Gregory W. Lyons

Localization of Acoustic Sources in Shock-Containing Jet Flows Using Phased Array Measurements

In this paper, we shall examine the feasibility of using time-resolved hybrid RANS-LES (HRLES) simulation data to perform noise source localization studies on a hot jet from a conic nozzle operating at pressure matched conditions. The source localization will be performed using a traditional delay-and-sum beamforming technique. This technique requires time-resolved data on a phased array of microphones located in the acoustic fareld of the jet; this data will be obtained by coupling the HRLES simulation with a Ffowcs Williams and Hawkings equation noise prediction code. Using insights gained from experimental beamforming, we shall show that beamforming using CFD data is a feasible, and potentially less expensive and time-saving, alternative to constructing complicated phased array systems for performing these calculations on experimental data.

Acoustic source indicators using LES in a fully expanded and heated supersonic jet.

A Large Eddy Simulation of a fully expanded heated supersonic jet flow is examined using acoustic source indicators based on simplification to the Lighthill tensor. This is done in an effort to explore the utility of such methods in defining a rationalized indicator that would help guide future experiments using high-speed PIV, and the accuracy of this rational approach is determined by a detailed comparison with the full Lighthill source term. Finally a wave-number frequency analysis is performed along the mixing layer in order to identify areas of acoustic efficiency, and investigate the importance of the local convective speed of the large-scale structures.

A measurement system for the study of nonlinear propagation through arrays of scatterers

Various experimental challenges exist in measuring the spatial and temporal field of a nonlinear acoustic pulse propagating through an array of scatterers. Probe interference and undesirable high-frequency response plague typical approaches with acoustic microphones, which are also limited to resolving the pressure field at a single position. Measurements made with optical methods do not have such drawbacks, and schlieren measurements are particularly well suited to measuring both the spatial and temporal evolution of nonlinear pulse propagation in an array of scatterers. Herein, a measurement system is described based on a z-type schlieren setup, which is suitable for measuring axisymmetric phenomena and visualizing weak shock propagation. In order to reduce directivity and initiate nearly spherically-symmetric propagation, laser induced breakdown serves as the source for the nonlinear pulse. A key component of the schlieren system is a standard schliere, which allows quantitative schlieren measurements ...

Nondimensional analysis of wind noise and atmospheric surface-layer properties

Wind noise is a prominent limitation to the signal to noise ratio of acoustic sensors. Realistic expectations of signal detectability can be generated by predicting the noise floor prior to a sensor deployment; however, a relationship must be established between the wind noise measured by a sensor and atmospheric surface-layer properties. Under conditions of horizontal homogeneity and quasi-steadiness, Monin-Obukhov similarity theory relates friction velocity, temperature scale, and roughness length to the near-surface profiles of mean wind speed and turbulent intensity, which in turn are known to govern wind noise. It is expected that the ratio of one-third octave band root-mean-square sound pressure to the turbulent flux of momentum, Strouhal number, and dimensionless elevation have a nondimensional relationship that collapses wind noise data as a function of Monin-Obukhov parameters. In order to establish such a relationship, we analyze a dataset of wind noise recorded in Spring 2018 within the Army Research Laboratory’s Meteorological Sensing Array on the Jornada Experimental Range, New Mexico. This dataset consists of continuous recordings of ambient noise at several sites on audio microphones up to 20 m above ground level, co-located with a suite of high-fidelity meteorological instruments, including sonic anemometers.

Inferring atmospheric surface-layer properties from wind noise in the nocturnal boundary layer

Uncertainties in the state of the atmosphere, to a large extent, limit the prediction accuracy of outdoor sound propagation. In particular, event sound propagation requires accurate knowledge of wind speed and temperature profiles, spatially averaged over the path of propagation. In a stable quasi-steady nocturnal boundary layer, wind speed and temperature gradients follow a scaling that is asymptotically independent of altitude and depends on Monin-Obukhov similarity parameters. Since these parameters describe the near-surface profiles of wind speed and turbulent intensity, which in turn are known to govern wind noise, it is anticipated that a connection exists between Monin-Obukhov parameters and the statistics of wind noise. It is expected that these parameters may be inferred from wind noise sensed by screened microphones. Ambient noise data collected in the southwest U.S. are analyzed for the purpose of examining whether Monin-Obukov similarity parameters may be inferred from wind noise. We explore the consequences of establishing inferences with a priori distributions for the similarity parameters, and utilizing wind noise data from microphones at one or more altitudes.Uncertainties in the state of the atmosphere, to a large extent, limit the prediction accuracy of outdoor sound propagation. In particular, event sound propagation requires accurate knowledge of wind speed and temperature profiles, spatially averaged over the path of propagation. In a stable quasi-steady nocturnal boundary layer, wind speed and temperature gradients follow a scaling that is asymptotically independent of altitude and depends on Monin-Obukhov similarity parameters. Since these parameters describe the near-surface profiles of wind speed and turbulent intensity, which in turn are known to govern wind noise, it is anticipated that a connection exists between Monin-Obukhov parameters and the statistics of wind noise. It is expected that these parameters may be inferred from wind noise sensed by screened microphones. Ambient noise data collected in the southwest U.S. are analyzed for the purpose of examining whether Monin-Obukov similarity parameters may be inferred from wind noise. We explore t...

