Directivity patterns of laser-generated sound in solids: Effects of optical and thermal parameters

In the present paper, directivity patterns of laser-generated sound in solids are investigated theoretically. Two main approaches to the calculation of directivity patterns of laser-generated sound are discussed for the most important case of thermo-optical regime of generation. The first approach, which is widely used in practice, is based on the simple modelling of the equivalent thermo-optical source as a mechanical dipole comprising two horizontal forces applied to the surface in opposite directions. The second approach is based on the rigorous theory that takes into account all acoustical, optical and thermal parameters of a solid material and all geometrical and physical parameters of a laser beam. Directivity patterns of laser-generated bulk longitudinal and shear elastic waves, as well as the amplitudes of generated Rayleigh surface waves, are calculated for different values of physical and geometrical parameters and compared with the directivity patterns calculated in case of dipole-source representation. It is demonstrated that the simple approach using a dipole-source representation of laser-generated sound is rather limited, especially for description of generated longitudinal acoustic waves. A practical criterion is established to define the conditions under which the dipole-source representation gives predictions with acceptable errors. It is shown that, for radiation in the normal direction to the surface, the amplitudes of longitudinal waves are especially sensitive to the values of thermal parameters and of the acoustic reflection coefficient from a free solid surface. A discussion is given on the possibility of using such a high sensitivity to the values of the reflection coefficient for investigation of surface properties of real solids.Comment: 14 pages, 7 figure

Ground vibrations from high-speed railways: Prediction and mitigation

This book aims to present in one volume the views of leading international experts in ground vibrations from high-speed railways on the current status of the problem of generation and propagation of ground vibrations from high-speed trains and to discuss possible ways of reduction of their environmental impact. The book describes mainly the results of recent academic research, and it is pitched largely at an advanced level. It is assumed that the ideal reader will have a university background in engineering, physics, or applied mathematics. However, some of the chapters have been written in a more practical language, and they are expected to be understood by a more general audience. The intended readership of the book is rather wide. These are scientists and engineers working on prediction and remediation of railway noise and vibration, environmental consultants investigating particular situations associated with environmental impact of railways, local authorities, designers of new railway lines, etc. It will be also useful for university students, railway enthusiasts, and for all members of a general public concerned about topical environmental issues

Remarks on reply to comments on Chapter 12 of "Railway Noise and Vibration: Mechanisms, Modelling and Means of Control", by D. Thompson (with contributions from C. Jones and P.-E. Gautier), Elsevier, 2009

Remarks on reply to comments on Chapter 12 of "Railway Noise and Vibration: Mechanisms, Modelling and Means of Control", by D. Thompson (with contributions from C. Jones and P.-E. Gautier), Elsevier, 200

Ground vibrations from tracked vehicles: theory and applications

Over the last decade, much of attention was paid to the use of traffic-induced ground vibrations for the purposes of remote detection and monitoring of heavy military vehicles, such as tanks and armoured personnel carriers. A promising method of detection and identification is the one using the information extracted from ground vibration spectra generated by heavy military vehicles, often termed as their seismic signatures. The aim of the present work is to discuss the analytical approach to the prediction and interpretation of ground vibration spectra from heavy military vehicles. The analysis in this work is limited to tracked vehicles only. The modelling of track periodic irregularities (discontinuities) is considered from the point of view of their effect on the resulting dynamic forces applied from a vehicle to the ground and on the generated ground vibration spectra. Also, the effects of layered ground structure and of changes in wheel tyre compliance are considered for different values of ground and tyre parameters. It is shown that the obtained ground vibration spectra contain clearly identifiable peaks at frequencies associated with some characteristic vehicle parameters, such as track periodicity, distance between the nearest wheel axes, and vehicle speed. Although the relative amplitudes of these peaks vary with changes in layered structure, track irregularity profile, or tyre compliance, their positions on the frequency axis are robust and depend on track periodicity, distance between the nearest wheel axes, and vehicle speed only. A discussion follows on possible use of these spectral peaks for determination of these vehicle parameters and thus for vehicle identification

Localized vibration modes propagating along edges of cylindrical and conical wedge-like structures

Localized vibration modes propagating along edges of cylindrical and conical wedge-like structure

Generation of low-frequency ground vibrations by sound waves propagating in underground gas pipes

