Selected Publications
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.
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.
This letter describes how a landmark 1960s supersonic jet noise experiment influenced subsequent noise models. A discrepancy in other researchers' application of Potter and Jones's axial decomposition of the sound power generated from a laboratory-scale jet can be traced to an erroneous plot in the original report. Whereas most jet noise research indicates the dominant sound power is generated upstream of the supersonic core tip, propagation of this error in the ubiquitous NASA SP-8072 report has caused rocket noise modelers for five decades to disproportionately allocate sound power generation to the subsonic flow.
Multiple International and Federal regulations stipulate the acquisition of aircraft noise be conducted using inverted pressure microphones over a round ground board. Ground boards are used to provide an acoustically hard reflecting surface, limiting the effects of the potentially absorptive local ground. To determine the ground board effects on the measured acoustic signal, a measurement campaign was undertaken at NASA Langley Research Center. The experiments included multiple ground board configurations placed on top of a sand pit, in an otherwise anechoic chamber. Ground board configurations included a microphone inverted and offset over a ground board and a microphone offset and flush mounted in the ground board. White noise was used to investigate the ground board effects on the recorded signal. Normal impedance measurements were acquired to determine the reflection coefficients of the sand and ground boards. Results indicate that both microphone configurations perform adequately up to 10 kHz. When the ground substrate is a soft material and the sound comes in at angles near grazing incidence, the signal is attenuated above 1 kHz. Additionally, the plastic material used to construct the ground boards was found to be acoustically hard between 0.1 – 3.0 kHz, and likely extending to higher frequencies.
This paper describes noise measurements taken of the new Boeing T-7A Red Hawk trainer aircraft, which uses a single F404 afterburning turbofan engine. The extensive measurement satisfies the American National Standards Institute/Acoustical Society of America standard S12.75-2012 for ground run-up for future environmental impact assessment and includes additional locations around the aircraft to understand exposure by maintenance personnel. A large near-field array was also deployed to shed light on phenomena that are not generally seen in the measurement of laboratory-scale jets, such as the presence of spatiospectral lobes. Initial data analysis shows they are of high fidelity and contain similar phenomena as other recent high-performance jet aircraft noise measurements, including evidence of large and fine-scale noise radiation, broadband shock-associated noise, spatiospectral lobing at multiple engine powers, an upstream shifting of overall level directivity with engine power, and appreciable shock content in the measured waveforms. Further analysis of this dataset will add to the understanding of full-scale, high-speed jet noise and allow comparisons to similar numerical simulations and laboratory-scale measurements.