Evidential value of voice quality acoustics in forensic voice comparison

Voice recordings in forensic voice comparison casework typically involve speech style mismatch and are separated by days or weeks, but studies that aim to empirically validate the evidential value of speech features rarely include systematic comparisons on contemporaneous vs. non-contemporaneous recordings and match vs. mismatch in speech style. This study addresses this gap and focuses on the acoustics of laryngeal voice quality, since voice quality has been reported to be one of the most popular and useful features for forensic voice comparison. 75 male speakers aged 18–45 were selected from a forensically-oriented database of Australian English speakers in Sydney/New South Wales. The evidential strength of a number of spectral tilt and additive noise parameters were tested under the Bayesian likelihood-ratio framework. Results show that system performance using these parameters as input were stable across 50 replications. When speech style is controlled for, VQ parameters yielded promising results and better system validity was achieved when using more VQ parameters. However, they offered limited speaker-discriminatory value when speech style mismatch is involved, and non-contemporaneous recordings only led to a small decline in performance. Overall, forensic practitioners should be cautious when using spectral tilt measures and additive noise measures as speaker discriminants in forensic casework.