Determining the Likelihood Ratio From Perceptual Attributes of Voice

Abstract

Purpose: To provide voice experts with a method for determining the likelihood ratio (LR) from the perceptual evaluation of distinctive voice attribute scores. The proposed method aims to obtain the similarity and typicality judgments made by forensic voice experts (FVEs) during the comparison of attributes in voice pairs. Method: It is based on the scoring method for LR calculation. In the first stage, 17 perceptual attributes grouped into six vocalic categories are specified. A novel graphical interface is used to obtain discriminative responses both globally and for each attribute from ten pairs of test sentences produced by the same and different speakers. The FVEs should discriminate whether the attributes are similar or different in each pair and should indicate the degree to which the attributes are present. In addition, for six specific attributes, the FVEs must decide whether the attribute is typical or atypical in the reference population. In the second stage, the mean score obtained in the first stage is converted to LR using probability density functions of listeners' responses to 1680 same/different speaker pairs discriminated for female and male speakers. Results: The responses of the FVEs to the test pairs show the discriminatory power of the attributes, the incidence of the typicality factor on the final score and the performance of each FVE. With the application of the probability density functions obtained for the responses to pairs of the same or different origin taken from the reference population, the final scores are converted into LRs that are compared with the true conditions of each pair. Conclusions and future work: The application of the developed system allows the global and discriminated evaluation of the perceptual attributes with high agreement in the comparison of pairs of voices. Obtaining the LRs allows associating the perceptual evaluation method with the automatic methods that are used nowadays. The responses of the FVEs taken as a reference, will allow training and evaluating the performance of young FVEs.