In-vehicle application of common speech intelligibility metrics

Binaural speech-to-noise loudness ratio at the speech reception threshold in vehicles

“…For the SII, scores > 0.75 are usually taken as indicating good speech communication, while poor communication systems have an SII < 0.45 5 . STI scores for speech in car noise have been interpreted in the following way: > 0.75 -good or excellent speech communication; 0.6 to 0.75 -good communication; 0.45 to 0.6 -fair communication; 0.3 to 0.45 -poor communication; <0.3 -almost no intelligibility 23,24,25 .…”

“…Figure 1 shows the experimental setups, using a mid-sized sedan, for speech signal calibration and semi-anechoic vehicle dynamometer speech and noise measurements used in a study of Samardzic and Novak 24 . A head and torso simulator (HATS) was used to estimate the spectra of the speech and noise at the listener's ears.…”

“…Also, there are no recommendations about how to combine metric values across ears. This may be one factor that contributes to the finding that these metrics do not give accurate predictions of the intelligibility of speech in vehicle noise 23,24,25,35,36,37 .…”

“…In addition, the outer ear acts as a directional filter that can change the sound pressure level at the eardrum by +15 to 30 dB, depending on the frequency and direction of sound incidence 34 , and this can introduce strong inter-aural level differences. To accurately characterize the signals reaching each ear, it is essential to perform binaural measurements either using in-ear microphones or, as in the study of Samardzic and Novak 24 , using a HATS.…”

“…The present analysis utilizes the raw data, specifically the speech and noise spectra at the SRT, of Samardzic et al 22 . In the present study, the SII and STI were calculated using the spectrum of the speech material actually used for the SRT measurements, whereas previously 23,24,25 the SII metric was calculated based on the speech spectrum for normal vocal effort, as specified in the ANSI standard for the SII 5 . The calculations of the STI at the SRT (Table 2) have not previously been published.…”