Atmospheric Acoustics Research Articles

Experimental characterization of littoral atmospheric acoustics: Concurrent meteorological and acoustic observations.

The aim of this work is to describe a rich set of acoustic transmission loss observations that were completed in a coastal environment. The data library, enumerated in detail and publicly posted, is comprised of pitch-catch acoustic transmission loss measurements along with concurrent high spatial resolution meteorological observations. The meteorological parameters include near-surface temperature profiles, vertical wind speed profiles in the acoustic propagation direction, and significant wave height estimates. The acoustic source is positioned on an anchored vessel such that the first several hundred meters of the acoustic range is over open water with one microphone array positioned at the shore. A second microphone array is placed several hundred meters inland along the same source-to-receiver heading. The path between the two acoustic arrays is uniform salt marsh vegetation. Observations were made during seven sessions, which represent a variety of atmospheric conditions. That variety of conditions allows for some experimental generalizations about transmission loss as a function of meteorological observations. These include (1) the relationship between vertical effective sound speed profile and transmission loss, and (2) the variability of acoustic pressure with turbulence over time and elevation.

Multiple Sound Scattering by Combinations of Cylindrically Symmetric and Linearly Invariant Anomalies

A number of previous papers have used the coupled-mode method to assess three-dimensional scattering by cylindrically symmetric anomalies (seamounts, hills) or anomalies which are invariant in a horizontal direction (wedges, ridges). This paper makes an extension to combinations of these two anomaly types. The upper and lower depth boundaries of the anomalies may be flat or irregular, and the sound source may be anywhere in the medium. After a discretization of the anomalies of the two types with laterally homogeneous rings and strips, respectively, an adaptation of the coupled-mode method yields the solution of the pertinent Helmholtz equation. The adaptation involves a combination of Fourier-series summations to handle the ring anomalies and adaptive wavenumber integrations to handle the strip structure. For each anomaly, recursively computed reflection matrices relate the expansion coefficients for incoming and outgoing normal modes. Iterative solution of a linear equation system for the amplitudes of the scattered cylindrical waves from the ring anomalies, involving formulas for transformation between plane and cylindrical waves, yields an expansion of the field. Related expansions allow isolation of partial waves, multiply scattered among the anomalies. The paper includes examples from underwater acoustics and atmospheric acoustics.

Sound propagation and scattering in turbulent media

This presentation overviews sound propagation and scattering in turbulent media and pertinent acoustic remote sensing of turbulence. Although sound propagation through a turbulent atmosphere is mainly considered, the presented results apply to other media such as oceanic turbulence. Formulations are provided for various statistical characteristics of acoustic signals such as the scattering cross section, the variances of the phase and log-amplitude fluctuations, the spatial, temporal, and cross-frequency coherences, and the probability density functions. Monte Carlo simulation of sound propagation in a random medium is explained. Specifics of acoustic signals in turbulent media as compared to electromagnetic waves are highlighted. For example, sound waves are scattered by both the sound speed and medium velocity fluctuations, which are scalar and vector random fields, respectively, with different statistical properties. Also, fluctuations in the acoustic refractive index are much stronger than those in electromagnetic propagation, and sound can be scattered by humidity (or salinity) fluctuations. Finally, the Markov approximation, which is widely used for electromagnetic waves, might not be applicable for some statistical characteristics of acoustic signals. It is explained how these specifics of sound propagation are addressed in atmospheric acoustics.

Coupled-mode field computations for media with locally reacting irregular boundaries.

