The Variability of Vowels' Formants in Forensic Speech

Speech analysis plays a pivotal role in the exploitation of forensic recordings in resolving a wide range of questions. Although this topic may include a large set of methodologies based on varied digital features (MFCC, Centroid, Harmonicity, VOT, etc., cf. [1]), this work approaches the theme from a phonetic perspective, taking into consideration the vowel formants, and focusing on their variability and difficulty of measurement. Formants correspond to the resonant frequencies of the vocal tract and are, therefore, sensible to specific speaker-related variations such as age and sex. In this respect, formants' variation contributes to characterizing the subjective timbre of the person. For this reason, formants' values are largely used in forensics [2], with all the practical problems that come with them, and in particular when dealing with speaker recognition or discrimination [3], [4]. Indeed, formants' values correspond to specific frequencies of the sound signal and are usually reported in Hertz. They are, however, affected by numerous internal and external variables, so that although on average they are characteristic of the individual speaker, they always vary within frequency bands that cannot be defined in absolute terms [5]. How, then, is it possible to provide reliable answers to forensic questions? In essence, it is up to the specialist to determine if there are the conditions to carry out an analysis, and to understand, for example, whether the differences between formants' values could be ascribed to two different speakers or the difference is too subtle to justify this claim. What should be measurable today, and must be defined at the level of jurisprudence, is therefore the professionalism of the expert. However, this is still heterogeneous in different countries, although several codes of practice such as the International Association for Forensic Phonetics or engineering society ones have been validated.