Exponential ground impedance models and their interpretation

The authors compare the results of Donato's exponentially varying ground model, Attenborough's exponentially varying ground model and the rigid backed thin layer model. They show that these models produce similar results for slow variations. For rapid variations the results are quite different, but the basic theory used is only correct for the thin layer model. These results suggest that the exponentially varying models are not necessary for fitting ground impedance data

Steady state risetimes of shock waves in the atmosphere

A square wave shape is used in the Pestorius algorithm to calculate the risetime of a step shock in the atmosphere. These results agree closely with steady shock calculations. The healing distance of perturbed shocks due to finite wave effects is then investigated for quasi-steady shocks. Perturbed 100 Pa shocks require on the order of 1.0 km travel distance to return to within 10 percent of their steady shock risetime. For 30 Pa shocks, the minimum recovery distance increases to 3.0 km. It is unlikely that finite wave effects can remove the longer risetimes and irregular features introduced into the sonic boom by turbulent scattering in the planetary boundary layer

Atmospheric effects on the risetime and waveshape of sonic booms

Accurate prediction of human response to sonic booms from proposed HSCT aircraft depends on a knowledge of the waveshape and risetime of the boom at the ground. In previous work, we have developed a numerical technique to predict the combined effects of molecular absorption and finite wave distortion on the sonic boom as it propagates from the aircraft to the top of the turbulent boundary layer. We have more recently developed a scattering center based model to calculate the effects of turbulence on the sonic boom waveform as it propagates through this boundary layer. Calculations have been performed using single scales of turbulence and compared to measurements at Edwards AFB in the late 1960's. A model of the atmosphere involving two scales each for convective and mechanical turbulence has been developed and fit to meteorological data collected during JAPE 2. Scattering calculations employing this model underpredict the number of unperturbed waveforms. In order to develop a more realistic model of the atmosphere, the JAPE 2 meteorological data has been fit to a von Karman spectrum. Results of scattering using this multi-scale model will be presented. The combination of finite wave effects with turbulent scattering predictions includes the principal effects of the atmosphere on the sonic boom from the HSCT

Scattering of sound by atmospheric turbulence predictions in a refractive shadow zone

According to ray theory, regions exist in an upward refracting atmosphere where no sound should be present. Experiments show, however, that appreciable sound levels penetrate these so-called shadow zones. Two mechanisms contribute to sound in the shadow zone: diffraction and turbulent scattering of sound. Diffractive effects can be pronounced at lower frequencies but are small at high frequencies. In the short wavelength limit, then, scattering due to turbulence should be the predominant mechanism involved in producing the sound levels measured in shadow zones. No existing analytical method includes turbulence effects in the prediction of sound pressure levels in upward refractive shadow zones. In order to obtain quantitative average sound pressure level predictions, a numerical simulation of the effect of atmospheric turbulence on sound propagation is performed. The simulation is based on scattering from randomly distributed scattering centers ('turbules'). Sound pressure levels are computed for many realizations of a turbulent atmosphere. Predictions from the numerical simulation are compared with existing theories and experimental data

Acoustic end corrections for micro-perforated plates

© 2019 Acoustical Society of America. Micro-perforated plates (MPPs) are acoustically important elements in micro-electro-mechanical systems (MEMS). In this work an analytical solution for perforated plates is combined with finite element method (FEM) to develop formulas for the reactive and resistive end effects of the perforations on the plate. The reactive end effect is found to depend on the hole radius and porosity. The resistive end effect is found to depend on hole radius only. FEM is also used to develop an understanding of the loss mechanism that corresponds to the resistive end effects. The developed models can be used in optimization studies of the MEMS and MPPs

Comparison of measured and predicted pure tone propagation levels from JAPE-1: An evaluation of the performance of ASOPRAT

Joint Acoustic Propagation Experiment Phase One (JAPE-1 ) short range propagation data has been used to evaluate the performance of the Advanced Sound Propagation in the Atmosphere (ASOPRAT) prediction code. The pure tone short range data was Fourier analyzed giving the propagated pressure levels as a function of frequency. Meteorological profiles measured at the experimental site were used as input for the acoustic prediction routine ASOPRAT. Predicted and measured propagation levels are compared in decibels (dB) relative to one of the measurement positions for receivers on the line passing between the two thirty meter towers. Agreement between predicted and measured levels is very good. Source strength data was not available, hence the comparisons show good agreement as to the shape of the propagation loss curve not necessarily the propagation levels

Statistical and numerical study of the relation between weather and sonic boom characteristics

NASA measured sonic boom characteristics near Edwards Air Force Base from 11/66 to 1/67. Thirty four flights by an F-104 were recorded at an altitude of about 31,000 feet and flying speed of Mach 1.3. Forty two microphones were placed on the ground directly under the fight track. Each microphone recorded boom shape, rise time, peak overpressure, total boom duration, positive duration, and positive impulse

Corrected Tilt Calculation for Atmospheric Pressure-Induced Seismic Noise

In a literature search on the coupling of wind-generated pressure fluctuations into seismic noise, it was noticed that the expression for the angular tilt induced by pressure fluctuations in the seminal paper “A preliminary investigation into the relationship between long-period seismic noise and local fluctuations in the atmospheric pressure” by G. G. Sorrells was only valid at the surface. A search of the literature which cites the Sorrells paper was performed to see if any subsequent research corrected this error, and what effect the error might have on the research. A recent paper by Tanimoto and Wang notes the correct expression for the tilt, but employs the simpler erroneous expression in the research. In this paper, we develop the correct expression for effective measured displacements and approximate expressions analogous to those of Sorrells. The resulting magnitudes and decay with depth are then displayed and compared to Sorrells. Next, the results of the literature search are discussed. The results of three papers are identified as potentially modified by the correction of the tilt calculation. Finally, it is noted that the majority of the papers referenced are not affected by the correction, since many of the papers are for near-surface displacements of very low frequencies

Acoustic frequency response method for the measurement of fast adsorption – Diffusion processes. Theoretical treatment

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Corrected Tilt Calculation for Atmospheric Pressure-Induced Seismic Noise

In a literature search on the coupling of wind-generated pressure fluctuations into seismic noise, it was noticed that the expression for the angular tilt induced by pressure fluctuations in the seminal paper “A preliminary investigation into the relationship between long-period seismic noise and local fluctuations in the atmospheric pressure” by G. G. Sorrells was only valid at the surface. A search of the literature which cites the Sorrells paper was performed to see if any subsequent research corrected this error, and what effect the error might have on the research. A recent paper by Tanimoto and Wang notes the correct expression for the tilt, but employs the simpler erroneous expression in the research. In this paper, we develop the correct expression for effective measured displacements and approximate expressions analogous to those of Sorrells. The resulting magnitudes and decay with depth are then displayed and compared to Sorrells. Next, the results of the literature search are discussed. The results of three papers are identified as potentially modified by the correction of the tilt calculation. Finally, it is noted that the majority of the papers referenced are not affected by the correction, since many of the papers are for near-surface displacements of very low frequencies