1. 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 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 Use of simultaneous phase information from microphone arrays to estimate the direction of the signal arrival. Background  Microphone Arrays 30-channel arrays 32-channel arrays  Conventional thought: Microphone does not change position even if it is unable to pick up a sound well. Goal of the Research A good combination of active-operation microphone and signal processing. System Overview Active Microphone Microphone (Focal point) Parabolic reflector Rotation manually Parabolic Reflection Board  rotate together  perform signal processing  seek to locate the direction of the sound source The reflector and its associated microphone Focal point Signal Parabolic surface -d H P Q 2d s1 s2 O Directrix Parabolic surface Focal point Distance difference between path s1 and s2 to the focal point: QP+PO = QP+PH = 2d d : distance of the focal point s1 : Direct sound s2 : Reflection sound Time difference to the focal point: (depending only on ‘ d ’) a : sound speed “The signal is coming from the front of the parabolic surface.” Observed signal at the focal point  In the frequency domain Direct sound Reflection sound Reflection coefficient  Power spectrum The use of parabolic reflector can increase the power gain of the signal arriving from the front of the parabolic reflector according to. 90 deg 120 deg 150 deg When the sound source is located in front of the parabolic surface, any wave is reflected toward the 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. Signal Power Observed by Parabolic Reflection (Assume no background noise) Time difference between direct path and reflection path is equal to 2d/a wherever the signal reflects. (There is no delay among reflection waves.)

2. “The signal is not coming from the front of the parabolic surface.” Signal Power Observed by Parabolic Reflection When the input signal is coming from degrees, the direction of the reflected signal at the parabolic surface is off degrees from PO. Focal point P O Tangential line (Reflected signal) degrees Selection of Direction Having Maximum Power Microphone (Focal point) Parabolic reflector Signal Rotation manually 1. The microphone is set up at the focal point. 2. The microphone rotates and the power of the target signal at each angle is calculated. 3. The direction having maximum power is selected as the sound source direction. 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. No reflection waves will travel toward the focal point. The power gain will not increase. Experiments Experiment Conditions Parabolic Reflector: Small parabolaLarge parabola Diameter12cm24cm Focal length3cm9cm Photo Microphone: omnidirectional microphone Source signal: white noise (5 sec) Results Target source: 90 degrees Performance of parabolic reflector Performance on different sound-source distances 40cm mic. with reflector speaker 100cm 70cm Small parabola mic.Large parabola mic.  The use of parabolic reflector improves the normalized power to 15 dB at 90 degrees, and the power decreases as the direction of the microphone becomes farther from the direction of the target sound source. Estimation of Sound Source Direction Summary Future Work  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.  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. Microphone Loud speaker 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. Experiment environment (real room environment) Reflector: Small parabola Sound source distance: 70cm The power was normalized so that the minimum was 0 dB.  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.