Coupled-mode methods have been used in underwater acoustics to compute three-dimensional sound propagation and scattering. Significant computational simplifications are possible for media with a lateral variation restricted to cylindrically symmetric anomalies, such as seamounts, and also for media which are invariant in one of the horizontal directions. Typically, the upper and lower depth boundaries of the medium have then been horizontal and flat. This paper generalizes the discrete coupled-mode method with a reflection- (or scattering-) matrix formulation to media with irregular and locally reacting boundaries. Horizontal and vertical segments thereby approximate a sloping boundary. Incorporation of the boundary conditions in a correct way necessitates modifications of the basic equation systems. Additional coupling matrices appear, involving integration of normal-mode products over the depth increments for adjacent regions of the medium. The paper includes three computational examples. One is from underwater acoustics with an island that rises above the sea level. The other two are from atmospheric acoustics with sound propagation over a locally reacting irregular ground surface. Using nonlinear optimization, it is possible to select a suitable artificial absorbing medium termination for a mode representation of the field.

Two Chebyshev Spectral Methods for Solving Normal Modes in Atmospheric Acoustics.

The normal mode model is important in computational atmospheric acoustics. It is often used to compute the atmospheric acoustic field under a time-independent single-frequency sound source. Its solution consists of a set of discrete modes radiating into the upper atmosphere, usually related to the continuous spectrum. In this article, we present two spectral methods, the Chebyshev-Tau and Chebyshev-Collocation methods, to solve for the atmospheric acoustic normal modes, and corresponding programs are developed. The two spectral methods successfully transform the problem of searching for the modal wavenumbers in the complex plane into a simple dense matrix eigenvalue problem by projecting the governing equation onto a set of orthogonal bases, which can be easily solved through linear algebra methods. After the eigenvalues and eigenvectors are obtained, the horizontal wavenumbers and their corresponding modes can be obtained with simple processing. Numerical experiments were examined for both downwind and upwind conditions to verify the effectiveness of the methods. The running time data indicated that both spectral methods proposed in this article are faster than the Legendre-Galerkin spectral method proposed previously.

Evaluation of numerical predictions of sonic boom level variability due to atmospheric turbulence.

A numerical model of full-scale N-wave sonic boom propagation through turbulence is described based on the nonlinear Khokhlov-Zabolotskaya-Kuznetzov (KZK) propagation equation and the most advanced turbulence model used in atmospheric acoustics. This paper presents the first quantitative evaluation of a KZK-based model using data from the recent Sonic Booms in Atmospheric Turbulence measurement campaigns, which produced one of the most extensive databases of full-scale distorted N-waves and concurrent atmospheric parameters. Simulated and measured distributions of the perceived level (PL) metric, which has been used to predict public annoyance due to sonic booms, are compared. For most of the conditions considered, the present model's predictions of the PL variances agree with the measurement to within normal uncertainty, while about half of the mean value predictions agree. The approximate PL distribution measured for high turbulence conditions falls within about 2 dB of the simulated distribution for nearly all probabilities. These favorable results suggest that the KZK-based model is sufficiently accurate for approximating the N-wave PL distribution, and the model may therefore be useful for predicting public reaction to sonic booms in turbulent conditions.

Assessing Raspberry Shake and Boom Sensors for Recording African Elephant Acoustic Vocalizations

Acoustic sensors are increasingly being used in ecological and conservation research, but the choice of sensor can be fraught with trade-offs. In this work we assess the performance of the Raspberry Shake and Boom (RS&B) sensor package for detecting and monitoring African elephants (Loxodonta africana). This is the first documented test of this particular unit for recording animal behavior; the unit was originally designed for detecting tectonic earthquakes and low frequency (<50 Hz) atmospheric acoustics. During a four day deployment in South Africa we tested five RS&B units for recording acoustic and seismic vocalizations generated by a group of African elephants. Our results highlight a varied degree of success in detecting the signals of interest. The acoustic microphone recorded fundamental frequencies of low-frequency (<50 Hz) harmonic vocalizations that were not clearly recorded by more sensitive instruments, but was not able to record higher frequency harmonics due to the low sampling rate (100 Hz). The geophone was not able to consistently record clear seismic waves generated by vocalizations but was able to record higher harmonics. In addition, seismic signals were detected from footsteps of elephants at <50 m distance. We conclude that the RS&B unit currently shows limited potential as a monitoring tool for African elephants and we propose several future directions and deployment strategies to improve the sensitivity of the sensor package.

