Active Microphone with Parabolic Reflection Board for Estimation of Sound Source Direction

Tetsuya Takiguchi, Ryoichi Takashima and Yasuo ArikiOrganization 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 togetherperform signal processingseek 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

DirectrixParabolic 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 sounds2 : Reflection sound

Time difference to the focal point:

ad /2(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

)()()( tsAtstx :)(ts Direct sound

:)( ts Reflection sound

:A Reflection coefficient

)()()( 2 SeASX j

Power spectrum

The use of parabolic reflector can increase the power gainof the signal arriving from the front of the parabolic reflector according to .)(H

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.)

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

“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

mi mX

Mi

2|);(|log

1maxargˆ

Microphone(Focal point)

Parabolic reflector

SignalRotation 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 targetsignal observed at a microphone angle i, the power spectrumis obtained at frame m.

i

No reflection waves will travel toward the focal point. The power gain will not increase.

Experiments

Experiment Conditions

•Parabolic Reflector:

Small parabola Large parabola

Diameter 12cm 24cm

Focal length 3cm 9cm

Photo

•Microphone: omnidirectional microphone

•Source signal: white noise (5 sec)

Results

-5

0

5

10

15

20

0 30 60 90 120 150 180Direction [degree]

Nor

mal

ized

Pow

er [

dB] with parabolic

reflector (small parabola)

without parabolicreflector

•Target source: 90 degrees

•Performance of parabolic reflector

-5

0

5

10

15

20

25

0 30 60 90 120 150 180Direction [degree]

Nor

mal

ized

Pow

er [

dB

]

40cm

70cm

100cm

-5

0

5

10

15

20

25

0 30 60 90 120 150 180Direction [degree]

Nor

mal

ized

Pow

er [

dB

]

40cm

70cm

100cm

•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 parabolaSound 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 wellwhen the distance from the sound source was short.