The understanding of communications between passengers in an automobile as well as vehicle information systems to the driver is critical. A comprehensive study was conducted to evaluate speech intelligibility inside a vehicle for person to person communication using the Speech Transmission
Index (STI). This metric provides a measure of the understanding or intelligibility of speech and has a value which varies from 0 for completely unintelligible to 1 for perfect intelligibility. For this study, several vehicle operating conditions, road surfaces and talker and listener configurations
were compared in terms of the STI metric to gain a better understanding of these influences on in-vehicle person to person communication. Individual contributions of background noise and interior vehicle acoustics to the STI were also investigated. The results of this study found that higher
in-vehicle sound pressure levels from engine and road noise are not necessarily associated with lower speech intelligibility. A wide range of STI values considered to be significant from subjective descriptions of intelligibility provided in the IEC EN 60268-16:2003 standard corresponding
to those values were observed. These observations were for particular vehicle operating conditions, various configurations of the talker and the listener locations inside the vehicle, as well as between the ears of the listener at any particular location inside the vehicle. It was found that
the rate of change of STI varied in unsteady background noise due to variable speed operation, at any given speed. It is recommended that subjective testing in a simulated vehicle environment be pursued to investigate any correlation of results with the STI metric. Also recommended is to determine
the significance of the findings in terms of human perception of speech intelligibility inside a vehicle, and further investigate the appropriateness of using the STI metric, which is one of the most comprehensive speech intelligibility metrics available for a complete assessment of factors
influencing in-vehicle person to person communication. Lastly, the effects of reverberation on reducing speech intelligibility were found to be negligible.
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