On: “Dispersion of body waves in layered media” by I. N. Gupta (GEOPHYSICS, August, 1966, pp. 821–823)

Geophysics ◽  
1967 ◽  
Vol 32 (1) ◽  
pp. 124-125 ◽  
Author(s):  
Yosio Nakamura
Keyword(s):  
Time Delay ◽  
High Frequency ◽  
Layered Medium ◽  
Short Note ◽  
Low Frequency ◽  
Body Waves ◽  
Frequency Limit ◽  
Dispersive Effect ◽  
The Subject ◽  
Low Frequency Waves

In his short note, Gupta has shown that the dispersive effect of a finely layered medium may be responsible for some of the anomalous observations in explosive and earthquake investigations. In his note, the phase velocity of waves propagating perpendicular to a horizontally stratified structure at its low-frequency limit is compared with that at the high-frequency limit, and consequently a time delay for low-frequency waves has been denoted. The following discussion shows by a further calculation that a still greater time delay can be expected in other frequencies. The result will be of further help for the interpretations given in the subject note.

Correspondence between the low- and high-frequency limits for anisotropic parameters in a layered medium

Geophysics ◽  
2009 ◽  
Vol 74 (2) ◽  
pp. WA25-WA33 ◽  
Author(s):  
Mercia Betania Costa e Silva ◽  
Alexey Stovas
Keyword(s):  
Wave Propagation ◽  
Layer Thickness ◽  
High Frequency ◽  
Layered Medium ◽  
Homogeneous Medium ◽  
Low Frequency ◽  
Single Layer ◽  
Vertical Axis ◽  
Frequency Limit ◽  
High Frequency Limit

Wave propagation in a layered medium when the wavelength is much greater than each layer thickness (low frequency) produces a response equivalent to that of wave propagation in an equivalent single-layer medium. This equivalent medium is transversely isotropic with symmetry about a vertical axis (VTI), and the elastic parameters are computed with the Backus averaging technique. Conversely, when the wavelength is comparable to each layer thickness (high frequency), the directional dependence of the phase velocity in the transmission response also can be simulated by replacing the layered medium with a single homogeneous medium with properties derived from a time average. It then can be treated approximately as a VTI medium. To compute the medium parameters, a method based on fitting the traveltime parameters is used. We investigated the relationship between Thomsen’s anisotropic parameters [Formula: see text] and [Formula: see text] computed for the equivalent medium in the low-frequency limit and for the homogenized medium in the high-frequency limit. In our experiments, we used a medium in which layers of only two isotropic materials alternate repeatedly. For the high-frequency limit, we obtained solutions for PP- and SS-wave propagation.

Evaluation of Special Hearing Aids for Deaf Children

1971 ◽  
Vol 36 (4) ◽  
pp. 527-537 ◽  
Author(s):  
Norman P. Erber
Keyword(s):  
Hearing Aids ◽  
High Frequency ◽  
Hearing Aid ◽  
Low Frequency ◽  
Acoustic Stimulation ◽  
Deaf Children ◽  
Frequency Range ◽  
Frequency Limit ◽  
Unpublished Studies ◽  
Published Research

Two types of special hearing aid have been developed recently to improve the reception of speech by profoundly deaf children. In a different way, each special system provides greater low-frequency acoustic stimulation to deaf ears than does a conventional hearing aid. One of the devices extends the low-frequency limit of amplification; the other shifts high-frequency energy to a lower frequency range. In general, previous evaluations of these special hearing aids have obtained inconsistent or inconclusive results. This paper reviews most of the published research on the use of special hearing aids by deaf children, summarizes several unpublished studies, and suggests a set of guidelines for future evaluations of special and conventional amplification systems.

Preferential acceleration of heavy ions from thermal velocities

1968 ◽  
Vol 46 (10) ◽  
pp. S638-S641 ◽  
Author(s):  
D. B. Melrose
Keyword(s):  
Frequency Spectrum ◽  
High Frequency ◽  
Heavy Ions ◽  
Crab Nebula ◽  
Low Frequency ◽  
High Frequency Waves ◽  
Hydromagnetic Waves ◽  
The Crab Nebula ◽  
Low Frequency Waves

The acceleration of ions from thermal velocities is analyzed to determine conditions under which heavy ions can be preferentially accelerated. Two accelerating mechanisms involving high-and low-frequency hydromagnetic waves respectively are considered. Preferential acceleration of heavy ions occurs for high-frequency waves if the frequency spectrum falls off faster than (frequency)−1. For the low-frequency waves heavy ions are less effectively accelerated than lighter ions. However, very heavy ions can be preferentially accelerated, the abundances of the very heavy ions being enhanced by a factor Ai over the thermal abundances. Acceleration of ions in the envelope of the Crab nebula is considered as an example.

