Near field acoustic holography with microphones mounted on a rigid sphere

Spherical near field acoustic holography (spherical NAH) is a technique that makes it pos-sible to reconstruct the sound field inside and just outside an acoustically transparent spherical surface on which the sound pressure is measured with an array of microphones with negligible scattering. This is potentially very useful for source identification.

On the other hand a rigid sphere is somewhat more practical than an open sphere, and it is possible to modify the existing spherical NAH theory so that a similar sound field reconstruction can be made with an array of microphones flush-mounted on a rigid sphere. Rigid spheres with flush-mounted microphones are also used for beamforming, and it is known that they are advantageous compared with open spheres for this application.

However, whereas beamforming is a far field technique NAH is a near field technique, and spherical NAH based on a rigid sphere is only valid if it can be assumed that the sphere has a negligible in-fluence on the incident sound field, and this is not necessarily a good assumption when the sphere is very close to a radiating surface.

This paper describes the modified spherical NAH theory and examines the matter through simulations and experiments.