Low-frequency Propagation Research Articles

Low-frequency non-reciprocal sound propagation features in thermoacoustic waveguide.

Thermoacoustic waveguides are systems of hollow tubes and thermally graded porous segments that can operate as active materials where acoustic waves receive energy from an external heat source. This work demonstrates that by adjusting the pore geometry several unique low-frequency propagation features arise from the complex-valued band structure of periodic thermoacoustic waveguides that reflect into the acoustic pressure field within finite-length systems. Numerical methods have been employed to model waveguides with porous segments constituted by cylindrical inclusions (parallel pins). In periodic structures, a critical frequency emerges where the sign of the refractive index in one direction of propagation changes, thus zero- and negative-unidirectional refractive index, unidirectional energy transport, and amplification/attenuation crossover effects may take place. On the other hand, the study of the acoustic pressure field shows that, for wave packets with either direction of propagation, finite-length waveguides may behave as active acoustic metamaterials with zero- or negative-refractive index. The acoustic pressure field in the waveguide, generated by an upstream source, may exhibit increasing amplitude and phase recovery farther away from the source, mimicking the field created by a downstream source, propagating upstream in a non-active medium.

A 3-D Global FDTD Courant-limit Model of the Earth for Long-time-span and High-altitude Applications

A new global 3-D finite-difference time-domain (FDTD) model is introduced to simulate electromagnetic wave propagation around the Earth, including the lithosphere, oceans, atmosphere, and ionosphere regions. This model has several advantages over existing global models, which include grids that follow lines of latitude and longitude and geodesic grids comprised of hexagons and pentagons. The advantages of the new model include: (1) it may be run at the Courant-Friedrichs-Lewy (CFL) time step (as a result, it is termed the Courant-limit model); (2) subgrids may be added to specific regions of the model as needed in a straight-forward manner; and (3) the grid cells do not become infinitely larger as the grid is extended higher in altitude. As a result, this model is a better candidate than the others for investigating electromagnetic phenomena over long time spans of interest and for investigating atmosphere-ionosphere-magnetosphere coupling. The new model is first described and then validated by comparing results for extremely low frequency (ELF) propagation attenuation with corresponding analytical and measurement results reported in the literature.

Does the moon meddle with the lower ionosphere?

At night free electrons of the lowest ionosphere are either lost by recombination with positive ions, or by attachment to neutrals thus forming negative ions. Whereas for ionisation photons of at least UV energy and/or energetic particles are required, visible light suffices to detach electrons from neutrals. One could compare mesurements of nights with and without moonlight, but one could never rule out that other factors such as temperature might also have changed between the two nights. Recent total lunar eclipses provide the opportunity to test the influence of moonlight assuming that the mesosphere does not change otherwise in the night under consideration. Results of several low frequency propagation paths are analysed, some of which indeed show ionospheric signatures of the transient disappearance of the moonlight.

Empirical Parameterization of Broadband VLF Attenuation in the Earth‐Ionosphere Waveguide

AbstractThe Earth‐ionosphere waveguide (EIW) determines the propagation of Very Low Frequency (VLF; ∼3−30 kHz) waves. Characterizing the waveguide is a longstanding challenge due to its large spatial scale and the complex variability of the lower ionosphere. Here we apply a novel linear basis function regression technique to characterize attenuation in the EIW using broadband measurements of lightning‐generated radio waves. The process begins with defining a basis function set, which ideally encompasses a feature set that can predict the variability seen in VLF attenuation properties. With this basis set defined, a system of linear equations is then constructed using sensor pair observations to eliminate the dependency on source amplitude in each observation. Using this formalism, an empirical attenuation model for broadband signals from lightning is constructed and the dependence on attenuation properties with the boundary conditions is explored. The empirically derived results show attenuation rates over ice that are 12 dB/Mm higher compared to paths over saltwater. Well‐known east/west attenuation rate asymmetry stemming from anisotropic reflection coefficients of the ionosphere is also demonstrated and investigated. For example, under a daytime ionosphere, westward‐propagating waves suffer up to 2.8 dB/Mm greater attenuation compared to eastward‐propagating waves. The trained model is used for propagation corrections in a global lightning locating system (LLS), but this technique can be expanded to further study VLF attenuation rates by employing different sets of basis functions.

