Mismatched distances from speakers to telephone in a forensic-voice-comparison case

In a forensic-voice-comparison case, one speaker (A) was standing a short distance away from another speaker (B) who was talking on a mobile telephone. Later, speaker A moved closer to the telephone. Shortly thereafter, there was a section of speech where the identity of the speaker was in question - the prosecution claiming that it was speaker A and the defense claiming it was speaker B. All material for training a forensic-voice-comparison system could be extracted from this single recording, but there was a near-far mismatch: Training data for speaker A were mostly far, training data for speaker B were near, and the disputed speech was near. Based on the conditions of this case we demonstrate a methodology for handling forensic casework using relevant data, quantitative measurements, and statistical models to calculate likelihood ratios. A procedure is described for addressing the degree of validity and reliability of a forensic-voice-comparison system under such conditions. Using a set of development speakers we investigate the effect of mismatched distances to the microphone and demonstrate and assess three methods for compensation.