High-resolution continuous scan acoustical holography applied to high-speed jet noise

Acoustical holography is a technique that has been used extensively to characterize the surface velocities and acoustic pressures of coherently vibrating structures such as engines and gearboxes. For aeroacoustic noise sources such as jets with multiple distributed, partially correlated source mechanisms, scan-based techniques using reference and response transducers and singular value decomposition (SVD) have been applied to acoustical holography to decompose these noise sources into partial fields. Partial fields can reconstruct an overall sound field and also provide a near-field representation of the source that may aid in understanding the physics of jet noise. This paper presents model-scale experimental evidence that advanced signal processing techniques can enable the generation of high resolution acoustical holograms of the hydrodynamic and acoustic near-fields of high-speed jets with a reasonable number of microphones. Two key innovations discussed in the paper are: (1) the construction of partial fields using continuously moving response (i.e., "hologram") microphones and spatially fixed reference microphones with Chebyshev-spaced sampling points to achieve sufficient averaging, and (2) the computation of transfer functions using a method of canonical coherences. This method uses a tensorial formulation of coherence analysis to filter off all signal components at or below the noise floor. The current experimental evidence is to be used in the development of a full-scale acoustical holography system to be demonstrated on a military jet aircraft.