Seismogenic ULF/ELF Wave Phenomena: Recent Advances and Future Perspectives

There has been enormous progress in the field of electromagnetic phenomena associated with earthquakes (EQs) and EQ prediction during the last three decades, and it is recently agreed that electromagnetic effects do appear prior to an EQ. A few phenomena are well recognized as being statistically correlated with EQs as promising candidates for short-term EQ predictors: the first is ionospheric perturbation not only in the lower ionosphere as seen by subionospheric VLF (very low frequency, 3 kHz f 30 kHz)/LF (low frequency, 30 kHz f 300 kHz) propagation but also in the upper F region as detected by ionosondes, TEC (total electron content) observations, satellite observations, etc, and the second is DC earth current known as SES (Seismic electric signal). In addition to the above two physical phenomena, this review highlights the following four physical wave phenomena in ULF (ultra low frequency, frequency Hz)/ELF (extremely low frequency, 3 Hz frequency 3 kHz) ranges, including 1) ULF lithospheric radiation (i.e., direct radiation from the lithosphere), 2) ULF magnetic field depression effect (as an indicator of lower ionospheric perturbation), 3) ULF/ELF electromagnetic radiation (radiation in the atmosphere), and 4) Schumann resonance (SR) anomalies (as an indicator of the perturbations in the lower ionosphere and stratosphere). For each physical item, we will repeat the essential points and also discuss recent advances and future perspectives. For the purpose of future real EQ prediction practice, we pay attention to the statistical correlation of each phenomenon with EQs, and its predictability in terms of probability gain. Of course, all of those effects are recommended as plausible candidates for short-term EQ prediction, and they can be physically explained in terms of the unified concept of the lithosphere-atmosphere-ionosphere coupling (LAIC) process, so a brief description of this coupling has been carried out by using these four physical parameters though the mechanism of each phenomenon is still poorly understood. In conclusion, we have to emphasize the importance of more statistical studies for more abundant datasets sometimes with the use of AI (artificial intelligence) techniques, more case studies for huge (M greater than 7) EQ events, recommendation of critical analyses, and finally multi-parameters observation (even though it is tough work).

STUDY ON THE DISPERSION CHARACTERISTICS OF WOOD ACOUSTIC EMISSION SIGNAL BASED ON WAVELET DECOMPOSITION

Artificial AE sources were generated on the surfaces of Ulmus pumila, Zelkova schneideriana, Cunninghamia lanceolata, and Pinus sylvestris var.mongolicaLitv. specimens. The AE transverse wave signal was decomposed into 3-layers detail signalsby wavelet decomposition and reconstructed, and it was calculated based on correlation analysis. Then thelongitudinal wave speed was calculated according to the time-difference-of-arrival (TDOA) method, and the wood dispersion phenomenon was studied. The results showed that thedispersion phenomenon of Ulmus pumilawas obvious. The propagation speed of high-frequency signal was 2.38 times that of low-frequency signal. The ratio of high and low frequency propagation speed of soft wood was 1.72 and 1.73. The dispersion degree of Zelkova schneideriana was the weakest, and the propagation speed of the high frequency was 1.25 times of the low one. The ratios of longitudinal and transverse wave speedsof the four specimens were 4.59, 4.07, 4.24 and 4.2, respectively.

Study of acoustic propagation across an oceanic front at the edge of the New England shelf.

Propagation of low-frequency sound across a warm core ring-enhanced oceanic front at lower horizontal grazing angles is presented. The data were collected from an experimental source tow conducted during the New England Shelf Break Acoustics (NESBA) experiments in spring 2021. The 3 h tow track provides spatiotemporal measurements of acoustic propagation through the front across varying geometries. Coincident oceanographic measurements are used to estimate the strong temperature gradient of the water column and three-dimensional (3D) sound speed field. Two-dimensional (2D) adiabatic mode and full-field sound propagation models are utilized to investigate the acoustic sensitivity to the frontal structure. Then, the joint effects of acoustic ducting and bathymetric slope refraction are examined using 3D sound propagation models. Key components of the measured acoustic impulse response are captured in the 3D numerical model, and the sensitivity of low-frequency propagation to the front geometry is demonstrated.

