Supersonic acoustic intensity with statistically optimized near-field acoustic holography

The concept of supersonic acoustic intensity was introduced some years ago for estimating the fraction of the flow of energy radiated by a source that propagates to the far field. It differs from the usual (active) intensity by excluding the near-field energy resulting from evanescent waves and circulating energy in the near-field of the source. This quantity is of concern because it makes it possible to identify the regions of a source that contribute to the far field radiation, which is often the ultimate concern in noise control. Therefore, this is a very useful analysis tool complementary to the information provided by the near-field acoustic holography technique. This study proposes a version of the supersonic acoustic intensity applied to statistically optimized near-field acoustic holography (SONAH). The theory, numerical results and an experimental study are presented. The possibility of using particle velocity measurements instead of conventional pressure measurements is examined. The study indicates that the calculation of the supersonic intensity based on measurement of the particle velocity is more accurate than the one based on sound pressure measurements. It also indicates that the method based on SONAH is somewhat more accurate than the existing methodology based on Fourier transform processing.