Effective Sound Speed Profile Research Articles

Inferring the Speed of Sound and Wind in the Nighttime Martian Boundary Layer From Impact‐Generated Infrasound

AbstractThe properties of the first kilometers of the Martian atmospheric Planetary Boundary Layer have until now been measured by only a few instruments and probes. InSight offers an opportunity to investigate this region through seismoacoustics. On six occasions, its seismometers recorded short low‐frequency waveforms, with clear dispersion between 0.4 and 4 Hz. These signals are the air‐to‐ground coupling of impact‐generated infrasound, which propagated in an low‐altitude atmospheric waveguide. Their group velocity depends on the structure of effective sound speed in the boundary layer. Here, we conduct a Bayesian inversion of effective sound speed up to 2,000 m altitude using the group velocity measured for events S0981c, S0986c and S1034a. The inverted effective sound speed profiles are in good agreement with estimates provided by the Mars Climate Database. Differences between inverted and modeled profiles can be attributed to a local wind variation in the impact→station direction, of amplitude smaller than 2 m/s.

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.

Experimental study on outdoor sound propagation under various meteorological conditions

From the winter of 2018 through the spring of 2021, outdoor sound propagation experiments were repeatedly conducted three times a day on weekdays at a glider airfield in Hokkaido, Japan. In each experiment, impulse responses at the points of 155 m and 300 m from sound sources were measured. The ground surface condition of the experimental field was grass-covered in summer and snow-covered in winter. The monthly averages of sound pressure levels under upwind, downwind, and calm conditions were calculated every month. The data show that the seasonal variation depends on wind conditions, propagation distance, and frequencies. At the point of 300 m from sound sources, the seasonal variations are small under downwind conditions, whereas the range of variations is about 10 dB at high frequencies under upwind conditions and calm conditions. The relationship between vector wind speed and excess attenuation is discussed focusing on differences in atmospheric stability, snowfall, and snow cover conditions, respectively. We also introduce trial calculation results assuming the logarithmic-liner effective sound speed profiles and the parameters of turbulence proposed in previous studies.

Quantification of spatial and seasonal trends in the atmosphere and construction of statistical models for infrasonic propagation

SUMMARY Infrasonic waves are influenced by variations in the density, pressure and temperature as well as the ambient winds. Modelling infrasonic propagation can be challenging due to the dynamic nature of the atmosphere as well as the sparseness of measurements which result in variability and notable uncertainty. A framework is presented to quantify spatial and seasonal trends in atmospheric structure via analysis of the effective sound speed profile and identification of temporal trends in the middle atmospheric waveguide produced by the circumpolar vortex winds. Seasonal definitions identifying typical atmospheric structures during the summer, winter and spring/fall transition periods are identified using atmospheric data from 2010 through 2020. Seasonal trend analysis is conducted for a number of locations across the contiguous United States to quantify spatial variations in atmospheric structure that impact infrasonic propagation. For each season and location, empirical orthogonal function analysis is used to reduce the historical archive of atmospheric data into a smaller representative set that can be analysed using numerical tools more efficiently. Infrasonic ray tracing and finite-frequency modal propagation analyses are applied to construct propagation path geometry and transmission loss statistics which are useful in localization and yield estimation for infrasonic sources, respectively. An example application is detailed in which transmission loss statistics are combined with an explosive source model and noise statistics to quantify the capability of a network to detect nearby sources.

Spatial clustering of sound speed distributions

It is increasingly important to our understanding of the effects of climate change on our oceans that we can model sound propagation in the future environments that are being predicted. This brings conflicting demands to increase the fidelity of sound propagation models, to account for a greater number of dynamic ocean properties, and their efficiency, in particular, to account for the spatial and temporal distribution of sound speed data. In this paper, different spatial clustering methods are developed to group similar sound speed data into a small number of clusters based on acoustically relevant properties of the sound speed structure. The clusters then represent the sound speed distribution, derived from thousands of individual sound speed profiles, with a small number of effective sound speed profiles which have similar properties. This is illustrated on data for the Norwegian Sea where the performance of each method is evaluated, using measures of the separation between clusters, and by comparing sound speed predictions using profiles sampled from each cluster and using the effective sound speed profile. We show that spatial clustering is an effective tool for reducing the number of sound speed profiles that are needed to represent their spatial distribution.

