Dynamic perception process of the three dimensional spatial hearing

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In the present study, a new spatial hearing control system, in which the head-related transfer functions (HRTF) are switched over by sensing the motion of listener's head was first developed. This system is consisted of multiple DSP chips of 50 MFLOPS in speed and realizes very short delay time of only 13 ms in switching HRTFs against the head motion. By using the above mentioned system, the psychometric measurement of the detection threshold and the difference limen (DL) of the delay time in the switching of HRTF become possible. As results, it was first showed that the detection threshold is around 50 ms and the DL suddenly increase for delay time longer than 100 ms; these finding are new, interesting and important ones. Then, a psychoacoustical experiments were conducted to examine the human ability of sound localization under two conditions of head moving. In one condition, listeners were just allowed to move their heads while in the other condition, listeners were requested to move their heads actively to locate the sound image as precisely as possible. Low-passed noise, High-passed noise as well as wide-band noise with duration from 100 ms to 3 s were used as sound stimuli. As a result, the effect of active head movement was particularly effective to reduce localization errors for the low-passed noise. Moreover, the loci of the head movements were recorded with a motion sensor. The results showed some interesting tendencies such as clear vertical motions during the localization of a sound image on the horizontal plane.