ANU College of

Engineering & Computer Science

Recording and Reproducing Large Sound FieldsBy Prasanga Samarasinghe*, ANU CECS ASP Group, prasanga.samarasinghe@anu.edu.au. Supervisors: Thushara Abhayapala†, Mark Poletti‡.

1 Introduction

Recording and reproduction of spatial sound fields over a large area is an

unresolved problem in acoustic signal processing.

As the frequency increases and as the region of interest becomes large the

number of microphones/loudspeakers needed in effective

recording/reproduction increases beyond practicality.

Our intention is to minimise these numbers with the use of distributed

arrays of higher order microphone/loudspeaker units.

2 Practical Applications

1.1 Cancelling the noise pollution in mining towns

1.2 Creating virtual realities (Ex. Experience a real sporting encounter

in your living room)

3 Theory

A 2D or 3D sound field in space may be described with respect to a defined

origin based on several representations and the commonly used ones are,

Several approaches to the problem of accurately

recording/reproducing spatial sound fields are,

Wave Field Synthesis approach based on the Kirchhoff-Helmholtz integral

Inverse method

Ambisonics approach

Spherical arrays of omnidirectional microphones/loudspeakers

All of the above approaches fail when applied in large sound fields due to

the inherent restriction in recording/reproducing higher order harmonic

components.

Our approach ….

It is based on effectively capturing/recreating the sound field coefficients in

the harmonic representation with the use of realizable higher order

microphone/loudspeaker units.

4 Solution

4.1 Work done so far

A successful 2D recording system has been theoretically formulated using

higher order microphones.

The system is capable of accurately recording interior sound fields

It’s ability of successfully recording exterior sound fields beyond source

locations has also been demonstrated.

A similar design for the reproduction of large 2D sound fields with the

use of higher order loudspeaker units has been carried out by Mark

Poletti and Thushara Abhayapala.

4.2 Future work

Extend current findings in to the three dimensional space in order to

facilitate any natural field.

Work on more realizable approaches to design higher order microphone

and loudspeaker units.

6 References[1] Poletti, Mark and Abhayapala, Thushara D, ‟Spatial sound reproduction systems using higher

order loudspeakers”, in IEEE International Conference on Acoustics, Speech and Signal

Processing (ICASSP), 2011, pp. 57 – 60.

[2] Samarasinghe, Prasanga N, Abhayapala, Thushara D and Poletti, Mark, (forthcoming 2011)

‟Spatial sound field recording over a large area using distributed higher order microphones” in

IEEE Workshop on Applications of Signal Processing to Audio and Acoustics (WASPAA)

[3] Poletti, Mark, ‟Three-dimensional surround sound systems based on spherical harmonics”, J.

Audio Engin. Soc., 53(11), pp 1004-1025, November 2005

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* Advanced Signal Processing Group, College of Engineering & Computer Science, Australian National University

† Research School of Engineering, College of Engineering & Computer Science, Australian National University

‡ Industrial Research Limited, PO Box 31-310, Lower Hutt, New Zealand

Can we ever get rid of the

mass noise pollution

happening in mining

communities? Yes

The existing sound field

over the entire mine can be

recorded and a new

inverting sound field can be

produced in order to cancel

it out.

Do you hear the true sounds

from conventional stereo

systems or even the

surround sound systems ?No you are made to believe the

sound is generated at the speaker

positions and recordings are static

We are trying to recreate

that original sound exactly

as it is, enabling you to

listen to anywhere in the

field you wish to, even a

conversation between two

umpires in a cricket match.Figure 2: A Cricket field

Figure 1: A Mining Neighbourhood

The Kirchhoff-Helmholtz integral

A Taylor series expansion

Cylindrical and Spherical

harmonics representation,

Ex. The pressure at a 2D interior sound

field can be represented as

Unknown Known

Figure 3: Sound field representation

Figure 6: Exterior field : Recorded sound field for M=1 and the existing sound field of f=575Hz

Figure 4: Interior field : Recorded sound field for M=1 and the existing sound field of f=140Hz

Figure 5: Interior field : Recorded sound field for M=3 and the existing sound field of f=340Hz

Figure 7: Exterior field : Recorded sound field for M=3 and the existing sound field of f=1.37kHz

A higher order

microphone of order ‘M’

A higher order

loudspeaker of order ‘M’

Extracts sound field

coefficients up to order ‘M’

Reproduces sound field

coefficients up to order ‘M’