No abstract

The use of conventional metrics to quantify the perception of nonlinearly propagated noise has been studied. Gaussian noise waveforms have been numerically propagated both linearly and nonlinearly, and from the resulting waveforms, several metrics are calculated. These metrics are overall, A-, C-, and D-weighted sound pressure levels, perceived noise level, Stevens Mark VII perceived loudness, Zwicker loudness, and sharpness. Informal listening demonstrations indicate that perceived differences in annoyance between linearly and nonlinearly propagated waveforms are substantial. Because the metrics studied seem inadequate in representing the perceived differences, rigorous subjective testing is encouraged to properly quantify and understand these differences.

A distinctive feature of many propagating, high-amplitude jet noise waveforms is the presence of acoustic shocks. Metrics indicative of shock presence, specifically the skewness of the time derivative of the waveform, the average steepening factor, and a new wavelet-based metric called the shock energy fraction (SEF), are used to quantify the strength and prevalence of acoustic shocks within waveforms recorded 10-305 m from a tethered military aircraft. The derivative skewness is more sensitive to the presence of the largest and steepest shocks, while the ASF and SEF tend to emphasize aggregate behavior of the entire waveform. These metrics are applied at engine conditions ranging from 50% to 150% engine thrust request, over a wide range of angles and distances, to assess the growth and decay of shock waves. The responses of these metrics point to significant shock formation occurring through nonlinear propagation out to 76 m from the microphone array reference position. Although these strongest shocks decay, the metrics point to continued nonlinear propagation in the far-field, out to 305 m. Many of these features are accurately characterized using a nonlinear propagation scheme based on the Burgers equation, but this scheme fails to account for multipath interference and significant atmospheric effects over the long propagation distances, resulting in an overestimation of nonlinearity metrics.

The prediction of acoustic scattering from hard surfaces such as the fuselage or deck near an exhausting jet aircraft or rotorcraft is important to attempts at predicting the spatial effects of various engine and nozzle configurations. A time-domain equivalent source method similar to Lee et al.'s method is presented for predicting acoustic scattering using pressure gradient predictions obtained from the G1A formulation. In contrast to Lee's method, the approach proposed here involves using coincident source and control points, leading to improved performance for some geometries. The greatest advantage appears to be for planar surfaces, where interaction between panels is avoided. Validation cases considered include a series of planar segments for analytical validation as well as cases to validate the use of the G1A inputs in connection with the proposed method, and a series of scattering cases using input from an acoustic data surface.

Attempts to reduce the noise from high-performance military aircraft requires an understanding of the different jet noise generation mechanisms. The primary noise sources originate from interactions between turbulent mixing noise associated with large and finescale turbulent structures and the ambient air. A nonideally expanded jet also contains broadband shock-associated noise. A three-way decomposition of the spectral density measured near a tied-down F-35B quantifies the contribution from each type of noise. The decomposition is performed on noise from a ground-based, linear array of microphones, approximately 8 m from the estimated shear layer, which spanned an angular aperture of 35° to 152° (relative to engine inlet). This large spatial aperture allows for a detailed investigation into the spatial variation in broadband shock-associated noise and fine and large-scale turbulent mixing noise. The spectral decompositions match the measured spectral levels with three main exceptions: 1) the F-35B noise contains multiple spectral peaks in the maximum radiation region, 2) nonlinear propagation increases the high-frequency spectral levels, and 3) the low-frequency levels in the maximum radiation region are less than those predicted by the large-scale similarity spectrum. The main peak of the F-35B broadband shock-associated noise, evident from 35°-70°, has the same characteristic shape and variation in peak frequency as overexpanded, laboratory-scale jets. The F-35B broadband shockassociated noise peak level and width exhibit different trends than laboratory-scale BBSAN and those recently reported for the F/A-18E [