The hypothesis is examined about sources of disturbing low-frequency hums arising from buried gas or petrol pipes in which turbulent flows of gas or liquid generate sound waves of high amplitude propagating in pipe-lines as in waveguides. Theoretical investigation of this problem shows that if the velocities of sound inside the pipes (450 m/s for methane) are higher than the velocities of Rayleigh surface waves in the ground (typically 300-600 m/s) then ground Rayleigh waves are efrectively generated by sound waves propagating inside the pipes, the mechqnism of generation being similar to that of sonic boom from supersonic jets. The Rayleigh waves then propagate to buildings and cause building vibration and structure-borne noise. centralfrequencies of generated Rayleigh wave spectra are in the range of 5-20 Hz and depend on pipe-depth. The amplitudes of ground vibration velocity may achieve 70 dB (relative to lTs m/s). This is quite enough to annoy some people both due to the direct impact of vibrations and to structure-borne noise. The results obtained may contribute to a fuller understanding of the noture of low-frequency hums

Basic principles of sound radiation and scattering

The book gives a brief account of the theory of radiation and scattering of sound in liquids and gases. General principles of radiation and scattering of acoustic waves are considered, including Huygens’ principle, the reciprocity theorem, the problem of the existence and uniqueness of solutions. Acoustic fields generated by some complicated radiators are analysed in detail. Basic definitions and facts relating to the scattering of sound by an infinite cylinder, sphere, gas bubbles in liquids, etc. are considered as well. Much of attention is paid to the general theory of scattering with respect to the scattering of acoustic waves. These include the method of boundary integral equations and the methods based on the approximate solutions of the equations of Lippmann - Schwinger type. Considered are also energy conservation issues at wave scattering and the problems linked to causality and Kramers - Kronig relations. The book is intended for students and researchers working in the field of acoustics. (Abstract translated from the Russian)

Generation of ground vibration boom by high-speed trains

Railway-generated ground vibrations cause significant disturbance for residents of nearby buildings even when generated by conventional passenger or heavy-freight trains [1,2]. If train speeds increase, the intensity of railway-generated vibrations generally becomes larger. For modern high-speed trains the increase in ground vibration intensity is especially high when train speeds approach certain critical velocities of waves propagating in a track-ground system. The most important are two such critical velocities: the velocity of Rayleigh surface wave in the ground and the minimal phase velocity of bending waves propagating in a track supported by ballast, the latter velocity being referred to as track critical velocity. Both these velocities can be easily exceeded by modern high-speed trains, especially in the case of very soft soil where both critical velocities become very low. As has been theoretically predicted by the present author [3,4], if a train speed v exceeds the Rayleigh wave velocity cR in supporting soil a ground vibration boom occurs. It is associated with a very large increase in generated ground vibrations, as compared to the case of conventional trains. The phenomenon of ground vibration boom is similar to a sonic boom for aircraft crossing the sound barrier, and its existence has been recently confirmed experimentally [5,6] (see also chapter 11). The measurements have been carried out on behalf of the Swedish Railway Authorities when their West-coast Main Line from Gothenburg to Malmö was opened for the X2000 high-speed train. The speeds achievable by the X2000 train (up to 200 km/h) can be larger than lowest Rayleigh wave velocities in this part of Sweden characterised by very soft ground. In particular, at the location near Ledsgärd the Rayleigh wave velocity in the ground was around 45 m/s, so the increase in train speed from 140 to 180 km/h lead to about 10 times increase in generated ground vibrations [5] (see chapter 11). The above mentioned first observations of ground vibration boom indicate that now one can speak about “supersonic” (“superseismic”) or, more precisely, “trans-Rayleigh” trains [7-9]. The increased attention of railway companies and local authorities to ground vibrations associated with high-speed trains stimulated a growing number of theoretical and experimental investigations in this area (see, e.g. [10-13]). 2 If train speeds increase further and approach the track critical velocity, then rail deflections due to applied wheel loads may become essentially larger. Possible very large rail deflections around this speed may result even in train derailment, thus representing a serious problem for train and passenger safety [6,14-16]. From the point of view of generating ground vibrations outside the track, these large rail deflections can be responsible for an additional growth of ground vibration amplitudes, as compared to the above mentioned case of ground vibration boom [7,9,17]. In the present paper we review the current status of the theory of ground vibration boom from high-speed trains. Among the problems to be discussed are the quasi-static pressure generation mechanism, effects of Rayleigh wave velocity and track wave resonances on generated ground vibrations, effects of layered geological structure of the ground, and waveguide effects of the embankments. The results of theoretical calculations for TGV and Eurostar high-speed trains travelling along typical tracks are compared with the existing experimental observations