Atmospheric Sound Propagation Research Articles

Atmospheric Sound Propagation over Rough Sea: Numerical Evaluation of Equivalent Acoustic Impedance of Varying Sea States

This work presents a numerical study on atmospheric sound propagation over rough water surfaces with the aim of improving predictions of sound propagation over long distances. A method for generating pseudorandom sea profiles consistent with sea wave spectra is presented. The proposed method is suited for capturing the logarithmic nature of the energy distribution of the waves. Sea profiles representing fully developed seas for sea states 2, 3, 4, and 5 are generated from the Elfouhaily et al. (ECKV) sea wave spectra. Excess attenuation caused by refraction and surface roughness is predicted with a parabolic equation (PE) solver. A novel method for estimating equivalent effective impedance based on PE predictions at different sea states is presented. Parametric expressions using acoustic frequency and significant wave height are developed for effective surface impedances. In this work, sea surface roughness is on a scale comparable with the acoustic wavelength. Under this condition, the acoustic scattering is primarily incoherent. This work shows the limitations of using an equivalent surface impedance in such incoherent scattering cases.

Highlights from the technical committee on noise

The Technical Committee on Noise (TCNS) includes researchers and industry practitioners interested in understanding noise generation from a variety of sources, its propagation, and its impact on structures, objects, and people. The goal of reducing the impact of this noise, or unwanted sound, has led to innovative model development, measurement techniques, mitigation strategies, and input towards the establishment of regulations. Community noise definition and abatement is of particular interest and motivates the advancement of research on a variety of topics, such as transportation noise from traditional and new vehicles, hearing protection, jet and rocket noise, building systems noise and vibration, atmospheric sound propagation, soundscapes, and low-frequency sound. The diverse technical background of TCNS members is essential, given the interdisciplinary nature of this research, and it is mirrored in the topics of special sessions typically held jointly with other ASA technical committees.

Predicting excess attenuation: An investigation of turbulence in a littoral environment

This work presents an effort to effectively represent turbulence in a numerical model of atmospheric sound propagation in a near-shore environment. Model results are compared with a set of atmospheric transmission loss measurements made in a pitch-catch configuration with a stationary source 500 m from shore. High resolution temperature profiling and scanning Doppler LIDAR wind profiling measurements in the source-to-receiver direction are concurrent with the acoustic transmission loss measurements at the shoreline. These meteorological measurements inform the input parameters used in the parabolic equation. The numerical predictions are compared to the measured transmission loss values to evaluate model performance with a variety of different approaches to implement turbulence parameters.

High spatial resolution long-term temperature profiling to inform near-shore atmospheric sound propagation

Atmospheric sound propagation depends on factors such as surface characteristics and meteorological parameters. Effective numerical modeling of atmospheric sound propagation requires reliable and realistic input parameters. One of the most influential factors is the air temperature profile. This work examines the application of experimentally determined air temperature profiles in acoustic modeling of littoral environments. Long term measurements of near-surface air temperature profiles over water and marsh grass are presented. Data have been recorded using temperature loggers affixed to two permanent 7 m masts. The loggers were positioned with a 1 m vertical spacing. A common approach to characterize the near-surface boundary layer uses Monin–Obukhov Similarity Theory (MOST). This work compares high spatial resolution air temperature measurements with measurement-informed profiles based on MOST. The development of a more experimentally grounded set of assumptions will ultimately contribute to the improvement of atmospheric acoustic transmission loss models in near shore environments.

Influence of mesoscale meteorological observations on near shore excess attenuation prediction

This work focuses on numerical modeling of atmospheric sound propagation in a near-shore environment that is informed by concurrent meteorological observations. The atmospheric acoustic measurement layout places the source approximately 500 m from shore with one acoustic receiver array at the shoreline and a second acoustic receiver array approximately 350 m inland. The space between the shoreline and the inland receiver array consists of highly uniform marsh grass vegetation. The concurrent meteorological observations include high resolution temperature profiling at two locations (one over land and one over water) along with scanning Doppler LIDAR wind profiling measurements in the source-to-receiver direction. These meteorological measurements inform the input parameters used in a parabolic equation solver. The numerical predictions are compared to the measured transmission loss values to evaluate model sensitivity to input parameters.

Measuring excess attenuation in littoral environments: Evolution of a measurement system

Atmospheric sound propagation measurements over long distances are generally lacking in the literature. This work presents the evolution of the system deployed to collect high spatial and temporal resolution synchronized acoustic and meteorological data. Specifically, the mature configuration includes: a long-range acoustic device capable of source levels up to 140 dB; up to four 7-channel acoustic arrays; two 7-m temperature profiling masts; and a scanning Doppler LIDAR system for volumetric wind profiling at ranges up to several kilometers. This work presents lessons learned over the various iterations of the system such as the design and evolution of the excitation signal used; physical configuration of the components of the system; logistics and planning concerns; post-processing challenges; and quality control considerations.