Atmospheric Boundary Layer as a Laboratory for Modeling Infrasound Propagation and Scattering in the Atmosphere

In this work, we show that large-scale processes of the long-range propagation and scattering of infrasound signals in stratospheric and thermospheric waveguides are to some extent similar to the smaller scale processes of waveguide propagation and scattering of acoustic signals in stably stratified ABL. Moreover, we note a resemblance between the zeroth order tropospheric mode and the Lamb mode that is observed for larger nuclear explosions. The results of physical modeling of the long-range propagation of infrasound signals in the atmosphere by studying the propagation of the acoustic pulses from detonation sources in the stably stratified atmospheric boundary layer (ABL) are presented. Such modeling became possible due to continuous measurement of the wind velocity and temperature profiles in the ABL by Doppler sodar and temperature profiler, respectively. The resemblance between the propagation of the infrasound signals from surface explosions (20–70 t TNT) in the “shallow” tropospheric wave guide and propagation of Lamb mode from nuclear explosions is found. Also, the propagation of infrasound signals in the stratospheric wave guide is modeled by studying the propagation of acoustic pulses in the ABL during morning hours (after sunrise), when an effective sound speed profile in the ABL is similar to that in the stratospheric wave guide. The instantaneous vertical profiles of the effective sound speed and wind velocity fluctuations in the thin layers of the stably stratified ABL located at different heights (up to 700 m) above the ground and at different distances from the source (up to 2.25 km) have been retrieved using the wave forms and travel times of the recorded arrivals of the acoustic pulses from the detonation sources. The effect of scattering of acoustic pulses from the fine-scale layered structure of the stably stratified ABL is studied to model the effect of scattering of infrasound signals from the fine-scale layered structure of the stratosphere.

Use of the parabolic equation method for local infrasound propagation predictions

Local infrasound (<150 km) propagation is strongly influenced by range- and time-dependent meteorological conditions. The Parabolic Equation computational method provides an efficient and flexible tool for predicting infrasound propagation in this regime. Over these longer distances, the influence of spatial and temporal variations in the meteorological conditions can be significant. Meteorological conditions extracted from simulations using the Weather Research and Forecasting (WRF) model are incorporated by updating the effective sound speed profile as a function of propagation range. By examining horizontal grid increments between 1 and 15 km and temporal resolutions between 4 and 60 seconds, we begin to quantify the potential variability in predictions at ranges of up to 150 km, for frequencies between 1 and 20 Hz. This procedure will inform a broader discussion of the application of the PE method to infrasound prediction. [Work funded by the Assistant Secretary of the Army (Acquisition, Logistics, and Technology) [ASA(ALT)] under 62784/T40. Distribution Statement A: Approved for public release; Distribution is unlimited.]Local infrasound (<150 km) propagation is strongly influenced by range- and time-dependent meteorological conditions. The Parabolic Equation computational method provides an efficient and flexible tool for predicting infrasound propagation in this regime. Over these longer distances, the influence of spatial and temporal variations in the meteorological conditions can be significant. Meteorological conditions extracted from simulations using the Weather Research and Forecasting (WRF) model are incorporated by updating the effective sound speed profile as a function of propagation range. By examining horizontal grid increments between 1 and 15 km and temporal resolutions between 4 and 60 seconds, we begin to quantify the potential variability in predictions at ranges of up to 150 km, for frequencies between 1 and 20 Hz. This procedure will inform a broader discussion of the application of the PE method to infrasound prediction. [Work funded by the Assistant Secretary of the Army (Acquisition, Logistics, an...

Analysis of the conditions for the occurrence of "the voice of the sea" using infrasound observations in marine waters

We study the conditions for the occurrence of "the voice of the sea" in the infrasound range can occur and determine its parameters from infrasound measurements in the Black Sea water area conducted in 2011 and 2016 and in the water area of the Sea of Okhotsk in 2017. To this end, we compared the observations of different parameters (mean correlation, spectra of acoustic arrivals, direction and phase velocity) of the high-frequency infrasound (1–10 Hz), recorded in Katsiveli (Crimea) in 2011 and 2016. We performed a detailed study of the wind conditions in the Black Sea water area during the measurement period, as well as the conditions for the propagation of acoustic waves along the direction of their arrivals.&#x0D; In both cases the atmospheric vortices in the direction of the arrival of infrasound were detected, which caused changes in wind velocity vector above the sea surface. The infrasound recorded in 2011 was caused by the interaction of two differently rotating atmospheric vortices over the sea. In 2016, a vortex was observed to the West from the registration point. The possibility of generating "the voice of the sea" due to the rotation of the wind velocity, which causes a nonlinear interaction of surface waves propagating in opposite directions to each other, and the formation of their 2nd harmonic in the form of a standing surface wave is discussed.&#x0D; From the analysis of the wind speed and wind direction profiles along the infrasound arrival path, as well as the acoustic pressure fields calculated by the parabolic equation method for the effective sound speed profiles in the direction of infrasound propagation, the most probable areas of infrasound generation were determined. In both cases these areas coincided with the zones in which the wind speed drops to zero, and the direction of the wind changes to the opposite.&#x0D; An example of simultaneous detection from one direction of the microbaroms with frequencies 0.2–0.3 Hz and "the voice of the sea" with a higher frequency of 5.5 Hz is given.

