Cylindrical acoustical holography applied to full-scale jet noise

Abstract

Near-field acoustical holography methods are used to predict sound radiation from an engine installed on a high-performance military fighter aircraft. Cylindrical holography techniques are an efficient approach to measure the large and complex sound fields produced by full-scale jets. It is shown that a ground-based, one-dimensional array of microphones can be used in conjunction with a cylindrical wave function field representation to provide a holographic reconstruction of the radiated sound field at low frequencies. In the current work, partial field decomposition methods and numerical extrapolation of data beyond the boundaries of the hologram aperture are required prior to holographic projection. Predicted jet noise source distributions and directionality are shown for four frequencies between 63 and 250 Hz. It is shown that the source distribution narrows and moves upstream, and that radiation directionality shifts toward the forward direction, with increasing frequency. A double-lobe feature of full-scale jet radiation is also demonstrated.

Intensity analysis of the dominant frequencies of military jet aircraft noise

“…Recently, discrete frequency peaks were discovered, 4,11 from what appear to be two mutually-incoherent sources which exhibit distinct directionalities. 13 As seen in Fig. 3, the spectrum shifts from a maximum at about 250 Hz at 120° to 125 Hz at 140°, without a smooth transition between these frequencies.…”

Multisource statistically optimized near-field acoustical holography

“…In general, M-SONAH could be used in NAH applications where the sound field is generated by multiple sources of interest, where an additional noise source of known location and shape interferes with the source of interest, or even where scattering off an object alters the sound field. Preliminary applications of M-SONAH to reconstruct the sound field of a high-performance military jet in the presence of a large reflecting surface have been reported by Wall (2013) and Wall et al (2013aWall et al ( , 2013b, and more detailed investigations are underway.…”

“…In the second experiment, a measurement was simulated far from the source plane of two parallel, finite line arrays. This simulation was motivated by past NAH investigations of a long, military jet noise source in the presence of a rigid ground reflection (Wall, 2013;Wall et al, 2013aWall et al, , 2013b. Here, M-SONAH, planar SONAH, and cylindrical SONAH (Cho et al, 2005) are all used to reconstruct the field to demonstrate the capabilities of M-SONAH when the hologram is far from the source.…”

The “sound Of Freedom”: Characterizing Jet Noise From High-Performance Military Aircraft

“…55,56 The measurement "holograms" are the individual planes of data in the F-22A OASPL maps in Figure 7, constructed from the rectangular array in Figure 6 and the stationary, ground-based linear "reference" array. By matching wave functions to the measured holograph pressures, a model of the field is generated, and the predicted pressures at any other location can be calculated.…”