Comparison of seismic anisotropy of sedimentary strata at low- and high-frequency limits

Geophysics ◽  
2019 ◽  
Vol 85 (1) ◽  
pp. MR1-MR10 ◽  
Author(s):  
Fuyong Yan ◽  
De-Hua Han ◽  
Tongcheng Han ◽  
Xue-Lian Chen
Keyword(s):  
High Frequency ◽  
Seismic Anisotropy ◽  
Sedimentary Rocks ◽  
Low Frequency ◽  
Layered Media ◽  
Ray Theory ◽  
Frequency Limit ◽  
Anisotropic Properties ◽  
Velocity Anisotropy ◽  
Backus Averaging

The layer-induced seismic anisotropy of sedimentary strata is frequency-dependent. At the low-frequency limit, the effective anisotropic properties of the layered media can be estimated by the Backus averaging model. At the high-frequency limit, the apparent anisotropic properties of the layered media can be estimated by ray theory. First, we build a database of laboratory ultrasonic measurement on sedimentary rocks from the literature. The database includes ultrasonic velocity measurements on sandstones and carbonate rocks, and velocity-anisotropy measurements on shales. Then, we simulate the sedimentary strata by randomly selecting a certain number of rock samples and using their laboratory measurement results to parameterize each layer. For each realization of the sedimentary strata, we estimate the effective and apparent seismic anisotropy parameters using the Backus average and ray theory, respectively. We find that, relative to Backus averaging, ray theory usually underestimates the Thomsen parameters [Formula: see text] and [Formula: see text], and overestimates [Formula: see text]. For an effective layered medium consisting of isotropic sedimentary rocks, the differences are significant. These differences decrease when shales with intrinsic seismic anisotropy are included. For the same sedimentary strata, the seismic wave should perceive stronger seismic anisotropy than the ultrasonic wave.

scholarly journals Propagation of high frequency waves in the quiet solar atmosphere

2008 ◽  
pp. 87-99 ◽  
Author(s):  
A. Andic
Keyword(s):  
Solar Atmosphere ◽  
High Frequency ◽  
Low Frequency ◽  
Magnetic Structures ◽  
Partial Heating ◽  
Fabry Perot ◽  
High Frequency Waves ◽  
Propagation Of Waves ◽  
The Waves ◽  
Low Frequency Waves

High-frequency waves (5 mHz to 20 mHz) have previously been suggested as a source of energy accounting for partial heating of the quiet solar atmosphere. The dynamics of previously detected high-frequency waves is analyzed here. Image sequences were taken by using the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Izana, Tenerife, with a Fabry-Perot spectrometer. The data were speckle reduced and analyzed with wavelets. Wavelet phase-difference analysis was performed to determine whether the waves propagate. We observed the propagation of waves in the frequency range 10 mHz to 13 mHz. We also observed propagation of low-frequency waves in the ranges where they are thought to be evanescent in the regions where magnetic structures are present.

scholarly journals On the Noise Hazard Assessment within the Intermediate Range of the High Audible and the Low Ultrasonic Frequencies

2016 ◽  
Vol 41 (2) ◽  
pp. 331-338 ◽  
Author(s):  
Antoni Śliwiński
Keyword(s):  
Hazard Assessment ◽  
High Frequency ◽  
Low Frequency ◽  
Frequency Range ◽  
Research Activity ◽  
High Frequency Audiometry ◽  
Noise Assessment ◽  
The Subject ◽  
The One ◽  
Assessment Procedures

AbstractIn parallel to the ultrasonic noise assessment procedures and research activity in the field there have appeared several papers in the domain of so called high-frequency audiometry which covers the range of frequencies 8-20 kHz. They are important for recognizing the harmfulness and hazard of the audible high frequency sound components in the same range as the one of the low frequency ultrasonic noise. On the other hand there exists a certain inconsequent situation in the general approach to the problem of ultrasonic noise hazard assessment in work places environment which concerns the convention to include the frequency range of 10-20 kHz to the domain of ultrasonics. The range consists of one third octave bands of central frequencies: 10, 12.5, 16, 20 kHz and conventionally is called low frequency ultrasonic noise though at least the components of the two lowest bands are naturally audible by a majority of population (mainly young people).The paper presents a discussion related to some achievements of the two domains and some conclusions which could be useful for a more consequent description of the subject and could be taken into account in the future regulations for the ultrasonic noise assessment in work places environment.

