Auditory detectability of vocal ageing and its effect on forensic automatic speaker recognition

The comparison of non-contemporary speech samples is common in forensic speaker recognition cases. It has yet to be established however, to what extent the time interval between non-contemporary samples can increase before a problem is created for forensic automatic speaker recognition. This paper presents results of a human listener test designed to evaluate the detectability of vocal ageing over increasing intervals of up to 30 years. Subsequently, a forensic automatic speaker recognition evaluation of 15 ageing males at increasing intervals of up to 60 years is presented. It is shown that at intervals of around 10 years, the average detectability of vocal ageing by humans is just above chance. As the interval rises to 30 years, vocal ageing is detected 90% of the time. In the automatic system, vocal ageing is manifested as a drop in intra-speaker likelihood ratios (LRs) as the time interval between non-contemporary samples increases. At an interval of 30 years, LRs for the vast majority of intra-speaker comparisons fall below a value of 100 commonly interpreted as ‘moderate support’ on a verbal LR scale. Our findings indicate that at a time-lapse of 30 years, vocal ageing creates significant problems for forensic automatic speaker recognition.