Poster of the 2nd International Symposium on Ambisonics and Spherical Acoustics

May 6-7, 2010, Paris, France

Improving sound ﬁeld reproduction in a small roombased on higher-order Ambisonics with 157-loudspeaker array
T. Okamoto1, D. Cabrera2, M. Noisternig3, B. FG Katz4, Y. Iwaya1 and Y. Suzuki1
1Tohoku University, Japan, 2The University of Sydney, Australia, 3IRCAM, France, 4LIMSI-CNRS, France

1. Introduction
For realizing high deﬁnition 3D sound ﬁeld
reproduction, we have developed a 157
loudspeaker array system [1,2] in a smallroom based on higher-order Ambisonics [3].
The features of this system are
1. no loudspeaker is installed on the ﬂoor
andthe arrangement is on rectangular grid2. 157 channels are controlled by 4 PCs
3. reverberation time of this room is 0.2 s
Therefore, we discuss about
1. appropriate decoding order
2. inter-channel synchronicity
3. acoustics of the reverberation
for realizing more precise sound ﬁeld
reproduction.
[1,2] T. Okamoto et. al 2007 and 2009[3] M. A. Poletti 2005
Surrounding 157 loudspeaker array system using 4 PCs, 14 units of A/D D/A and splitter

2. Appropriate decoding order

3. Synchronization

The general Ambisonics approach is deﬁned by a sphericalharmonics expansion of the free ﬁeld without taking the radialsolutions of the wave equation. The decoding matrix
C+ iscalculated from the set of spherical harmonics between the sweet
spot and each loudspeaker position. The sweet spots is located in
the center of the room for a seated person's ear height with z=1.26
m. The accuracy of the decoding matrix was estimated from thecondition number of Eq. (1). We implemented HOA of order 5which represents an over-determined system, i.e. it can be solved
by singular value decomposition (SVD) / pseudo-inverse.

The latency was measured using a Time Stretched Pulse (TSP) signal[4], a variant of the swept-sinusoid. Deconvolution of the TSP signal
from the recorded signals for each channel provides an impulse
whose temporal position is equivalent to the delay. The results showthat the output signals from all D/A channels from the same PC werecompletely synchronous at the 1-sample level. However, the output
signals between diﬀerent PCs were not synchronous at the 1-sample
level and the delay timing varies. The unsigned maximum latency was
51 samples (= 1.1 msec), requiring additional delays to compensate
for this.
[4] Y. Suzuki et. al 1995

Y00 (θ1 , φ1 )−1+  Y1 (θ1 , φ1 )C=..
.
YNN (θ1 , φ1 )Y00 (θ2 , φ2 )Y1−1 (θ2 , φ2 )..
.

······..
.

YNN (θ2 , φ2 )···

+Y00 (θK , φK )Y1−1 (θK , φK ) ..
.

(1)

YNN (θK , φK )70

condition number [dB]z=1.26 [m]60504030201001234567891011decoding order

The relationship between condition number of Eq. (1)and HOA decoding order

System latency measurement process using TSP signal

4. Acoustics of the reverberation environment
HOA reproduction requires free ﬁeld condition. However, the measured mid-frequency reverberation
time RT30 of the reproduction room is about 0.2 s. Therefore, additional sound absorption material wasinstalled and the reverberation time was reduced a point where it could not be measured. Moreover, byadapting a measurement method that is normally used for qualifying anechoic rooms, we found thereproduction room was nearly free ﬁeld within a 1 m radius from the sweet spot [5].
[5] D. Cabrera et. al 2010 (that will be presented at the International Symposium on Room Acoustics(ISRA) in Melbourne on Aug. 2010)

Acknowledgements: This study was partly supported by the GCOE program (CERIES) of the Graduate School of Engineering, Tohoku Universityand Grant-in-Aid of JSPS for Specially Promoted Research (19001004).