Selected Publications
Crackle perception in supersonic jet noise is attributed to the presence of acoustic shocks in the waveform. This study uses an event-based beamforming method to track shock events as they propagate from the near to the far field of a high-performance military aircraft operating at afterburner. Near-field events are propagated via a nonlinear model and compared with far-field measurements. Comparisons of overall sound pressure level and spectra validate the use of the nonlinear model. The skewness of the time-derivative pressure waveform, or derivative skewness, a metric indicative of jet crackle perception, is greatly overpredicted for nonlinearly propagated waveforms. Cross-correlation coefficients of waveform segments centered about the near-field beamformed events reveal that for farther aft angles, near-field events are more related to far-field measurements. Waveform observations show that shock-like events in the near field that are more spiked in nature tend not propagate into the far field. However, near-field, large-derivative events with broader, high-pressure peaks nonlinearly steepen and form shocks in the far field that are likely responsible for crackle perception.
This study investigates source-related noise characteristics of the Falcon 9, a modern launch vehicle with a high operational tempo. Empirical prediction of the noise characteristics of launched rockets has long been a topic of study; however, there are relatively few comparisons with high-fidelity, far-field data, and historical inconsistencies persist. Various quantities are considered: overall directivity, overall sound power, maximum overall sound pressure level (OASPL), and peak frequency. The noise directivity of the Falcon 9 vehicle is shown to be between two disparate ranges given in the historical literature, but the observed peak directivity angle is well represented using convective Mach number concepts. A comparison between mechanical and acoustic power yields a radiation efficiency is consistent with the literature. Two independent methods of predicting maximum OASPL produce results accurate within 2 dB, even at distances of several kilometers. Various scaling parameters are calculated for observed spectral peak frequency and connect these measurements with prior observations. Finally, the impact of terrain shielding on levels and spectra is assessed. These determined source characteristics of the Falcon 9 vehicle provide a connection to prior launch vehicle acoustics studies, which helps identify useful models and methods for understanding rocket 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.
Outdoor acoustic data often include non-acoustic pressures caused by atmospheric turbulence, particularly below a few hundred Hz in frequency, even when using microphone windscreens. This paper describes a method for automatic wind-noise classification and reduction in spectral data without requiring measured wind speeds. The method finds individual frequency bands matching the characteristic decreasing spectral slope of wind noise. Uncontaminated data from several short-timescale spectra can be used to obtain a decontaminated long-timescale spectrum. This method is validated with field-test data and can be applied to large datasets to efficiently find and reduce the negative impact of wind noise contamination.
Atmospheric acoustic waves from volcanoes at infrasonic frequencies (0.01–20 Hz) can be used to estimate source parameters for hazard modeling, but signals are often distorted by wavefield interactions with topography, even at local recording distances (<15 km). We present new developments toward a simple empirical approach to estimate attenuation by topographic diffraction at reduced computational cost. We investigate the applicability of a thin screen diffraction relationship developed by Maekawa [1968, doi: https://doi.org/10.1016/0003-682X(68)90020- 0]. We use a 2D axisymmetric finite-difference method to show that this relationship accurately predicts power losses for infrasound diffraction over an idealized kilometer-scale screen; thus validating the scaling to infrasonic wavelengths. However, the Maekawa relationship overestimates attenuation for realistic volcano topography (using Sakurajima Volcano as an example). The attenuating effect of diffraction may be counteracted by constructive interference of multiple reflections along concave volcano slopes. We conclude that the Maekawa relationship is insufficient as formulated for volcano infrasound, and suggest modifications that may improve the prediction capability.
Jones et al. [J. Acoust. Soc. Am. 146, 2912 (2019)] compared an elevated (1.5 m) acoustical measurement configuration that used a standard commercial windscreen for outdoor measurements with a ground-based configuration with a custom windscreen. That study showed that the ground-based measurement method yielded superior wind noise rejection, presumably due to the larger windscreen and lower wind speeds experienced near the ground. This study further examines those findings by attempting to decouple the effects of windscreens and microphone elevation using measurements at 1.5 m and near the ground with and without windscreens. Simultaneous wind speed measurements at 1.5 m and near the ground were also made for correlation purposes. Results show that the insertion of the custom windscreen reduces wind noise more than placing the microphone near the ground, and that the ground-based setup is again preferable for obtaining broadband outdoor acoustic measurements.