Active Microphone with Parabolic Reflection Board for Estimation

Active Microphone with Parabolic Reflection Board for Estimation of Sound Source Direction Tetsuya Takiguchi, Ryoichi Takashima and Yasuo Ariki Organization of Advanced Science and Technology, Kobe University Introduction Background In everyday life, if an interesting sound is almost inaudible, people usually adjust the angle of their ears so that their ears are facing the sound direction. Signal processing is occurring in the brain as one changes the angle of his/her ear. Conventional Method £Microphone Use of simultaneous phase information from microphone arrays to estimate the Arrays direction of the signal arrival. £Conventional thought: Microphone does not change position even if it is unable to pick up a sound well. 30 -channel arrays Goal of the Research 32 -channel arrays A good combination of active-operation microphone and signal processing. System Overview Signal Power Observed by Parabolic Reflection Active Microphone Parabolic reflector “The signal is coming from the front of the parabolic surface. ” s 1 : Direct sound s 2 : Reflection sound Microphone (Focal point) Rotation manually Signal 2 d H Distance difference between path s 1 and s 2 to the focal point: QP+PO = QP+PH = 2 d P -d d : distance of the focal point Q O s 2 s 1 Focal point Time difference to the focal point: 90 deg 120 deg a : sound speed (depending only on ‘d’) 150 deg The reflector and its associated microphone £rotate together £perform signal processing £seek to locate the direction of the sound source Parabolic Reflection Board Parabolic surface Time difference between direct path and reflection path is equal to 2 d/a wherever the signal reflects. (There is no delay among reflection waves. ) Observed signal at the focal point When the sound source is located in front of the parabolic surface, any wave is reflected toward the focal point. Parabolic surface Directrix (Assume no background noise) Direct sound £ In the frequency domain £ Power spectrum Reflection sound Reflection coefficient Focal point When the sound source is not located in front of the parabolic surface, no reflection waves will travel toward the focal point. The use of the parabolic reflection board enables us to find the power difference between the target direction and non-target direction at the focal point. The use of parabolic reflector can increase the power gain of the signal arriving from the front of the parabolic reflector according to.

Estimation of Sound Source Direction Signal Power Observed by Parabolic Reflection Selection of Direction Having Maximum Power “The signal is not coming from the front of the parabolic surface. ” When the input signal is coming from degrees, the direction of the reflected signal at the parabolic surface is off degrees from PO. 1. The microphone is set up at the focal point. Tangential line 2. The microphone rotates and Microphone the power of the target signal (Focal point) at each angle is calculated. Signal 3. The direction having maximum power is selected as the sound source direction. P degrees O No reflection waves will travel toward the focal point. The power gain will not increase. Parabolic reflector Focal point (Reflected signal) Rotation manually By applying the short-term Fourier transform to the target signal observed at a microphone angle i, the power spectrum is obtained at frame m. Experiments Results Experiment Conditions • Microphone: omnidirectional microphone • Performance of parabolic reflector • Parabolic Reflector: Small parabola Sound source distance: 70 cm Small parabola Large parabola Diameter 12 cm 24 cm Focal length 3 cm 9 cm The power was normalized so that the minimum was 0 d. B. Photo £The • Experiment environment (real room environment) mic. with reflector use of parabolic reflector improves the normalized power to 15 d. B at 90 degrees, and the power decreases as the direction of the microphone becomes farther from the direction of the target sound source. • Performance on different sound-source distances Small parabola mic. 40 cm Large parabola mic. speaker Microphone 70 cm Loud speaker 100 cm • Source signal: white noise (5 sec) • Target source: 90 degrees The angle of the microphone with parabolic reflector is changed manually from 0 degrees to 180 degrees at an interval of 10 degrees. The average power of the target signal at each angle was calculated in five seconds. Summary £A sound-source direction estimation method using a single microphone only. £The new proposed method using parabolic reflector is able to estimate the sound source direction without any measurement in advance. New concept: Active microphone is a good combination of an active-operation microphone and signal processing. £The shape of the average power for the large parabola sharpens up at 90 degrees in comparison with that with small parabola because of the difference of the focal distance of parabolic reflectors. £When the large parabola was used, the power gain increased as the distance increased, with the graph taking on a very sharp shape. The sound source did not form the ideal sound wave (plane wave) and the sound wave were not reflected well when the distance from the sound source was short. Future Work £Investigate the performance in noisy environments, such as with multiple sound sources. £Research for short signals and nonstationary signals (for example, speech). £Detail research for the form of the parabolic reflector.