A maximum likelihood estimator for spectral models of acoustic noise processes

The power spectral density of time series from many acoustic phenomena can vary rapidly through several decades of magnitude, particularly for noise processes. This property can complicate evaluation of spectral density models with respect to a non-parametric estimate of the spectral density, also known as the periodogram. Typical measures, such as the sum of squares of the residuals, can suffer from bias errors and oversensitivity to large values. A log-likelihood function is here presented for a periodogram derived from a sequence of independent, circularly-symmetric complex normal Fourier components. The bias-corrected Akaike’s information criterion of an expected-value spectral model is shown to be a useful measure for multi-model selection. A maximum-likelihood estimator is formed for spectral model parameters with respect to a known periodogram, along with approximate confidence intervals for the parameter estimates. Results from the maximum-likelihood estimator are compared with weighted and unweig...

Calculations of low-frequency wind noise along a low two-dimensional hill surface

Measurement of outdoor sound propagation is often limited by wind noise, i.e. pressure fluctuations from atmospheric turbulence, especially for infrasound and low audible frequencies. Over a flat ground surface, the spectral density of wind noise can be predicted by the shear-turbulence mechanism for static pressure fluctuations using a mirror flow atmospheric turbulence model for the inhomogeneous surface-blocking effect. This study moves beyond a flat ground model to consider the effects of flow distortion by weak topography on surface wind noise, specifically, flow over a low two-dimensional hill, free from separation, at large Froude number. The integral solution for the pressure Poisson equation is used as a starting point, with turbulence modeled by the mirror flow in surface-following coordinates. The perturbation analysis of Hunt et al. [Q. J. R. Meteorol. Soc. 114(484), 1435–1470 (1988)] is used to model the mean shear rate as a function of elevation and distance over the hill. For upwind, downwi...

Weather focused challenges for continuous monitoring of military noise

Domestic military installations generate high levels of noise due to testing and training which leads to annoyance and complaints from surrounding communities. This necessitates continuous noise monitoring to provide decision makers with the information they need to proactively manage their noise environment. Due to the diverse climates in which military testing and training are conducted (e.g., desert, tundra, and rainforest), monitoring equipment that can operate in a variety of environmental conditions with minimal maintenance and low power consumption is needed. Using existing technologies as a baseline, various iterations of a low-cost acoustic monitor were designed to meet these constraints while minimizing initial investment cost, improving the mean time between failures, and increasing overall system capability. This paper will describe the system developed to provide a rapid deployment option that is robust to extreme temperatures, humidity, and destructive wildlife. A review of operational logs collected during multiple deployments was used to evaluate system performance against benchtop and off-the-shelf solutions. This data demonstrate the reliability of the monitoring stations and the sustainability of their hardware.Domestic military installations generate high levels of noise due to testing and training which leads to annoyance and complaints from surrounding communities. This necessitates continuous noise monitoring to provide decision makers with the information they need to proactively manage their noise environment. Due to the diverse climates in which military testing and training are conducted (e.g., desert, tundra, and rainforest), monitoring equipment that can operate in a variety of environmental conditions with minimal maintenance and low power consumption is needed. Using existing technologies as a baseline, various iterations of a low-cost acoustic monitor were designed to meet these constraints while minimizing initial investment cost, improving the mean time between failures, and increasing overall system capability. This paper will describe the system developed to provide a rapid deployment option that is robust to extreme temperatures, humidity, and destructive wildlife. A review of operational logs ...

Improving practices through analysis of long-term microphone calibration records

A microphone’s history of calibration is a useful source of information, as an independent analysis of the data can reveal more than simply the metrological traceability. The U.S. Army Engineer Research and Development Center maintains over 13 years of calibration records from an inventory of over 280 Class 1 microphones, which were examined for historical validity and consistency. Analyzing trends in the laboratory reported sensitivities showed that 12.9% of active microphones exhibited four possible trends that could indicate potential microphone failure: sensitivity drift, sensitivity drift followed by instability, jump in sensitivity, and general instability. Many of the microphones that displayed these trends had uncertainties greater than a ± 1.0 dB tolerance inferred from ANSI S1.14 Class 1 microphone specifications, and an additional 10.7% of microphones showed errors in the calibration laboratory’s transcription process or testing procedures. Recommendations are put forth for best practices relat...

Validation of Short-Term Noise Assessment Procedures: FY16 Summary of Procedures, Progress, and Preliminary Results

Abstract : Army regulations require that installations assess the impacts of training range noise on wildlife and the human community. This work provides a summary of the work performed in fiscal year 2016 for the Environment, Safety and Occupational Health Short-Term Noise Assessment Procedure Demonstration/Validation project. This report describes the procedure used to generate the noise models output dataset, and then it compares that dataset to the benchmark, the Engineer Research and Development Centers Long-Range Sound Propagation dataset. It was found that the models consistently underpredict the measured values. Multiple topics were explored towards identifying possible sources of error. The peak-level calculation algorithm is found to perform adequately, but an alternative method is recommended for future use. Significant meteorological variability is found across the landscape, leading to challenges in estimating the correct propagation class for a particular scenario. A deeper investigation of the propagation class selection algorithm is continuing in FY17. Updates to the noise assessment tools are identified. Throughout this document, procedures used for calculations and analysis are included.