The Effects of Meridian Surface Shape on the Pressure Pulsation of a Multi-Stage Electric Submersible Pump

The influence mechanism of the internal pressure fluctuation propagation law of multi-stage submersible electric pump (ESP) is still unclear, which has been a major problem restricting the stable exploitation of deep-sea oil and gas. In order to investigate the effect of different meridian profiles on the pressure pulsation characteristics of three-stage submersible electric pumps, the unsteady Reynolds-averaged Navier–Stokes (URANS) method is used to numerically investigate it. The results show that the lower the pressure pulsation amplitude in the pump caused by the meridional shape that is more in line with the flow law, has a positive effect on the operation stability. The change of the shape of the meridian greatly affects the pressure pulsation law in the secondary and final pumps. The rotor–stator interaction causes the pressure pulsation amplitude of the monitoring point in the middle of the pump chamber to reach a peak value. By using continuous wavelet transform analysis, it is found that the regularity of 1–2 times frequency conversion is complicated due to multiple pulsation sources and low frequency propagation coupling between stages. At 3–6 times frequency, it is basically close to the pulsation rule of the blade frequency. The above research provides a basis for improving the operation stability of the ESP.

Evidence of 3D effects for along shelf acoustic propagation during SBCEX21

The 2021 Seabed Characterization Experiment (SBCEX21) was performed on the New England Shelf Break (NESB) in May 2021 with the main objective of characterizing the NESB sediment geoacoustic properties. In this talk, we focus on low-frequency propagation (f<100 Hz) along the 200 m isobath line. Time-frequency dispersion of normal modes is estimated using a SUS charge signal recorded on a distant single hydrophone TOSSIT system. Geoacoustic inversion is performed, but two-dimensional (2D) acoustic propagation models fail to capture the observed behavior of higher order acoustic modes, and predict much shorter travel times than are seen in the data. By introducing a three-dimensional (3D) model environment, both parabolic equation and modal ray calculations confirm that the gradual shelf slope has a nontrivial influence on modal travel times. This comparison between 2D and 3D simulations of SBCEX21 acoustics and experimental data emphasizes the importance of 3D effects from sloped bathymetry on understanding propagation in a shelfbreak environment. These effects will need to be accounted for when doing geoacoustic inversion of SBCEX21 along-shelf low-frequency data.

Spatial extent of the energetic electron precipitation region during substorms

The spatial coverage of the energetic electron precipitation (EEP) region during three substorms has been determined using a combination of ground-based instruments in North America. The primary datasets used to determine the presence or absence of EEP are riometer measurements of cosmic noise absorption (CNA); attenuation of the 10–11 MHz atmospheric noise in the D-region ionosphere determined using the Super Dual Auroral Radar Network (SuperDARN); amplitude variations in subionospheric very low frequency (VLF) propagation observed using the Antarctic-Arctic Radiation-belt (Dynamic) Deposition VLF Atmospheric Research Konsortium (AARDDVARK) VLF receivers, and total electron content measurements derived from global navigation satellite systems. Our results show that substorm EEP during moderate geomagnetic conditions can extend considerably equatorward or poleward of the average latitude limits reported in a previous statistical study of substorm EEP and an EEP model. This evidence is provided by the SuperDARN radars and the radar signature is not always accompanied by cosmic noise absorption measured by the riometers. Further work is required to determine which EEP energies and fluxes the SuperDARN radars are sensitive to, but we can conclude that EEP with sufficient flux to attenuate 10–11 MHz radiowaves by ∼5 dB reaches latitudes well equatorward of the statistical EEP latitude limits previously modelled for >30 keV electrons.

Elastic waves in media with bimodular nonlinearity taking into account the effects of reflection from shock fronts

A theoretical study of the propagation of longitudinal strong low-frequency and weak high-frequency elastic waves in non-dispersing solids with a bimodular nonlinearity is carried out, taking into account the effects of reflection from the shock fronts of the wave. Expressions are obtained for the waveform, as well as for the amplitudes, frequencies, and phases of the harmonic components of the perturbation reflected from the discontinuities of the nonlinear wave. Numerical and graphical analysis of the obtained solutions is carried out. It is noted that the experimental study of the effects of wave reflection from discontinuities can be used to determine the nonlinear parameter of the bimodular solids. Keywords: longitudinal elastic waves, bimodular nonlinearity, shock waves, reflection from a discontinuity.

