedure, significant cues for /θ/ which we can use in our synthesizer. Six monosyllabic nonsense words (e.g., /fiyk/, /θayk/) were recorded. Segments of approximately 30-ms duration from different locations of /θ/ and its following vowel were spliced into f-initial words. Eight subjects were asked to identify each stimulus as ‘‘th,’’ ‘‘f’’ or ‘‘indistinguishable.’’ In the /iy/ context, /f/-initial stimuli spliced with fricative-vowel transitions from /θ/ were perceived as /θ/ 55% of the time, while stimuli involving other vowel contexts and other splices tended to be perceived as /f/. This implies that a cue for /θ/ resides in this transition when followed by a high, front vowel, but that the cue is rather obscure in the 0–5 kHz region when other vowels follow.