Forensic Linguistics

The Law and Speech Sounds

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Two underlying assumptions
- Different spectogram renderings of the same word or sound from one individual would be similar to each other
- Different spectogram renderings of the same word or sound from different individuals would be different from each other
Problems
- Intra-speaker variability
- Inter-speaker similarity
- Significant or diagnostic similarities or differences
- Over-zealous application

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Procedures
- Phoneticians listening to and making notes on speech patterns in disputed and known recordings
- Auditory impressions were compared using a range of segmental and prosodic dimensions
Problems
- No serious attempts to explain its basis in the academic of professional literature until very recently
- Acoustically different data may create similar auditory impressions

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Auditory-phonetic examinations
- Segmental features
- Pitch, rhythmical and fluency features
- Voice quality
Acoustic-phonetic examinations
- Spectographic analysis
- Energy-frequency domain examinations
- Time domain examinations
Fundamental frequency examinations
- Creak transition points
- Intonation
- Jitter estimation

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Speaker identity cannot currently be established with total certainty
Influences on speech production:
- Deliberate voice disguise
- Tiredness
- Illness
- Intoxication
- Psychological states
- Changes in the communicative situation
Phonetician must be able to appraise the differences found using casework, experience and relevant research
If the proof of guilt is beyond reasonable doubt, it follows that forensic speaker identification is not a basis for conviction; an expert can only offer an opinion
The International Association of Forensic Phonetics has a Code of Practice to prevent experts over stating their opinions

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Authentication
- Evidence of tape tampering is sought by a variety of means
- Each case presents slightly different problems
- Depends on:
  - The nature of the speech materials and recording
  - The specific instructions accompanying the material
Determination of unclear or contested utterances
- Contacted by Met Police to:
  - Help in the investigation
  - Provide an expert opinion
Earwitness Testimony
- Ear Witnesses
  - Neglected area
  - Number of investigators are adamant in their opinion that voice parades are an adequate or inappropriate approach to speaker identification

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