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

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 arrays 32-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

Rotationmanually

Parabolic Reflection Board

Focal point 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 sounds2 : Reflection sound

O-d

HP Q

2d

s1

s2

a

d2Directrix

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

)()()( tsAtstx

)()()( 2 SeASX j

:)(ts:)( ts :ADirect sound

Reflection soundReflection coefficient

The use of parabolic reflector can increase the power gain of the signalarriving from the front of the parabolic reflector according to )(H

)()(1)()(22222 HSeASX j

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.

M

mi

imX

Mi

1 1

2);(log

1maxargˆ

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

Pow

er [

dB]

Frequency [Hz] 410x

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

0 5 10 15 20 245

10

15

20

25

30

35

40

45

50

55

60

　

　

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