Estimation of Sound Source Direction Using Parabolic Reflection Board 2008 RISP International Workshop on Nonlinear Circuits and Signal Processing (NCSP’08) 6-8 March, 2008 Watermark Hotel, Australia Tetsuya Takiguchi, Ryoichi Takashima and Yasuo Ariki Kobe University, Japan

Table of Contents Introduction Purpose of Sound-source-direction Estimation Conventional Technique Proposed Method Parabolic Reflection Board Active Microphone Experiments Summary and Future Work

Purpose of Sound-source-direction Estimation Noise suppression by forming directivity toward the target signal source Noise disturb the speech recognition Speech Noise If the system direction of the target signal source, … Directivity

Purpose of Sound-source-direction Estimation Search robot for disaster victims Estimation of speaker for the meeting system Help !! A is talking now. Sound-source-direction estimation technique is necessary for various systems

Conventional Techniques Microphone Arrays Use of simultaneous phase information from microphone arrays to estimate the direction of the signal arrival. 30-channel arrays32-channel arrays

Proposed Method Two or more microphones are necessary for conventional method It is difficult to estimate of the signal arrival using only a single microphone Goal: Sound-source-direction estimation using only a single microphone

Active microphone with Parabolic Reflector Proposed Method Microphone Parabolic reflector Diameter: 12cm The reflector and its associated microphone rotate together Rotation manually

Parabolic Reflection Board Focal point Any wave, where the sound source is located directly in front of the parabolic surface, is reflected toward the focal point. No reflection waves, where the sound source is not located directly in front of the parabolic surface, will travel toward the focal point.

Observed Signal at the Focal Point The signal is coming from directly in front of the parabolic surface 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 O-d H P Q 2d s1 s2 Directrix Focal point Parabolic surface Time difference for all reflection paths is equal to 2d/a. Time difference to the focal point: a : sound speed (depending only on ‘d’)

Observed Signal at the Focal Point The signal is coming from directly in front of the parabolic surface Observed signal at the focal point In the frequency domain Power spectrum Direct sound Reflection sound Reflection coefficient The use of parabolic reflector can increase the power gain of the signal arriving from the front of the parabolic reflector according to

Observed Signal at the Focal Point The sound source is not located directly in front of the parabolic surface. O: Focal point Parabolic surface P Tangential line (Input Signal) (Reflected Signal) degrees No reflection waves will travel toward the focal point!

Selection of Direction Having Maximum Power 1. A microphone is set up at the focal point. 2. The microphone rotates and the power of the target signal observed at each angle is calculated. 3. The direction having maximum power is selected as the sound source direction. Microphone i : angle of the parabolic reflector Rotation manually

Experiment Conditions 90cm 60cm 30cm Parabola Reflector Loud speaker Microphone Target source: 90 degrees Source signal: white noise (5 sec) The angle of the microphone with the parabolic reflector is changed manually from 0 degrees to 180 degrees at an interval of 10 degrees.

Average Power Versus Angle of Microphone Average log-power spectrum at 90 degrees is maximum value. The power decreases as the direction of the microphone becomes farther from the direction of the target sound source.

With / Without the Parabolic Reflector with reflector without reflector (The directivity of the microphone is set up opposite the sound source)

3D Power Spectrum of the Observed Signal Power [dB] Frequency [Hz] Angle of mic. [degree] Effect is not so great for the low-frequency components of the signal. Power spectrum becomes larger as the angle of parabolic reflector is closer to 90 degrees. Low-frequency: diffraction of the sound wave

Power spectrum of the signal observed without reflector The shape of the spectrum is not flat.

Summary A sound-source-direction estimation method using a single microphone only. New Proposed Method : Active microphone with parabolic reflection board is able to estimate the sound source direction using only a single microphone. In future work : research for short signal (for example, speech) form of the parabolic reflector

Comparison with Any Reflector