Effect of Human Speaker's Head Rotation on Speech Transmission Index in Vehicle Sound Environment

The speech transmission index (STI) is one of most common objective speech intelligibility metrics. The value of STI metric ranges from 0, indicating unintelligible speech, to 1, indicating excellent speech intelligibility, which could be influenced by signal to noise ratio (SNR), reverberation (or echo), and driving environment. In addition, a passenger talker could actually rotate his / her head to the driver when he / her talks to driver, which could induce the influence of speaker directivity on the STI. In current work, we measured several groups of binaural impulse responses on a human subject, under different orientations of a human speaker, and then calculate the corresponding STI values. Finally, we analyze the variation of STI caused by the human speaker's orientations, and give some advices on the measurement and evaluation method for STI in vehicle.