Modeling sea state induced atmospheric sound transmission loss

This work presents a numerical study on atmospheric sound propagation over rough rigid surfaces. The intent is to simulate acoustic propagation over water. Methods to estimate sea state induced atmospheric sound transmission loss and relative uncertainties are evaluated. In previous studies, a flat surface with an equivalent impedance was used to account for the effect of surface roughness on sound transmission loss. Equivalent impedances were estimated based on time-domain numerical simulations of atmospheric sound propagation above pseudorandom sea surfaces coherent with a Pierson-Moskowitz spectra. Estimation of equivalent impedances using time- and frequency- domain approaches are compared for cases up to sea state 4. Acoustic excess attenuation due to propagation over a rough surface was predicted by the authors, in a previous work, by correcting the excess attenuation of a propagation over a flat, perfectly reflecting surface. The correction factor was frequency and sea state dependent and included an additional term to account for the uncertainties characteristic of different sea states. Excess attenuation predictions estimated using the equivalent impedance approach are compared with those obtained from the authors’ previous study. Implications of the use of these methods for source detectability determinations are discussed.

Atmospheric acoustic propagation modeling using heterogeneous wind profiles along the acoustic path

This work presents atmospheric sound propagation predictions using a parabolic equation solver that accounts for heterogeneous wind profile distribution along the acoustic path. Transmission loss predictions using both homogeneous and heterogeneous wind speeds information are compared with data. A three-dimensional scanning Doppler LIDAR wind profiler captures real-time wind speed gradients at many locations along the acoustic propagation path providing the heterogeneous wind speed profiles. The wind measurements are concurrent with a pitch catch transmission loss measurements. A long-range acoustic device on an anchored pontoon sends known chirp sequences to a seven-channel receiver array at the water’s edge at ranges up to approximately one kilometer. Additional synchronized meteorological observations include temperature, humidity, and wind measured with anemometers. Key differences in model results are highlighted and an assessment of the value of the computational cost is presented.

Air temperature profiling over different littoral surfaces

There are many factors that must be considered in the study of atmospheric sound propagation over long distances; these include surface characteristics as well as wind speed and air temperature profiles. This work presents measurements of air temperature profiles over various surfaces. Building a catalogue of this type can enable more realistic case assumptions to be made in an atmospheric acoustic transmission loss model. Accurate air temperature profiles are critical because of the potentially significant impact on the transmission loss predictions. Air temperature profile measurements have been collected by solar radiation shielded temperature loggers mounted 1 m apart on a 7 m mast. Long duration measurements were taken to capture the variability in day and nighttime temperature differentials present over surfaces such as: gravel, vegetated shoreline, marsh grass, lawn grass, water, and asphalt. In particular, measurements performed off shore were done by placing the sensor array on the deck of a pontoon boat. The initial results revealed that this configuration captures a very low elevation warming due to re-radiation from the deck of a boat instead of the true over water temperature. This study highlights the influence of wind speed on the development of the near surface temperature inversions.

Gaussian beam tracing for three-dimensional sound propagation problems in subsonic moving medium

The study of sound propagation in moving medium is important in various fields, such as atmospheric sound and underwater acoustics. To address this problem, a three-dimensional Gaussian beam tracing model is developed for subsonic moving medium, based on the Helmholtz equation of velocity potential for high-frequency sound wave in a moving medium with arbitrary Mach numbers. The dynamic ray equations in the moving medium are derived by using the beam tracing method, and further the partial differential equation is transformed into ordinary differential equations, so as to be able to more efficiently and accurately calculate the three-dimensional sound field in the moving medium. The Gaussian beam tracing method reveals that the expansion of the beam in a moving medium is more complex than in a static medium, and the energy in the ray tube is not necessarily conserved. The model is applied to several problems, including point source sound propagation in a semi-infinite homogeneous medium, three-dimensional long-range sound propagation in horizontally layered atmospheres, and three-dimensional sound propagation in the Gulf Stream. The results of the point source sound propagation problem in the semi-infinite homogeneous medium verify the effectiveness and accuracy of the model. The results of the atmospheric sound propagation problem indicate that compared with the commonly used <i>N</i> × 2D method, the three-dimensional Gaussian beam tracing in a moving medium fully considers the effect of medium motion, especially the effect of crosswind, and can calculate the sound pressure field more accurately. Although the Mach number of the ocean current is very small, its effect cannot be ignored. The ocean current can quantitatively change the sound propagation mode and affect the convergence zone position. In some areas, the difference between calculation results with and without considering the ocean current is more than 5 dB. Moreover, the deviation of rays caused by lateral flow is much smaller, and even in the areas with complex terrain, the deviation becomes more obvious only after being reflected by the interface. Moreover, the influence of lateral flow on sound propagation is much smaller than that caused by flow velocity parallel to the propagation direction. In conclusion, the developed Gaussian beam tracing method provides an accurate and efficient approach for solving the sound propagation problem in subsonic moving media.

