A generalized cumulative noise exposure metric for community response analysis.

In community noise studies, there is often a desire to understand how the annoyance response to multiple noise events aggregates over a long period of time. Many cumulative response metrics, such as day-night level (DNL), are based on the idea that humans respond, on average, to the sum of frequency-weighted acoustic energy over time. This paper introduces a generalization of DNL that includes a parameter, b, that ranges between zero and one. When b equals zero, the metric returns the maximum level of the events. When b equals 0.5, the metric reproduces the equal-energy-based output of DNL. When b = 0, 0.5, and 1, the metric returns a value that more harshly penalizes the number of events. In this way, these common possible hypotheses are organized onto a single scale, one that may be used to craft effective noise mitigation techniques or implement regulations. The analysis is demonstrated in two ways: first, on synthetic datasets to show the utility and consistency of the metric, and second, on limited quiet-supersonic response data gathered during the Quiet Supersonic Flights 2018 community study.

Sounds to Astound: An acoustics outreach show.

Sounds to Astound is an acoustics demonstration show, produced for the community twice yearly by the Brigham Young University Student Chapter of the Acoustical Society of America. The free, interactive demonstration show explores the science of sound for a target audience of fifth- to eighth-grade students. Introductory acoustics concepts, such as longitudinal wave motion, wave properties, propagation effects, and standing waves, are taught with live demonstrations, animations, and videos. The goal of this paper is to inspire and encourage readers in their outreach efforts by describing the purposes of Sounds to Astound and technical details of several entertaining and educational demonstrations. Lessons learned from a decade of these student-produced shows serve as an aid for future efforts and highlight the benefits of outreach efforts, particularly for the students involved.

Evidence for nonlinear reflections in shock-containing noise near high-performance military aircraft.

Skewness values for the pressure time derivative are greater at ground-based measurements near a tactical aircraft than they are at nearby off-ground locations. A possible explanation for this phenomenon is the occurrence of nonlinear, irregular shock reflections at the ground. Propagation angle, source location, and corresponding angle of incidence relative to the ground are estimated using a two-point cross correlation of windowed shock events. Nonlinear reflections are likely to occur based on the combination of angles of incidence and measured shock strengths and cause a pressure increase at the shock that is greater than twice the free-field pressure. The associated pressure increase at the shocks appears to enhance shock-related metrics at the ground compared to off-ground locations.

Data-educed broadband equivalent acoustic source model for supersonic jet noise.

A broadband equivalent acoustic source distribution can be used to model the sound field near a high-speed jet. Such models must account for the spatiospectral variation of the sound levels. This work presents a technique for obtaining such a model using a spectral decomposition method associated with large and fine-scale turbulent mixing noise to create broadband equivalent source distributions for each noise type. The large-scale turbulent mixing noise is represented by frequency-dependent wavepackets, while the fine-scale turbulent mixing noise is modeled as a frequency-dependent incoherent, extended source distribution. This technique is applied to acoustical measurements from an ideally expanded, unheated Mach 1.8 jet. The wavepackets model the sound field levels in the maximum radiation region, but the second incoherent source distribution is required to obtain the levels at the other locations. The combination of the incoherent source distribution and the wavepacket provides a broadband, equivalent acoustic source representation that adequately models the sound field for Strouhal numbers between 0.04 and 0.25. At higher Strouhal numbers, better agreement is obtained when accounting for a frequency-dependent shift in the apparent acoustic source region. This frequency-dependent source region is more important closer to the jet than in the far field.

Broadband shock-associated noise from a high-performance military aircraft.

Broadband shock-associated noise (BBSAN) is a prominent noise component from nonideally expanded jets in the forward and sideline directions. BBSAN from laboratory-scale jets has been studied extensively, and spatial trends in BBSAN spectral peak characteristics-frequency, level, and width-have been established. These laboratory-scale trends are compared to those for BBSAN from a tied-down F-35B operated at four engine conditions. While the peak frequency varies as expected, both spatially and across engine condition, the peak level and width do not, pointing to the need for additional research into BBSAN for high-performance military aircraft.

Subjective rating of the jet noise crackle percept.

Results of the first formal perceptual study of jet crackle are presented. Prior studies examined noise waveform properties believed to be linked to the jet crackle percept from a physics perspective or using signal processing and informal subjective evaluation. This investigation involves 31 listeners that rated 15 jet noise waveforms with a category subdivision scaling test. Results reveal a strong log-linear correlation between the pressure waveform time derivative's skewness and crackle rating. A regression analysis establishes practical derivative skewness bounds for a five-point categorical crackle scale and results in the suggested definition of the crepit as the unit of crackliness.