Local infrasound propagation in three dimensions simulated with in situ atmospheric measurements

Local infrasound propagation is influenced by atmospheric conditions. The vertical gradients of local ambient temperatures and winds can alter the effective sound speed profiles in the atmosphere and dramatically change the focusing and defocusing behaviors of acoustic waves at local distances. Accurate prediction of local infrasound amplitude is critical to estimating explosion energies of natural and/or man-made explosions, and physics-based numerical simulation that accounts for three-dimensional propagation effects should be required for that purpose. The accuracy of a numerical modeling is, however, often compromised by the uncertainty of atmospheric parameters that are used for the modeling. In this study, we investigate the impacts of local atmospheric conditions on infrasound propagation using the data from chemical explosion experiments. In situ atmospheric conditions during the experiments are measured by a combination of (1) local radiosonde soundings, (2) Atmospheric Sounder Spectrometer for Infrared Spectral Technology (ASSIST), (3) surface weather stations, and (4) a wind LIDAR profiler, which can complement atmospheric profiles for numerical simulations and capture local atmospheric variability. We simulate three-dimensional local infrasound propagation using a finite-difference method with the local atmospheric measurements, and the accuracy of the numerical simulations are evaluated by the comparison with the field observations.

Simulating meteorological profiles to study noise propagation from freeways

Forecasts of noise pollution from a highway line segment noise source are obtained from a sound propagation model utilizing effective sound speed profiles derived from a Numerical Weather Prediction (NWP) limited area forecast with 1km horizontal resolution and near-ground vertical resolution finer than 20m. Methods for temporal along with horizontal and vertical spatial nesting are demonstrated within the NWP model for maintaining forecast feasibility. It is shown that vertical nesting can improve the prediction of finer structures in near-ground temperature and velocity profiles, such as morning temperature inversions and low level jet-like features. Accurate representation of these features is shown to be important for modeling sound refraction phenomena and for enabling accurate noise assessment. Comparisons are made using the parabolic equation model for predictions with profiles derived from NWP simulations and from field experiment observations during mornings on November 7 and 8, 2006 in Phoenix, Arizona. The challenges faced in simulating accurate meteorological profiles at high resolution for sound propagation applications are highlighted and areas for possible improvement are discussed.

Comparison and evaluation of physics-based outdoor sound propagation assessment schemes

Evaluating the effectiveness of outdoor sound propagation assessment schemes using experimentally collected data is a complex task. Specifically, when the acoustic data collected assume non-Gaussian distributions with differing sample sizes per assigned class, traditional parametric statistical techniques may not be used. As an alternative, this research introduces an original cost function to evaluate and compare various methods aimed at defining acoustic propagation classes based on meteorological measurements. Class assignments in each scheme are based on the atmospheric stability, the strength of the vertical effective sound speed gradient, and the vertical effective sound speed profile. The acoustic data included in this analysis were generated by a high-energy impulsive source and gathered at source-to-receiver distances of 1, 2, 4, and 8 km. The results indicate that the assessment scheme based on the strength of the effective sound speed gradient most effectively classifies the peak level sound propagation sampled in temperate and desert climate conditions at these distances.