XVII ШКОЛА-СЕМИНАР ИМЕНИ АКАДЕМИКА Л.М. БРЕХОВСКИХ «АКУСТИКА ОКЕАНА»

The short communication presents the results of the 17th School on Ocean Acoustics known as “L.M. Brekhovskikh School-Seminar” held online from October 19 to 23, 2020. The conference was combined with the XXXIII session of the Russian Acoustic Society. The conference was attended by 138 researchers from 25 research institutes form Russia, Abkhazia, Israel, USA, and Taiwan. Leading scientists gave 6 lectures. 59 oral and poster presentations were presented focusing on topical scientific and technical subjects: acoustic oceanology, sound propagation in the ocean, sound scattering and reflection, engineering, and signal processing, as well as bio-, geo- and atmospheric acoustics.

Acoustic source localization in imperfectly known environments using frequency-differencing techniques

Remote unknown sound-source localization is a challenging task with applications in a variety of fields, such as underwater acoustics, atmospheric acoustics, structural acoustics, bio-medical ultrasound, animal bio-acoustics, and seismology. In nearly all cases, the localization algorithm is only successful when there is good cross correlation between remotely measured and predicted acoustic fields, a possibility that exits at low frequencies and/or short ranges in imperfectly known environments. However, at sufficiently high frequency and/or long range, the requisite measured-to-predicted-field correlation might never be high enough for successful source localization. However, recent investigations of the frequency-difference autoproduct, a quadratic product of two complex field amplitudes having different frequencies, suggest that it may have the phase structure of an acoustic field at the difference frequency. Thus, using sufficiently low difference frequencies, unknown sources may be localized at ranges where conventional techniques are unsuccessful by correlating measured and predicted (ideal) autoproducts. The underlying formulation of frequency-differencing techniques is presented along with examples drawn from simulations, laboratory experiments, and ocean propagation measurements that involve frequencies from fractions of a Hertz to more than 100 kHz, and propagation distances from tens of centimeters to hundreds of kilometers. [Sponsored by ONR, NAVSEA, and NSF.]

Determining the Spatial Properties of a Turbulent Transverse Flow via Multi-Frequency Acoustic Probing

The results are presented from an experimental study of solving the inverse problem of recovering a turbulent flow’s velocity and its position in space by analyzing the spectrum of fluctuations in an acoustic signal of different frequencies intersecting the flow. The conditions for and accuracy of recovering turbulent flow characteristics are discussed in terms of a multifrequency signal propagating along one acoustic path. The study is performed using a muffled acoustic chamber with a jet of air serving as a turbulent flow. Despite the physical character of the experiments, their results can be applied to problems of atmospheric, aero-, and oceanic acoustics.

Correspondence between sound propagation in discrete and continuous random media with application to forest acoustics

Although sound propagation in a forest is important in several applications, there are currently no rigorous yet computationally tractable prediction methods. Due to the complexity of sound scattering in a forest, it is natural to formulate the problem stochastically. In this paper, it is demonstrated that the equations for the statistical moments of the sound field propagating in a forest have the same form as those for sound propagation in a turbulent atmosphere if the scattering properties of the two media are expressed in terms of the differential scattering and total cross sections. Using the existing theories for sound propagation in a turbulent atmosphere, this analogy enables the derivation of several results for predicting forest acoustics. In particular, the second-moment parabolic equation is formulated for the spatial correlation function of the sound field propagating above an impedance ground in a forest with micrometeorology. Effective numerical techniques for solving this equation have been developed in atmospheric acoustics. In another example, formulas are obtained that describe the effect of a forest on the interference between the direct and ground-reflected waves. The formulated correspondence between wave propagation in discrete and continuous random media can also be used in other fields of physics.

