S. Hales Swift

On the potential limitations of conventional sound metrics in quantifying perception of nonlinearly propagated noise

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.

An Efficient Time-Domain Equivalent Source Method for Acoustic Scattering

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 Lees 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.

A time-domain equivalent source method for acoustic scattering with coincident source and control points

T he ability to predict acoustic scattering from hard surface reflections in the vicinity of an exhausting jet is important to attempts at predicting the spatial effects of various engine and nozzle configurations. A time-domain equivalent source method is presented similar to Lee’s method for predicting acoustic scattering due to hard surface reflections using acoustic gradient predictions from the G1A formulation. In contrast to Lee’s method, the approach proposed here involves using source and control points that are coincident, leading to improvements in performance for some geometries. The most advantageous case appears to be for planar surfaces, where interaction between sources 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 our proposed method, and a series of scattering cases using an acoustic data surface as input to the model.

Examining the use of a time-varying loudness algorithm for quantifying characteristics of nonlinearly propagated noise (L)

A previous letter by Gee et al. [J. Acoust. Soc. Am. 121, EL1-EL7 (2007)] revealed likely shortcomings in using common, stationary (long-term) spectrum-based measures to quantify the perception of nonlinearly propagated noise. Here, the Glasberg and Moore [J. Audio Eng. Soc. 50, 331-342 (2002)] algorithm for time-varying loudness is investigated. Their short-term loudness, when applied to a shock-containing broadband signal and a phase-randomized signal with equivalent long-term spectrum, does not show a significant difference in loudness between the signals. Further analysis and discussion focus on the possible utility of the instantaneous loudness and the need for additional investigation in this area.

Acoustic Shock Formation in Noise Propagation During Ground Run-Up Operations of Military Aircraft

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.

Development and testing of a time-domain equivalent source method for acoustic scattering

T he prediction of acoustic scattering from hard surfaces such as the fuselage or deck near an exhausting jet aircraft or rotocraft is important to attempts at predicting the spatial effects of various engine and nozzle configurations. A time-domain equivalent source method has been recently introduced by our group for predicting acoustic scattering using pressure gradient predictions obtained from an integral acoustic formulation (G1A). The approach discussed here involves using coincident source and control points, leading to improved performance for some geometries. In this paper the performance of the method is evaluated further as applied to three groups of sphere cases, as well as several series of corner cases. These are compared with analytic solutions for an acoustic source outside a sphere and an acoustic source near a corner formed by two semi-infinite planes meeting at a right angle. Parameters relevant to method performance are evaluated including the number of panels per period, samples per period, and certain stability related parameters. Possible avenues of improvement are discussed, such as improvements to the boundary condition representation.

Inclusion of Broadband Shock-Associated Noise in Spectral Decomposition of Noise from High-performance Military Aircraft

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 [Tam et al., Journal of Sound and Vibration, Vol. 422, 2018, pp. 92-111]. The strengths and limitations of current models to represent the spatial variation in the spectral content of F-35B noise can guide research efforts to more fully understand the sound radiation from high-performance military aircraft.

Extending sharpness calculation for an alternative loudness metric input

Sound quality metrics help improve the psychoacoustic acceptability of devices and environments by modeling and thus enabling deliberate improvement of perceptual attributes. Sharpness as defined in DIN 45692 [(2009). Deutsches Institut für Normung, Berlin] requires inputs from Zwickers loudness metric [ISO 532-1 (2017). International Organization for Standardization, Geneva]. This letter demonstrates that sharpness can be formulated to accept specific loudness values from Moore and Glasbergs loudness metric [ISO 532-2 (2017). International Organization for Standardization, Geneva; ANSI S3.4 (2007). American National Standards Institute, Inc., Washington, DC]. Sharpness calculations using the two loudness metrics produce similar results. This method thus enables evaluation of sharpness as a straightforward add-on to standard loudness calculations using Moore and Glasbergs metric, for which sharpness calculations were not previously available.

Spatiotemporal analysis of high-performance military aircraft noise during ground run-up

Recent measurements of high-performance military aircraft noise have revealed that full-scale jet noise has features and structures that are still only partly understood, such as the presence of multiple acoustic radiation lobes in the aft direction at certain frequencies. Spatiotemporal analyses of a ground-based microphone array measurement of the noise from a tethered F-35 at various engine conditions are used to investigate these features of the sound field. The ground array covered an angular aperture of 35–152 degrees relative to the front of the aircraft. The large angular aperture allows for a detailed investigation of the correlation and coherence at frequencies exhibiting multi-lobe behavior. This spatiotemporal analysis yields further evidence of the characteristics of multi-lobe behavior in high-performance, full-scale jet noise. [Work supported by an Office of Naval Research grant, a USAFRL SBIR, and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07...

Three transformations of a crackling jet noise waveform and their potential implications for quantifying the “crackle” percept

In the 1975 paper by Ffowcs-Williams et al. on jet “crackle,” there are several potentially competing descriptors—including a qualitative description of the sound quality or percept, a statistical measure, and commentary on the relation of the presence of shocks to the sound’s quality. These descriptors have led to disparate conclusions about what constitutes a crackling jet, waveform, or sound quality. This presentation considers three modifications of a jet noise waveform that exhibits a crackling sound quality and initially satisfies all three definitions. These modifications alter the statistical distributions of primarily the pressure waveform or its first time difference in order to demonstrate how these modifications do or do not correspond to changes in the sound quality of the waveform. The result, although preliminary, demonstrates that the crackle percept is tied to the statistics of the pressure difference waveform instead of the pressure waveform itself.