Analysis of horizontally polarized shear waves on a highly inhomogeneous loaded bi-material plate

<abstract><p>The current manuscript critically examines the propagation of horizontally polarized shear waves on the dispersion of a highly inhomogeneous thin bonded bi-material plate when a load due to the Winkler's elastic foundation is prescribed. An analytical procedure of solution is deployed for the study; in addition to the exploitation of effective boundary conditions approach for the asymptotic examination. The overall inference of the current study is the realization of the fact that the vibrational displacements in both layers are enhanced by an increase in the inhomogeneity parameter; at the same time lessened with an increment in the foundation parameter. Moreover, a perfect approximation of the dispersion relation has been realized, with its validity extending to almost the entire low-frequency range. Lastly, the influence of the material inhomogeneity has been noted to affect fundamental mode, as against the presence of the foundation parameter which affects the first harmonic curve. More so, an increase in the two parameters narrows the chances of low-frequency propagation.</p></abstract>

The Micro‐Broadband Receiver (μBBR) on the Very‐Low‐Frequency Propagation Mapper CubeSat

AbstractThe very low frequency (VLF) propagation mapper (VPM) is a 6U CubeSat designed to measure VLF radio waves in Low‐Earth Orbit. The science goals of the VPM mission are to measure VLF signals broadcast by the DSX mission, and to study natural and anthropogenic signals (from lightning and VLF transmitters) in the near‐Earth space environment. The primary payload consists of an electric field dipole antenna deployed to 2 meters in length, and a magnetic search coil deployed 50 cm from the spacecraft. Signals from these two sensors are conditioned by analog electronics, sampled, and then processed digitally into downloadable data products. The VPM mission was launched in January 2020; science operations began in March 2020 and continued through September, when contact with the spacecraft was lost. This paper describes the mission goals and instrument designs in detail, as well as some examples of the VPM data set.

PLetma—Parallel software package for intensive infrasound simulation

The Plateform of the Laboratory of Studies and Modeling in Acoustics (PLetma) is a suite of scientific softwares developed by the partner institutions of LETMA since 2015. Currently under development, it contains four different models, namely 3D ray-tracing, 3D non-linear one-way equation, 3D normal modes and 2D direct Navier–Stokes, as well as user interfaces for model execution and data visualization. The aim of this package is to provide a set of complementary tools for the modeling of broadband infrasound signals in a moving inhomogeneous atmosphere. It allows to cover a large panel of low-frequency propagation problems arising in geophysics or engineering. Implemented with MPI and OpenMP, the package relies on parallelism to improve computational efficiency of simulations. PLetma provides the infrastructure for incorporation of existing propagation and variability modeling capabilities. Comparisons between the four methods have been successfully undertaken on a test case [Sabatini et al., JASA (2016)]. Sensitivity analyses have been conducted to understand the effect of environmental parameters on modeling of long-range infrasound propagation. [This work was conducted within the framework of LETMA (Laboratoire ETudes et Modélisation Acoustique), a Contractual Research Laboratory shared between CEA, CNRS, Ecole Centrale de Lyon, C-Innov, and Sorbonne Université.]

Modeling and observation of low-frequency propagation into warm core ring on the New England shelf slope

Analytical and computational modeling is used to examine propagation of low-frequency sound into a warm core ring oceanic front. When the source is placed outside the front and the receiver is placed inside it, the sound field consists of the refracted path and the horizontal lateral wave, which decays exponentially into the front. The analytical solution is presented in 2D (x, y) assuming an adiabatic vertical mode solution in z, wherein the vertical mode-dependence of the propagation paths into the front are considered. A 3D computational acoustic model examines the additional effects of sloping bathymetry and frontal geometry. The modeled results are compared to experimental measurements of into-front propagation during the March 2021 New England Shelf Break Acoustics Experiment using a low-frequency (fc = 115 Hz), shallow towed chirp signal received on an L-shape vertical and horizontal hydrophone array. [Work supported by the Office of Naval Research.]

The tune of the Universe: the role of plasma in tests of strong-field gravity

ABSTRACT Gravitational-wave (GW) astronomy, together with precise pulsar timing and long baseline interferometry, is changing our ability to perform tests of fundamental physics with astrophysical observations. Some of these tests are based on electromagnetic (EM) probes or electrically charged bodies, and assume an empty Universe. However, the cosmos is filled with plasma, a dilute medium which prevents the propagation of low-frequency, small-amplitude EM waves. We show that the plasma hinders our ability to perform some strong-field gravity tests, in particular: (i) nonlinear plasma effects dramatically quench plasma-driven super-radiant instabilities; (ii) the contribution of EM emission to the inspiral of charged black-hole binaries is strongly suppressed; (iii) EM-driven secondary modes, although present in the spectrum of charged black holes, are excited to negligible amplitude in the GW ringdown signal. The last two effects are relevant also in the case of massive fields that propagate in vacuum and can jeopardize tests of modified theories of gravity containing massive degrees of freedom.