Application of Propagation Modeling to Verify and Discriminate Ground-Truth Infrasound Signals at Regional Distances

An infrasound field campaign was performed in 2011/2012 utilizing single infrasound sensors along the great circle path between a known ground-truth source (Ariane 5 engine test facility, Lampoldshausen, Germany) and a regional receiver (German infrasound array IS26, Bavarian Forest) covering a distance of rough 320 km in total. The gathered recordings provide new insights in the infrasonic wave propagation at regional and near-source distances by comparing measured signals with modeling results within this study. Ray-tracing and parabolic equation approaches are utilized to model infrasound propagation from the ground-truth source to the line profile sensors and explain the obtained detections and non-detections. Modeling and observation results are compared by estimating their amplitude, quantifying amplitude deviations and also considering observed and calculated travel times and celerities. Modeling results show a significant influence of small-scale atmospheric variations in effective sound speed profiles on the propagation pattern, which results in varying tropospheric and stratospheric ducting behavior. A large number of gravity wave profiles are tested to investigate the influences of atmospheric dynamics on the infrasound wave field and improve the modeling results. The modeling is furthermore applied to a case of two potential, contemporaneous and closely spaced infrasound sources. Propagation modeling is used here to resolve the source ambiguity between a ground-based and a higher altitude source giving a strong preference to the latter with respect to the observed infrasonic signatures. The good agreement between modeling and observation results within this study successfully shows the benefit of applying infrasound propagation modeling to the validation of infrasound measurements, verification of ducting behavior and discrimination of infrasound sources.

Probing local wind and temperature structure using infrasound from Volcan Villarrica (Chile)

We use the continuous and intense (∼107W) infrasound produced by Volcan Villarrica (Chile) to invert for the local dynamic wind and temperature structure of the atmosphere. Infrasound arrays deployed in March 2011 at the summit (2826 m) and on the NNW flank (∼8 km distant at 825 m) were used to track infrasound propagation times and signal power. We model an atmosphere with vertically varying temperature and horizontal winds and use propagation times (ranging from 23 to 24 s) to invert for horizontal slowness (2.75–2.94 s/km) and average effective sound speeds (328–346 m/s) for NNW propagating infrasound. The corresponding ratio of recorded acoustic power at proximal versus distal arrays was also variable (ranging between 0.15 to 1.5 for the peak 0.33–1 Hz infrasound band). Through application of geometrical ray theory in a uniform gradient atmosphere, these ‘amplification factors’ are modeled by effective sound speed lapse rates ranging from −15 to +4 m/s per km. NNW‐projected wind speeds ranging from −20 m/s to +20 m/s at 2826 m and wind gradients ranging from −11 to +10 m/s per km are inferred from the difference between effective sound speed profiles and adiabatic sound speeds derived from local temperature observations. The sense of these winds is in general agreement with regional meteorological observations recorded with radiosondes. We suggest that infrasound probing can provide useful spatially averaged estimates of atmospheric wind structure that has application for both meteorological observation and volcanological plume dispersal modeling.

Upward refraction of sound propagating outdoors by a graded index sonic crystal noise barrier

Upward refraction of sound propagating outdoors by a finite set of two-dimensional cylinders has been studied in the homogenization region. All cylinders in the field are located above a ground plane and have their longitudinal axes orientated parallel to the ground. By increasing the cylinder radius with height, a graded index medium is created whose effective sound speed decreases over height. A cylindrical wave propagating inside a structure obeying such properties can be refracted upwards, consequently creating a zone of reduced pressure behind it. Acoustic phenomena for continuous graded index media (GRIN CM), predicted using the Finite Element Method (FEM), are demonstrated. Using modified homogenization formulas the results are then matched with a qualitatively equivalent graded index sonic crystal (GRIN SC), predicted with Multiple Scattering Theory (MST). It is shown that GRIN SC noise barriers potentially can be effective as broadband noise mitigation measures, with a barrier setup filling a cross sectional area not bigger than two meter squared. However, in order to gain a reasonable insertion loss, the barrier thickness must be chosen in accordance with the wavelength of the incoming field and should be in the order of a few wavelengths. Furthermore, the performance of these refractive structures is determined by the shape of the effective sound speed profile in relation to the point of excitation.

Impulse propagation in the nocturnal boundary layer: Analysis of the geometric component

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind jet lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal propagating in such a duct is received at long ranges from the source as an extended wave train consisting of a series of weakly dispersed distinct arrivals followed by a strongly dispersed low-frequency tail. The leading distinct arrivals have been previously shown to be well modeled by geometric acoustics. In this paper, the geometric acoustics approximation for the leading arrivals is investigated. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the distinct arrivals have been determined. The time delay between and relative amplitudes of the direct-refracted and single ground reflection arrivals have been investigated as parameters for an inversion scheme. A two parameter quadratic approximation to the effective sound speed profile has been fit and found to be in strong agreement with meteorological measurements from the time of propagation.