Green’s function retrieval by crosscorrelation and multidimensional deconvolution: Application to atmospheric acoustics

In this work, the Green’s function of an atmospheric acoustic propagation channel is extracted from experimental measurements. Of particular interest, is the accuracy of Green’s function retrieval methods in ideal and non-ideal situations. To this end, an emitting acoustic source (impulsive and audible) is placed at a distance of more than 90 meters from receiving tri-axis microphone arrays in open (ideal) and wooded areas (non-ideal) in southern Maryland. Green’s function retrieval methods are employed and investigated on the collected data. Green’s function retrieval by crosscorrelation has been successful in various applications despite the limitations of its lossless medium and equipartitioned wavefield assumptions. To overcome the violation of these assumptions, the multidimensional deconvolution has been proposed. Comparisons of the results between impulsive and audible sources using the two methods will be presented. The effects of multipath, source-receiver distance, temperature, and wind will be discussed.

Graduate Education in Acoustics at The Catholic University of America

The Catholic University of America (CUA) has a graduate program with a long history in acoustics dating back to the early 1930s. The acoustics program moved to the School of Engineering in the 1960s when there were strong needs in underwater acoustic studies to meet U.S. Naval applications. The end of the Cold War was concurrent with a decline in the CUA’s acoustics education that persisted into the 1990s. However, renewed interests in acoustical engineering, acoustic metamaterial, and environmental acoustic research has revived the acoustics research and education programs at the CUA in recent years. Currently, a variety of graduate level acoustic courses are being offered in the CUA’s Mechanical Engineering Department. Students can pursue a master’s degree or Ph.D. degree with research in acoustics or vibrations. The courses in the program include a two-course sequence in fundamentals in acoustics, and more focused courses in ocean acoustics, atmospheric acoustics, acoustic metrology, marine bioacoustics, nonlinear vibration, acoustic imaging, and acoustic metamaterials. In addition, CUA offers masters and Ph.D. programs to students who are interested in the field of acoustic research.

Simulation of Sound Waves Using the Lattice Boltzmann Method for Fluid Flow: Benchmark Cases for Outdoor Sound Propagation.

Propagation of sound waves in air can be considered as a special case of fluid dynamics. Consequently, the lattice Boltzmann method (LBM) for fluid flow can be used for simulating sound propagation. In this article application of the LBM to sound propagation is illustrated for various cases: free-field propagation, propagation over porous and non-porous ground, propagation over a noise barrier, and propagation in an atmosphere with wind. LBM results are compared with solutions of the equations of acoustics. It is found that the LBM works well for sound waves, but dissipation of sound waves with the LBM is generally much larger than real dissipation of sound waves in air. To circumvent this problem it is proposed here to use the LBM for assessing the excess sound level, i.e. the difference between the sound level and the free-field sound level. The effect of dissipation on the excess sound level is much smaller than the effect on the sound level, so the LBM can be used to estimate the excess sound level for a non-dissipative atmosphere, which is a useful quantity in atmospheric acoustics. To reduce dissipation in an LBM simulation two approaches are considered: i) reduction of the kinematic viscosity and ii) reduction of the lattice spacing.

Characterization of spark-generated N-waves in air using an optical schlieren method.

Accurate measurement of high-amplitude, broadband shock pulses in air is an important part of laboratory-scale experiments in atmospheric acoustics. Although various methods have been developed, specific drawbacks still exist and need to be addressed. Here, a schlieren optical method was used to reconstruct the pressure signatures of nonlinear spherically diverging short acoustic pulses generated using an electric spark source (2.5 kPa, 33 μs at 10 cm from the source) in homogeneous air. A high-speed camera was used to capture light rays deflected by refractive index inhomogeneities, caused by the acoustic wave. Pressure waveforms were reconstructed from the light intensity patterns in the recorded images using an Abel-type inversion method. Absolute pressure levels were determined by analyzing at different propagation distances the duration of the compression phase of pulses, which changed due to nonlinear propagation effects. Numerical modeling base on the generalized Burgers equation was used to evaluate the smearing of the waveform caused by finite exposure time of the high-speed camera and corresponding limitations in resolution of the schlieren technique. The proposed method allows the study of the evolution of spark-generated shock waves in air starting from the very short distances from the spark, 30 mm, up to 600 mm.

