Voice identification requires the forensic examiner to discover similarities, as well as differences, in all three areas of investigation.
Here are the step-by-step processes I use when conducting voice identification:
1. Visual examination of the original recording, analogue or digital. This includes examination of the physical characteristics of the tape itself (if analogue) or analogue or digital recorder. It is important to examine the cassette tape (standard, mini or micro) or other analogue or digital source to determine if there are visual signs of tampering or alteration.
2. Once the physical evidence has been examined, the next step is to load the recording in question into a forensic computer. Visual examination of the sound wave, sonogram and spectrograph reveal speech characteristics and patterns of verbal delivery as well as electronic characteristics. At this point, the recording has been digitized so forensic software can analyze and conduct various tests.
3. If possible, for authentication or voice identification, an exemplar or comparison recording should be made of the original recording to compare the original recording characteristics. This same forensic examination process that is applied to the evidence is also applied to the exemplar to determine that the characteristics are the same and the recording is from the same audio recorder.
4. When conducting voice identification, it is important to create an exemplar of the accused for audio comparison using as exact conditions and equipment as close as possible to the measurements taken from the evidence as outlined above. The speech must be the same as the speech on the evidence in order for the testing to be accurate. As an audio forensic expert, I often have to coach the accused into the same energetic voice tone and inflection as the evidence recording. However, it is still possible to compare speech if the exemplar is not as close to the evidence as I would like.
5. Critical listening skills are used to examine the speech pattern, pronunciation, voice tone and inflection, accent, dialect and specific speech characteristics (like a lisp or significant “s” delivery). There is a rhythm in how an individual speaks, and even if s/he is trying to disguise his/her speech (in an attempt to fool the forensic examiner), the rhythm and speech patterns as described above still show through. The expert must pay careful attention to the rhythm of spoken word formations. I listen to single words as well as phrases and sentences.