Using relative amplitude and travel times from geometric acoustics to determine nocturnal effective sound speeds.

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind structure lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal undergoes strong dispersion during propagation and is received at long ranges from the source as an extended wave train consisting of a series of distinct arrivals followed by a low frequency tail. The leading distinct arrivals have been shown to coincide with the direct and single reflection geometric ray paths. At a range of 2 Km from the source, these ray paths propagate through the lowest 50–70 m of the atmosphere while larger range arrivals reach higher into the atmosphere. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the arrivals can be determined. Arrival details can then be used to fit a low order Taylor series expansion of the effective sound speed profile. Approximations of the atmospheric conditions in the duct will be presented along with both predicted and measured arrival waveforms and meteorological data from the time of propagation.

Influence of short-term meteorological variations over flat grass-covered ground on noise level fluctuation and application of mean meteorological data to noise prediction

For environmental noise prediction, it is practicable to use meteorological data available from local meteorological observatories. However, these observations have limitations induced by the methods of measuring and data processing. Usually only mean meteorological values averaged over one 10 min period every hour are calculated. To apply these mean meteorological variables to noise propagation appropriately, we need to investigate the characteristics of both acoustic and meteorological parameters within the 10 min period. We made simultaneous measurements of both parameters over flat grass-covered ground and estimated effective sound speed profiles by similarity theory, using the meteorological data measured under conditions similar to those at local observatories. The changes in sound pressure level in periods around sunrise and sunset were similar and were smaller than those around culmination, in which fluctuations of approximately 20 dB were measured at higher frequencies at a distance of 100 m. Noise predictions by the parabolic equation method and sound speed profiles determined from instantaneous meteorological variations generally agreed with the measurements except in the time period around culmination. When we used 10 min mean meteorological values in combination with the parabolic equation method, we obtained reasonable agreement with the measurements at middle frequencies in time periods around sunrise and sunset.

Quantifying the uncertainty of geoacoustic parameter estimates in a dynamic environment using oceanographic data

This paper presents results of Bayesian matched field geoacoustic inversion in a temporally and spatially varying shallow water environment. The acoustic data were collected by a vertical line array at ranges of 1, 3 and 5 km in the SW06 (Shallow Water 06) experiment carried out in August 2006. The sound speed profiles measured at the array and at the source positions during the experiment showed significant variation in the thermocline in the middle part of the water column. The resulting uncertainty in the sound speed profile in the water impacts the performance of matched field inversion methods in estimation of geoacoustic model parameters. In previous work, an effective sound speed profile was estimated in the inversion, based on empirical orthogonal functions (EOFs) derived from a limited number of sound speed profiles measured at the time of the experiment. This paper explores the effect of using a larger number of profiles measured over several days in the SW06 experiment to include a greater diversity of ocean conditions in generating the EOFs. The inversion results for the two approaches are compared in terms of marginal probability distributions of the estimated geoacoustic parameters. (Work supported by Office of Naval Research).

Use of wind and turbulent stability for classifying mean refraction

The functional shapes of mean wind and temperature profiles in the boundary layer above the ground depend most strongly on the wind speed and turbulent stability scale size. By use of these two parameters and the wind direction, it is possible to substantially characterize the mean refraction for sound propagation to distances up to about 10 km. This presentation investigates ranges of values and the sensitivity of the vertical effective sound speed profiles to these variations and considers the mean refraction as an objective measure for classifying horizontal propagation.

Analysis of the effect of small currents on ray stability and caustic formation in a layered moving fluid medium

In studies of underwater sound it is usually sufficient to treat the medium of propagation as a static inhomogeneous medium. Including current and time-dependent changes adds to the overall understanding of the behavior of sound in underwater environments. The quiescent medium approach combines the movement of the fluid with the local sound speed to create an effective sound speed profile. Recently it has been demonstrated that the effects of small-scale fluid motion on the propagation of sound in the ocean can be of the same order of magnitude as that of the static sound speed profile [J. A. Colosi, J. Acoust. Soc. Am. 119, 705–708 (2006)]. The stability of the acoustic ray equation in a moving fluid medium depends on the first and second derivatives of the sound speed profile and fluid velocity [D. R. Bergman, Waves Random Complex Media 15(4), 417–435 (2005)]. In this presentation the effects of weak but rapidly changing currents on the stability and intensity of the acoustic field are investigated for environments with low Mach number. The main result will show that in real underwater environments small currents cannot be ignored when modeling or analyzing the propagation of sound.