Application of the finite element method to atmospheric sound propagation over impedance discontinuities

Outdoor noise propagation typically involves the propagation of sound over mixed ground conditions, including variations in the surface impedance. Diffraction occurs at the interface between different ground conditions, and this can significantly affect local sound attenuation. Popular approaches to modelling impedance discontinuities include the boundary integral method, parabolic equations, and semi-empirical methods. This paper presents an alternative approach that takes advantage of the generality of the finite element method. It was shown recently that the semi analytic finite element method can be used to solve the exact governing wave equation for atmospheric sound propagation in a complex vertically stratified medium. This allows for the inclusion of arbitrary temperature and wind profiles provided the problem is range independent. It is shown here that it is relatively straightforward to extend this approach to include multiple surface impedance discontinuities, provided each discrete surface is uniform in the axial direction. This is achieved using point collocation to enforce the axial boundary conditions over each discontinuity. Predictions are generated for single and multiple discontinuities, and comparisons are made against predictions obtained using other computational methods.

Evaluation of a gamma power, wrapped normal phase model for vertical and slanted atmospheric sound propagation

As sound propagates through the atmosphere, turbulence causes fluctuations in the signal power and phase. These fluctuations complicate the signal processing required to detect, locate, and identify elevated sources. This presentation evaluates a model that assumes a gamma distribution describes the power and a wrapped normal distribution describes the phase. The gamma distribution approximates a log-normal distribution when the normalized power variance is small compared to one and an exponential distribution when the normalized power variance is one. The wrapped normal distribution approximates a normal distribution when the phase variance is small and a uniform distribution when the phase variance is large. In 2018, we measured vertical and slanted sound propagation at the National Wind Technology Center (NWTC). The ground-borne source emitted tones in the range 600–3500 Hz, and several microphones were mounted to a meteorological tower up to 130 m high. Overall, the gamma power, wrapped normal phase model accurately approximated the NWTC data across a wide range of scattering regimes. This presentation also discusses the largest discrepancies.

Sea state induced sound transmission loss of atmospheric broadband sources

This work presents a numerical study conducted on atmospheric sound propagation over sea. In particular, it focuses on the sound pressure level prediction uncertainties induced by the water surface roughness. To quantify these uncertainties, the generalized terrain parabolic equation (GTPE) is used to model sound propagation above water surfaces at different seastates. Water roughness is pseudo-randomly generated using Pierson-Moskovitz ocean wave spectrum. Building on a previous result that has established a simple expression for predicting single frequency sound transmission loss as a function of sea state, this work extends the approach to broadband signal from 125 to 1000 Hz. This work presents relationships between fully developed sea states (up to sea state 4) and the uncertainties on sound pressure level predictions at distances up to 1000 m from the source. These relationships are presented for typical nocturnal thermal gradients and for different elevations from the water surface.

Spatial coherence of vertical atmospheric sound propagation

Atmospheric turbulence causes the amplitude and phase of sound waves to fluctuate, which reduces the coherence of acoustic signals. Spatial coherence describes the similarity of two signals at different points in space, and a better understanding of acoustic coherence could lead to improved target detection, tracking, and identification. This presentation compares theoretical predictions and measurements of the acoustic spatial coherence. The theoretical coherence is derived by combining sound propagation theory with turbulence models that include the effects of atmospheric shear and buoyancy instabilities. To be applicable to vertical and slanted propagation, the turbulence models use height-dependent variances and length scales for the fluctuations in temperature, shear-produced velocity, and buoyancy-produced velocity. Instrumentation on a 135-m meteorological tower at the National Wind Technology Center (Boulder, CO) provided the required model input data. The coherence measurements used a ground-based source and nine microphones attached to the same meteorological tower. Overall, the theoretical model accurately approximated the measured spatial coherences, especially when the atmospheric turbulence was fully developed (e.g., in the afternoon on sunny days). The largest disagreement occurred for measurements taken at dawn. For a source frequency of 3.4 kHz and microphones that are 130 m high and 1.5 m apart, the measured coherence was 0.3–0.6 for sunny conditions and 0.8–0.9 for cloudy conditions.

Synchronized acoustic and atmospheric measurement system for characterization of atmospheric sound propagation

Abstract The aim of the paper is to describe a portable, modular, and scaleable system for measuring concurrent acoustic transmission loss and atmospheric characteristics. This system has been developed specifically to inform an effort to improve the ability to implement high fidelity numerical predictions of acoustic transmission loss, particularly in acoustically complex outdoor ranges, such as those that occur in coastal areas. Such a system has broad possible applicability in many outdoor atmospheric acoustic monitoring scenarios.

