Assessment of Conversational Speech Quality Inside Vehicles, Concerning Influences of Room Acoustics and Driving Noises

This study considers the influences of room acoustics and driving noises in vehicle interiors on the subjectively perceived acoustical quality of conversations between passengers. A listening test with 25 participants was performed inside a laboratory to assess the impact of different vehicle interior transfer functions on the speech quality assessment in four predetermined dimensions. Idealized driving noises at three different vehicle speeds were presented simultaneously with speech samples to quantify the interferences of these noise conditions with varied signal-to-noise ratios. To minimize the influence of different human speakers, four talkers (two male and two female) were selected from commercially available audio books. The respective speech samples were adjusted in level and long-term average speech spectrum to the common values of conversational speech. The automatic reflex of raising one's voice in noisy environments, called "Lombard Effect" [1], was taken into account for an additional adjustment of speech levels while driving noises were present. A strong relationship between the speech-to-noise ratio and the test participants' evaluations was found. Thus, one can assume that the speech signals' attenuation or amplification caused by the different room acoustics of the tested vehicles play a more important role for a sufficient speech quality than the varied speech timbre or other parameters. Only at very high speech-to-noise ratios (>= 20 dB with A-weighting), room-acoustical parameters such as IACC or the reverberation time are more determining for the speech quality appreciation than the speech's sound pressure level.