Acoustical Holography-Based Analysis of Spatiospectral Lobes in High-Performance Aircraft Jet Noise

Holographic reconstructions of the sound field in the vicinity of a tied-down F-35 aircraft were achieved by applying multisource statistically optimized near-field acoustical holography to measurements taken at a linear ground array approximately parallel to the shear layer of the jet. The measured field, as well as reconstructions to locations where the field was not measured, show that the field can be described as a superposition of multiple lobes in the spatiospectral domain. These lobes are observed in the field when the aircraft is operated at a variety of engine powers, including afterburner. For a given engine power, as frequency is increased, the spatial lobes in the mixing-noise region shift aft in directivity until they disappear beyond the aperture of the measurement while new ones appear toward the sideline and shift aft with the others. At a fixed frequency, when engine power is increased, the forward-most spatial lobe increases in level more than the other lobes, which is a major factor in the observed forward shift in overall directivity with increasing engine power. Frequency-dependent raytracing of the spatial lobes gives insight into the directivity and apparent source locations for jet noise components as a function of frequency and engine power.