Experimental quantification of microphone windscreens and noise cones in a low speed wind tunnel

Wind noise is a common challenge in all types of atmospheric acoustics as well as in wind tunnel applications in the aerospace and automotive industry. The dynamic pressure field measured by the microphone includes both the acoustic pressure and the pressure induced by the turbulent air flow over the diaphragm. There are three types of microphone accessories that are commonly used to perform wind noise isolation: protective grid-caps (no isolation), windscreens, and nose-cones. In this study, each of these microphone accessories is tested in a wind tunnel using ¼” and ½” microphones in turbulent and laminar flows at flow speeds up to 55 mph and in both the head-on and parallel diaphragm orientations. Two types of flush mount microphones, a surface mount and a side-vented pressure microphone, are also evaluated. The effects of background flow noise on the microphone measurement is shown for all conditions and a case study is presented with an arbitrary acoustic source. Finally, recommendations are presented for which accessories are best used in different measurement situations.

Propagation of Sound in Sediments

The theories of sound propagation in sediments are more or less extensions of those applicable to atmospheric acoustics and to emulsions. The theoretical and experimental work of various investigators on the velocity and absorption of sound in sediments has been reviewed. The sound velocity in many high porous fine grained sediments has been found tobe less than reviewed.

ACOUSTIC WAVEFRONT TRACING IN INHOMOGENEOUS, MOVING MEDIA

We extend the Huygens wavefront tracing algorithm, which is a part of an open source Madagascar project, to sound propagation in inhomogeneous, moving media and apply it to a series of benchmark tasks. One set of tasks admits exact analytic solutions and serves the purpose of validation of the new algorithm. Another set of calculations demonstrates applicability of the algorithm to the studies of wavefront dynamics and stability in ocean and atmospheric acoustics. The method is based on a system of differential equations equivalent to the eikonal equation, but formulated in the ray coordinate system. In this paper, we present a first-order, two-dimensional discretization scheme that is interpreted very simply in terms of the Huygens' principle. The method has proved to be a convenient alternative to conventional ray tracing.

Acoustics research in Nanjing University–In memory of Professor Rong‐jue Wei.

Acoustics research in Nanjing University was initiated by Prof. Rong‐jue Wei in 1954, and developed as an Institute of Acoustics of Nanjing University in 1978, and then also a National Laboratory of Modern Acoustics in 1990. Under the leadership of Prof. Wei, the research of acoustics developed widely from audio acoustics to physical acoustics and ultrasonics, including electro‐acoustics, architectural acoustics, speech signal processing, molecular acoustics, atmospheric acoustics, acousto‐electronics, low temperature acoustics, nonlinear acoustics, photoacoustics, biological acoustics, environmental acoustics, and noise control. Most of the research fields and works were initial and creative in the country, and some of them have been continued and developed greatly. Up to now, the researches of the institute include five fields as follows: (1) Intensive acoustics: ultrasonic cavitation, somoluminescience, soliton and chaos; (2) Photoacoustics and laser ultrasonics: photoacoustic spectroscopy and imaging,...

On experience in using the method of morphological data analysis in atmospheric acoustics

A methodical system has been developed to solve the problem of isolating informative signals against the background of noise which is based on a morphological analysis. It is shown that the morphological method of analyzing infrasonic signals is an effective instrument to isolate infrasonic signals, to analyze the features of their forms, and to estimate their relative delays. The advantage that the morphological method has over the correlation method in analyzing short signal fragments recorded by infrasonic microphones spaced at distances that significantly exceed the signal wavelengths is shown.