Definition of appropriate indicator and threshold values for intelligibility: Application for rolling stock’s public address systems

The rolling stock’s public address systems are used to broadcast safety information from the driver cab or the train supervisor. EU legislation request an audible information using the STI-PA intelligibility indicator be at least 0.45 in the whole train. The purpose of the study is to assess the current intelligibility indicator according to the operating conditions of the rolling stock in comparison with other indicators in the literature, then to study the correlation between indicators and the passenger perception and to define threshold values and objective scales of comfort. The first step was to define a methodology to characterize the rolling stock’s public address systems. Secondly, two kinds of measurements were performed in a high speed train at several positions: the end-to-end public addresses system regarding its linear and non-linear characteristics in order to simulate distortion occurrence in the transmission channel at various operating condition. As a result of this study, standard intelligibility indicators showed poor correlations with intelligibility scores from the listening test, especially when the degradation is due to distortion. Fields of improvement are investigated using intelligibility indicators based on clean and degraded speech signal comparison.