Automotive voice microphones are commonly used in vehicle audio systems for hands-free communication. One important performance measure of such a voice microphone is the speech intelligibility (SI). Unfortunately, this measure cannot be directly derived simply using the microphone datasheet. Based on both a subjective method (ANSI S3.2-2009) and an objective method (ANSI S3.5-1997), this paper investigates the SI score judged by human listeners versus the speech intelligibility index (SII) score calculated from an established algorithm. Three typical automotive voice microphone designs differing in directivity and frequency response (FR) characteristics are tested. It is shown that no intelligibility differences are noticeable among three microphones at low background noise levels. With medium to high level and non-wind induced noises, the unidirectional or omnidirectional microphone with a rising FR exhibits certain advantage over the omnidirectional microphone with a flat FR. With wind turbulence inducted noises which are usually at high levels, the unidirectional microphone sees the greatest performance degradation. In Addition, an approximate linear correlation is demonstrated between the subjective SI and the objective SII scores for automotive applications, which enables the use of the simple SII method to predict the SI score without physically conducting the laborious SI test. This finding can guide engineers to choose cost-effective microphone technology solutions to meet specific application requirements in early design stages.
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