Atmospheric sound transmission loss uncertainties induced by sea roughness

This work presents a numerical study conducted on atmospheric sound propagation over sea. In particular, it focuses on the sound pressure level prediction uncertainties induced by the water surface roughness. To quantify these uncertainties, the generalized terrain parabolic equation (GTPE) is used to model sound propagation above water surfaces at different sea states. Water roughness is pseudo-randomly generated using an ocean wave spectrum. The GTPE predictions are compared with those obtained using the Crank-Nicholson parabolic equation (CNPE) solver. When using the CNPE the sea surface is flat and has a surface impedance equivalent to that of a rough surface. The use of the GTPE is less computationally efficient but provides insight on the detectability of an acoustic source at sea. This work presents relationships between fully developed sea states (up to sea state 4) and the uncertainties on sound pressure level predictions at distances up to 500 m from the source. These relationships are presented for typical diurnal and nocturnal thermal gradients and for different elevations from the water surface.

Technical Committee on Noise: Researchers and practitioners

The Technical Committee on Noise (TCNS) includes researchers and industry practitioners interested in understanding noise generation from a variety of sources, its propagation, and its impact on structures, objects, and people. The goal of reducing the impact of this noise, i.e., unwanted sound, has led to innovative model development, measurement techniques, mitigation strategies, and input towards the establishment of regulations. Community noise definition and abatement is of particular interest and motivates the advancement of research on a variety of topics, such as transportation noise, hearing protection, jet and rocket noise, building systems noise and vibration, atmospheric sound propagation, soundscapes, and low-frequency sound. The diverse technical background of TCNS members is essential, given the interdisciplinary nature of this research, and it is mirrored in the topics of special sessions typically held jointly with other ASA technical committees.

Developing efficient finite element techniques for atmospheric sound propagation

Applying the finite element method to problems in atmospheric sound propagation is challenging because the use of a standard approach, such as meshing the entire domain, quickly delivers a prohibitive problem size. The finite element method does, however, offer many advantages such as the ability to accommodate continuous variations in fluid properties, as well as scattering from obstacles of complex geometry. This means the method is ideally placed to provide benchmark solutions for more popular approaches. Accordingly, ideas for developing more efficient versions of the finite element method suitable for atmospheric sound propagation are explored here for two dimensional problems. These are based on the use of one dimensional normal mode expansions, and then mapping these on to two dimensional solutions for non-uniform obstacles. The normal mode solution for a range independent inhomogenous moving fluid is presented, and numerical mode matching techniques are introduced to demonstrate the inclusion of a point source. The extension of these mode matching methods to accommodate scattering from obstacles is then discussed, and the conditions necessary to deliver efficient finite element solutions are reviewed.

Atmospheric sound propagation in a moving fluid above an impedance plane: Application of the semi-analytic finite element method.

This article uses a normal mode approach to predict atmospheric sound propagation over a locally reacting impedance plane. The semi-analytic finite element method is used to compute the normal modes, which enables the exact governing wave equation for a moving fluid to be solved in two dimensions. A locally reacting surface is added using the general Ingard-Myers boundary condition, and the transmission loss is obtained for cylindrical and spherical spreading for range independent problems. The approach developed in this article will, in principle, converge toward the exact solution and so has the potential to provide benchmark predictions for complex, range independent, outdoor sound propagation problems. Predictions are shown to provide good agreement with benchmark solutions available in the literature, including those with a logarithmic wind velocity profile. Results are also reported for a combination of a logarithmic wind velocity profile and a temperature inversion for ranges of up to 5 km. Finally, transmission loss predictions are reported for a relatively wide frequency range, and it is concluded that finite elements can provide an alternative approach for computing range independent outdoor sound propagation that converges to the exact solution.

Energy estimation of explosion sound source based on atmospheric sound propagation theory

A method of estimating the explosion sound source energy is proposed by combining atmospheric sound field simulation with mid- and short-range overpressure amplitude attenuation model. Aiming at the problem that the traditional sound source energy estimation formula fails to make full use of the atmospheric parameters and the estimation error is too large, through numerically simulating the propagation loss in the atmosphere, the correction effectiveness of the atmospheric parameters to the sound source energy estimation is greatly improved and the estimation accuracy of the sound source energy is enhanced. In the surface chemical explosion experiment, the infrasound is used to receive the signal from 300 km to 2500 km away. A comparison of energy estimation effectiveness between the traditional energy estimation formula and the energy estimation method based on atmospheric sound field simulation of the explosion sound source is made. The experimental results indicate that this method is effective in improving the robustness of yield estimation in comparison with the traditional yield estimation method.