Evidence for the existence of locally-generated body waves in the short-period noise at the Large Aperture Seismic Array, Montana

1972 ◽  
Vol 62 (1) ◽  
pp. 13-29 ◽  
Author(s):  
H. M. Iyer ◽  
John H. Healy
Keyword(s):  
Statistical Analysis ◽  
Phase Velocity ◽  
High Frequency ◽  
Seismic Noise ◽  
Average Velocity ◽  
Low Frequency ◽  
Body Waves ◽  
Frequency Noise ◽  
High Frequency Noise ◽  
Short Period

Abstract The approximate hexagonal configuration of LASA subarrays enables their use as omnidirectional arrays. This property is used to study the phase velocity of short-period seismic noise at different frequencies. It is found that the noise in the low-frequency band consists mainly of surface waves traveling with average velocities in the range 3.0 to 3.5 km/sec. The high-frequency noise, in the band 0.45 to 1.0 Hz, has an average velocity of about 6.0 km/sec. It is quite likely that the high-frequency noise has the nature of locally-generated body waves. Statistical analysis of Pg velocities observed during a crustal refraction experiment at LASA lends support to this hypothesis.

Vibrational resonance in a discrete neuronal model with time delay

2014 ◽  
Vol 28 (16) ◽  
pp. 1450103 ◽  
Author(s):  
Canjun Wang ◽  
Keli Yang ◽  
Shixian Qu
Keyword(s):  
Time Delay ◽  
Input Signal ◽  
High Frequency ◽  
Phase Synchronization ◽  
Low Frequency ◽  
Neuronal Model ◽  
Vibrational Resonance ◽  
Frequency Signal ◽  
High Frequency Signal ◽  
Neuron System

The effects of time delay on the vibrational resonance (VR) in a discrete neuron system with a low-frequency signal and a high-frequency signal are investigated by numerical simulations. The results show that there exists a delay time that optimizes the phase synchronization between the low-frequency input signal and the output signal. VR is induced by the time delay. Furthermore, the time delay can improve the response to a low-frequency input signal. Therefore, the time delay plays a constructive role in the transmission of a low-frequency signal by inducing and enhancing VR.

Growth of low-frequency waves due to a photon beam in a magnetized electron–positron plasma

1997 ◽  
Vol 58 (2) ◽  
pp. 345-366 ◽  
Author(s):  
QINGHUAN LUO ◽  
D. B. MELROSE
Keyword(s):  
High Frequency ◽  
Random Phase ◽  
Low Frequency ◽  
Photon Beam ◽  
Radio Waves ◽  
Wave Interactions ◽  
Frequency Wave ◽  
Pair Plasma ◽  
Electron Positron ◽  
Low Frequency Waves

The effect of a beam of radio waves of very high brightness passing through a cold, magnetized, electron–positron plasma is discussed. The properties of the natural wave modes in such a plasma are summarized, and approximate forms for the nonlinear response tensor are written down. Photon-beam-induced instabilities of low-frequency waves in the pair plasma are analysed in the random-phase approximation. When three-wave interactions involve two high-frequency waves in the same mode and a low-frequency wave in a different mode, wave–wave interactions are similar to wave–particle interactions in that photons act like particles that emit and absorb low-frequency waves. The absorption coefficients for various low-frequency waves due to a photon beam are evaluated. In a pure electron–positron plasma, photon-beam-induced instabilities can be effective only when either the high-frequency or the low-frequency waves are strongly modified by the magnetic field. The growth of the low-frequency waves is most effective when the high-frequency photon beam has a frequency close to the cyclotron frequency.

Sign in / Sign up

Export Citation Format

Share Document