Упругие волны в средах с разномодульной нелинейностью с учетом эффектов отражения от ударных фронтов

A theoretical study of the propagation of longitudinal strong low-frequency and weak high-frequency elastic waves in non-dispersing solids with a bimodular nonlinearity is carried out, taking into account the effects of reflection from the shock fronts of the wave. Expressions are obtained for the waveform, as well as for the amplitudes, frequencies, and phases of the harmonic components of the perturbation reflected from the discontinuities of the nonlinear wave. Numerical and graphical analysis of the obtained solutions is carried out. It is noted that the experimental study of the effects of wave reflection from discontinuities can be used to determine the nonlinear parameter of the bimodular solids.

Evidence of critical dynamics in various electromagnetic precursors

A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on the study of fracto-electromagnetic emissions (fracto-EME) at the MHz band, the ultra-low frequency (ULF) magnetic field variations (<3 Hz) and the subionospheric very low frequency (VLF) propagation anomalies. We present recent analysis results for these electromagnetic signals using two independent methods which are known for their ability to uncover critical dynamics, the recently proposed method of critical fluctuations (MCF) and the natural time (NT) analysis method. Our results show that all three considered electromagnetic signals present critical characteristics from a few weeks up to a few days before the main shock occurrence. On the other hand, signatures for the departure from the critical (highly symmetrical) state towards a low symmetry state, a state during which there is high localization of the EQ preparatory process, have been identified in specific cases for the MHz fracto-EME as well as for the ULF magnetic field variations. Based on a multidisciplinary analysis, a four-stage model of EQ dynamics by means of fracto-EME in the MHz and kHz bands has recently been proposed. The hypothesis that the precursors considered in this article emerge during the spatially extensive phase of EQ preparation, which corresponds to the first stage of the abovementioned four-stage model, as well as their relation with the foreshock seismic activity are discussed.

MHD waves with Landau diamagnetic pressure and Pauli paramagnetizim in degenerate plasmas

Magnetosonic waves are studied in the presence of degenerate pressure due to Landau diamagnetic levels and Pauli spin magnetization with strong magnetic field in quantum degenerate electron- ion plasma. A linear dispersion relation of low frequency propagation wave in the direction of magnetic field is derived that strongly depends on the magnetic field while in classical regime this field has no such a role. In the presence of quantization of orbital motion and spin magnetization, new propagation modes of quantum plasmas are also explored. It is noted that quantum acoustic velocity and spin magnetization energy affect the Alfven mode propagation. The quantum effects are incorporated through the Bohm potential, Landau pressure due to Landau quantization of magnetic field and magnetization energy due to spin effect. The current model in the context of Landau diamagnetic pressure along with spin magnetization is sufficient for studying the astrophysical plasma environment existing in the compact systems e.g., white dwarfs and neutron stars.

Acoustofluidic equations governing fluid-elastic media with suspended particles and shear elastic properties

Understanding physics of complex media is generally helped by one's having a complete set of partial differential equations that govern the motion and interactions within the media. A single scalar wave equation is usually not sufficient for understanding all the acoustic properties of interest. An example of such a set of equations are those developed by M. A. Biot (1956) for low frequency propagation of compressional, shear, and other waves in poroelastic media. An analogous set of equations is here developed for a medium that has three primary phases—(1) a fluid, (2) a gossamer array of thin platelets held together by van-der-Waals forces, and (3) a disperse and somewhat random array of larger solid particles held in suspension by the gossamer array. Marine sediment mud is one example of such a medium. Because of the gossamer array. the medium resists static shear, and its compressibility is primarily caused by the fluid. Using various physical principles, one can formulate different elastical-mechanical equations for each of the three phases, or even for subcategories of the phases (such as for suspended particles within different size ranges). The phases interact via different physical mechanisms, such as viscous forces exerted by the fluid on the suspended particles. Examples are given for the use of these equations and of their implications. [This material is based upon research supported by, or in part by, the U. S. Office of Naval Research under award numbers N00014-15-2039, N00014-18-1-2439, and N00014-19-1-2636.]