17th May 2012, Acoustics 2012 @ Hong Kong Convention and Exhibition Centre

Acoustic privacy area generation
based on simple summation ofnumerous loudspeaker signals

Takuma OKAMOTO(1), Yukio Iwaya(2) and Yôiti Suzuki(3)(1) National Institute of Information and Communications Technology (NICT), Japan(2) Tohoku Gakuin University, Japan(3) Tohoku University, Japan1

Motivation
Audio secret sharing for 1-bit audio
N. Fujita, R. Nishimura and Y. Suzuki, Acoust. Sci. & Tech. 27, 3 (2006)
based on Secret sharing schema (A. Shamir 1979)

XOR
3 out of 3

2 out of 32

Motivation
Audio secret sharing for 1-bit audio
N. Fujita, R. Nishimura and Y. Suzuki, Acoust. Sci. & Tech. 27, 3 (2006)
based on Secret sharing schema (A. Shamir 1979)
Original speech can only be restored when all streams can be got
Privacy speech communication on network

Realizing these communications on actual sound field!!
Enhancing a speech signal at a point
Speech privacy technique on actual environment
Problem
XOR cannot be calculated on actual sound field and hearing
3

Previous methods
Speech enhancement techniques at a point
based on multiple points control
J.-W. Choi et al. (2002)
S. Enomoto et al. (2005)
based on beamforming
Y. Tamai et al. (2003)

Speech privacy techniques
based on masking

Proposed method

Techniques forenhancing objective signal
Objective signal can be listened at
outside of sweet spot

Techniques formasking objective signal
Spacial information is not taken intoaccount4

Previous method
Sound spot generation based on delay-and-sum
Y. Tamai et al. (2003)

r1s(t)z(D0 rN /c)r1riri・・・(D0 ri /c)・・・

z・・・s(t)・・・(D0 r1 /c)・・・z・・・s(t)rN

Sweet spotrN

Amplitude of signal is amplified N times at a sweet spot
Problem : Speech signal can be listened outside of a sweet spot5

Proposed method 0SNR ­20 dBs(t)nN (t)・・・

r1z(D0 rN /c)r1ri・・・ri・・・

z (D0 ri /c)・・・

ni (t)(D0 r1 /c)
・・・

z・・・

n1 (t)

rN

Sweet spotrN
6

Basis of proposed method 0Sound spot generation based on simple summation withrandom masking signals
xi (t) = s(t) + ni (t)Loudspeaker signal at i-th channel
1yi (t) = xi (t ri /c) Observed signal from i-th channel
riNNXX
1y(t) =ri · xi (t D0 ) = N s(t D0 ) +ni (t D0 ) Total observed signal
ri
i=1i=1Masking signals are mutually uncorrelated each other✓◆2
11tpn(t) =exp
22 22⇡✓◆2pN
1 ty(t) = N s(t) + p
exp= N s(t) + N n(t)22n2⇡n 2

We can only masking signal at each loudspeaker positionpSNR increases proportionally to N and the objective signal can be heard
only at the sweet spot

7

Introduction of reverse phase pairs
Problem in Proposed method 0
Masking signals can be listened at a sweet spot

Improved method : introduction of reverse phase pairs ofmasking signals
Masking signal components can completely be 0!!
Proposed method 1 : reverse phase pairs composed of set to each other
Proposed method 2 : reverse phase pairs composed of next to eachother8Performance evaluation
Computer simulations
Signal to Noise Ratio (SNR)
Speech Intelligibility Index (SII) ANSI S3.5-1997
Evaluate speech intelligibility as a score 0 to 1
Calculated from each SNR between speech and noise at each frequency
bandSimulation conditions
Sampling frequency : 48 kHz
Objective signal : a male voice from Speech Database by NTT-AT
Masking signals : Pink noise
SII conditions：1/3 octave band (18 bands)(center frequency:160 Hz to 8000 Hz)
Analysis bandpass filter ：FIR filter (128 samples)
9Linear array

10Linear array：Results

110110252504−5−106Y axis [m]Y axis [m]SNR04−5−106−158−4−20X axis [m]24−158−4−20−2[dB]

0X axis [m]24−20[dB]

Proposed 0

Proposed 211SII0.6

20.6

20.4

40.3
60.2

0.5

Y axis [m]Y axis [m]

0.5

0.4

40.3
60.2

0.1
8−4−20X axis [m]240Proposed 00.1
8−4−20X axis [m]240Proposed 211

Surrounding 157 array

Surrounding 157 loudspeaker array@R.I.E.C., Tohoku Univ.

12

Surrounding 112 array

112 chArrangement of loudspeakers
13Surrounding 112 array：Results
SNR
105000−10−15−1.5−1−0.5

X axis [m]00.510−5−10−20

Proposed 0−15−1−2[dB]

Y axis [m]

5−5−2

10

150−1101

Y axis [m]Y axis [m]1−1.5−1−0.5

X axis [m]00.510−5−10−20−2[dB]

Proposed 1−15−1−1.5−1−0.5

X axis [m]00.5

1−20[dB]

Proposed 2SII
10.6

0.5

0.5

0.5

0.3

0.4

00.30.2−10.1−2−1.5−1−0.5

X axis [m]

00.5

10

Proposed 0Y axis [m]0.60.4010.6

Y axis [m]Y axis [m]

10.4

00.3

0.2
−10.1

−2−1.5−1−0.5

X axis [m]

00.5

10

Proposed 10.2
−10.1

−2−1.5−1−0.5

X axis [m]

00.5

10

Proposed 214Concluding remarks
Acoustic privacy area generation based on simple summation
Using mutually uncorrelated masking signals
Introduction of reverse phase pairs for expansion of sweet spot
Reverse phase pairs composed of next to each other can increase SNR
around a sweet spot efficiently

Acknowledgement
This study was partly supported by the GCOE program (CERIES) of theSchool of Engineering, Tohoku University15