Speech intelligibility and speech quality (SI&SQ) of voice microphones used in automotive hands-free communication systems are affected not only by microphone acoustic characteristics but also by interactions between the microphone and the background acoustic field. Due to the complex acoustic environment inside a vehicle cabin which constantly changes with driving modes, it is not a trivial task to choose proper acoustic characteristics of voice microphones for different vehicle and cabin designs. To establish a relationship between microphone characteristics and SI&SQ performance in automotive applications, a study is conducted using three common types of automotive voice microphones. Their performance is evaluated using both subjective and objective metrics described in ANSI standards S3.2-2009, S3.5-1997 and ITU-T Recommendation P.862.2. It is found that the objective SI index (SII) and the subjective SI results correlate nearly linearly. Furthermore, when the original SII scores are weighted by the speech-to-noise ratio, the weighted SII (wSII) data also exhibit linear correlation relationship with the objectively calculated subjective mean opinion score. Because the SII/wSII calculation is significantly less complex than the SI or mean opinion score evaluation process, results from this study demonstrate that the SI/wSII may be conveniently used as a tool to guide automotive voice microphone designs and evaluations.
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