Forensic Phonetics: Speech & Document Analysis

Forensic Phonetics and Document Examination

• Forensic phonetics involves speech as evidence, from transcription to speaker identification.
• This field can provide information about a speaker's background and opinions on voice recordings.
• It can also used to design voice lineups and assess whether a suspect's voice matches that of a criminal.

Transcription in Legal Contexts

• Transcriptions of tape-recorded evidence are common in court cases involving criminal activity.
• Few court transcriptions are made by qualified phoneticians and are typically used when disputes arise.
• Transcription errors often stem from the transcriber's expectations rather than what was spoken.

Acoustic Analysis of Speech

• Speech is complex, with combinations of noises, musical notes, and frequencies
• Vocal cords vibrate to produce sound modifying airflow through the vocal tract
• The perceived pitch of a sound depends on the frequency of vibrations.
• Vocal cords alter pitch by adjusting length and thickness.
• Vowels are combinations of several pitches, modified by vocal tract amplifications.
• Formants are notes/pitches, with acoustic analysis focusing on the first three.

Speaker Profiling

• Speaker profiling aims to gather information about a suspect based on their voice, including regional and social accent.
• One early case successfully identified the Yorkshire Ripper regionally.
• Disguised accents can be detected through phonetic analysis, revealing a speaker's true origin.

Speaker identification by professionals

• Speaker identification compares a recording of a crime to voices of suspects.
• The goal is to determine if any suspect voices match the criminal’s voice.
• Variations always exist between speech samples, even from the same speaker under the same conditions.
• Forensic phoneticians distinguish between within-speaker and between-speaker differences.
• Ideal features for discrimination include high between-speaker variability, low within-speaker variability, and resistance to disguise.

Auditory and Acoustic Analysis Traditions

• Two primary methods exist for analyzing speech samples: auditory (British) and acoustic (US)
• A combination of both methods is preferred.
• The British tradition emphasizes descriptive phonetics and ear training.
• The use of the International Phonetic Alphabet (IPA) is important for accurate transcription.

Voiceprints

• Voiceprints are visual representations of speech, but they are not as reliable as fingerprints.
• The method involves visually matching spectrograms of known and suspect speakers.
• The flaw is that it involves subjective assessment of similarity between spectrograms by eye.
• Significant within-speaker variation exists, making accurate matching difficult.

Voice Lineups

• Voice lineups assist witnesses in identifying a suspect's voice.
• Naïve speaker recognition is evaluated by professionals, considering individual abilities and memory decay.
• Witnesses using auditory perception are less reliable than eyewitnesses.
• Recognizing familiar voices can be challenging, and voices are harder to learn than faces.
• Recognition success varies depending on voice familiarity, with errors occurring even with familiar voices.
• Listeners may have difficulty recognizing familiar speakers' voices.
• Stress at the time of hearing the voice may enhance memory but telephone quality reduces accuracy.

The work of the document analyst

• Document analysts include graphologists and handwriting analysis
• Graphologists claim to link handwriting to character and employment suitability, but are not reliable.
• Handwriting analysts focus on letter forms (graphemes) and graphetic variation.
• Each writer has a unique handwriting style
• A significant portion of the work is with disputed signatures.
• Copied signatures may reveal quality issues or divergence from graphic shapes.

ESDA

• Handwriting analysis suffers from subjectivity, unlike Electro-Static Document Analysis (ESDA)
• The Acronym ESDA stands for 'Electro-Static Document Analyser' and 'Electro-Static Detection Apparatus' developed in 1978
• ESDA visualizes indentations from handwriting on paper and can reveal writing on underlying sheets.
• ESDA was demonstrated in the Paul Dandy case (1987) where it exposed fabricated police interview records.
• ESDA supported claims of police fabricating verbal evidence.