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Forensic Speaker Comparison or Voice Comparison is the science of comparing two, or more, speech samples to give an opinion as to whether they are consistent with originating from the same speaker or not. In general this would mean comparing a known voice reference sample, taken from a police interview or other undisputed recording, with the recorded speech from a questioned recording, e.g. a hoax 999 call.

Consistency

First, a decision is made concerning whether the known and questioned samples are compatible, or consistent, with having been produced by the same speaker. In considering consistency we assess the degree to which observable features are similar or different. In judging the samples we would regard differences as weighing against compatibility unless they can be explained by models of acoustic, phonetic or linguistic variation (e.g. by reference to differential channel characteristics, sociolinguistic, psychological and/or physical factors). The presence of substantive non-explained differences between the samples would be taken as grounds for judging them to be inconsistent with having been produced by the same speaker. In some cases it may not be possible to reach a decision on consistency. The outcome of this first decision process is one of the following judgements:

Consistent

Not consistent

No decision

Consistent Outcome – Distinctiveness

The decision that two samples are consistent with having been spoken by the same person does not necessarily entail that they were. Unless the cluster of features leading to the consistency decision is distinctive or unusual, they will be shared by a substantial number of other people in the population. Therefore it is necessary to proceed to evaluate the distinctiveness of the features common to both sets of samples. A convenient way of assigning an overall rating to the distinctiveness of the common features is to locate them on a five-point scale, as follows:

Exceptionally Distinctive – the possibility of the observed combination of speech characteristics being shared by another speaker is considered to be remote

Highly Distinctive – it is unlikely that any other speaker would share the observed combination of speech characteristics

Fairly Distinctive – very few speakers would be likely to share the observed combination of speech characteristics

Moderately Distinctive – a small number of speakers would be likely to share the observed combination of speech characteristics

Not Distinctive – a significant number of speakers might be expected to share the observed combination of speech characteristics

Not Consistent Outcome

Where the two samples are not consistent it is appropriate to state the belief that the samples are spoken by different speakers. This may be stated with a degree of confidence appropriate to the exigencies of the data.

Inconclusive Evidence

It may alternatively be necessary to state that the available evidence does not allow a judgement to be made with confidence. This will be the conclusion reached when the evidence is too limited or of too poor a quality to sustain a firmer opinion.