Sound Field Visualization using Spherical Holography

Beamforming based on measurements with a spherical microphone array is a recognized technique for localization of noise sources. The method is very efficient for getting a quick overview of the positions of the most dominant sources. However, when more detailed information about the radiation from a source is needed, acoustical holography is normally more suitable, because it provides the actual acoustical quantities (pressure, velocity and/or intensity) at positions on or near the source surface, whereas beamforming only gives a directional map of contribution at the array position. Even though spherical arrays are most often used for beamforming they can also be applied for holography purposes. This means that with the same hardware beamforming can be used to identify a noise source, and then the array can be moved closer to the source to get a clearer picture of the radiation by using holography. Also, beamforming has a poor directional resolution at low frequencies, whereas holography performs best at low-to-mid frequencies, which means that the two methods supplement each other well in the frequency domain. This paper presents some results (simulations and real measurements) based on holography and beamforming using a rigid sphere with flush mounted microphones.