This study provides reference data on vowel devoicing in Japanese spontaneous speech for forensic and other speech investigations. We analysed the running speech of 226 speakers in order to examine how the places of origin of the speakers and their parents influence the occurrence frequency of vowel devoicing. According to the dialect distribution map of vowel devoicing, we classified the speakers into two dialects: the dialect where vowel devoicing occurs frequently (DF) and that where devoicing occurs infrequently (DIF). The results showed that DF speakers with DF parents devoiced the vowels most frequently, while DIF speakers with DIF parents devoiced them the least. The devoicing of speakers with parents of different dialects was in between. Some speakers, irrespective of whether they were DF or DIF, showed a percentage of vowel devoicing that contradicted their dialects while keeping the accentuation and intonation of their dialects. We further examined within-speaker variability in vowel devoicing. We found that the speakers who devoice vowels frequently showed consistent devoicing, whereas those who devoice them infrequently showed occasional and inconsistent devoicing. Forensically, we should not simply judge a speaker’s dialect by using the occurrence frequency of vowel devoicing
instead, we should also look at its reproducibility and other dialect-dependent features.