摘要:

Currently available films, filmloops, slides, overhead transparencies, and microcomputer software are listed for those who teach courses involving vibration and sound. The topics covered and addresses of the suppliers are given.

ISSN:0001-4966    

DOI:10.1121/1.391769

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A model of acoustic propagation in solid media has been derived. It is a one‐way wave equation based on a high‐order approximation, called a Padé approximation, to the square‐root function. The physical properties of the environment are modeled as stratified, thin, homogenous layers. The model can be applied to obtain an approximate solution in range‐dependent environments by allowing the properties of the layers to vary in range. Furthermore, the effects of rough or sloping reflecting interfaces can be approximated using an equivalent reflector, consisting of two thin layers whose properties vary in range.

ISSN:0001-4966    

DOI:10.1121/1.391770

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Using the plane‐wave decomposition of the incident field and theT‐matrix method, scattering by a partially illuminated, doubly periodic, doubly infinite surface is investigated. The general expressions thus derived are simplified for the case when the illuminated area is large. Numerical examples illustrating the applicability of the procedure are also given.

ISSN:0001-4966    

DOI:10.1121/1.391771

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A time‐dependent concentrated force applied obliquely on the surface of a plate generates elastic waves in the plate. The determination of the location, orientation, and time history of the force from the transient wave records is referred to as the inverse source problem of elastic waves. This paper presents an iterative method of deconvolution which determines the orientation and time‐dependent amplitude of the force from the transient response of the plate surface at a minimum of two locations, the source location being given. Numerical results are presented for forces with various orientations and time histories, and for synthetic data both with and without noise.

ISSN:0001-4966    

DOI:10.1121/1.391772

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The time harmonic Green function for a point load in an unbounded fluid‐saturated porous solid is derived in the context of Biot’s theory. The solution contains the two compressional waves and one transverse wave that are predicted by the theory and have been observed in experiments. At low frequency, the slow compressional wave is diffusive and only the fast compressional and transverse waves radiate energy. At high frequency, the slow wave radiates, but with a decay radius which is on the order of cm in rocks. The general problem of scattering by an obstacle is considered. The point load solution may be used to obtain scattered fields in terms of the fields on the obstacle. Explicit expressions are presented for the scattering amplitudes of the three waves. Simple reciprocity relations between the scattering amplitudes for plane‐wave incidence are also given. These hold under the interchange of incident and observation directions and are completely general results. Finally, the point source solution is Fourier transformed to get the solution for a load which is a delta function in time as well as space. We obtain a closed form expression when there is no damping. The three waves radiate from the source as distinct delta function pulses. With damping present, asymptotic approximations show the slow wave to be purely diffusive. The fast and transverse waves propagate as pulses. The pulses are Gaussian‐shaped, which broaden with increasing time or radial distance.

ISSN:0001-4966    

DOI:10.1121/1.391773

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The fundamental acoustic logging problem of a pulsed point source surrounded by both vertical and horizontal boundaries is solved with the hybrid method. The hybrid method yields the complete synthetic waveform including the head wave and the normal mode arrivals. The essence of the hybrid method consists in converting the head wave branch‐cut contribution into discrete modes which, together with the normal modes, form a complete basis for the solution in each region. Boundary conditions at the horizontal bed boundary then couple the modes between different regions and enable the solution to be expressed in terms of reflection and transmission matrices of modes. Numerical results are illustrated as a function of the formation parameters. Transmission coefficients of head waves are calculated. The advantages of the hybrid method are also discussed.

ISSN:0001-4966    

DOI:10.1121/1.391774

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The importance of the spatial extent of applied forces in the topic of structural radiation is studied through formulation of the sound power radiated by a line force acting on an infinite elastic beam. The expression for sound power is integrated numerically and the results examined as a function ofkL, the acoustic length of the force, and the wavenumber ratio. Below coincidence the power was found to be generally proportional to (kL)−2. Near and at coincidence, sufficiently large force scales have a significant effect on the power produced. In particular, when the length of the force is an integral multiple of the critical wavelength, the coincidence peak is completely suppressed. Above coincidence, minima in the second power occur when the force length is a multiple of the wavelength of the free bending wave. These minimal values of power are shown to be nearly equal to the power radiated far below coincidence. These results are discussed in terms of the coupled structural/acoustic wavenumber response function and the wavenumber spectrum of the applied force. In addition the relative effects of light fluid loading on the radiation at coincidence are presented.

ISSN:0001-4966    

DOI:10.1121/1.391775

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Measurements of acoustic impedance have been made on a series of small, circular orifices (0.1–0.3 mm radius) in thin plates (0.038 and 0.38 mm thickness). Both real and imaginary components, over the frequency range 0.6–3 kHz, were obtained using an impedance tube technique. The zero frequency limit of resistance was measured with a flow resistance device. Only small amplitude acoustic signals were considered. For the range of orifice radii and frequencies selected the boundary layer thickness is comparable to the radius, and the usual high‐frequency expressions for orifice impedance do not apply. Within the limits of experimental error (approximately 12% for resistance and 6% for reactance) the measured values are in reasonable agreement with Thurston’s general but approximate theory [G. B. Thurston, J. Acoust. Soc. Am.24, 653–656 (1952)]. The measurements support the simple idea thatbothreal and imaginary components of orifice impedance can be calculated assuming an effective total end correction 16a/3π, whereais the orifice radius.

ISSN:0001-4966    

DOI:10.1121/1.391776

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

An experimental investigation of the propagation of sound in the presence of an elevated temperature inversion has been conducted on a farm site in southern Alberta. The refractive focusing of sound in the inversion layer was observed to cause shadow zones and regions of sound enhancement in which sound levels were up to 20 dB lower and 20 dB higher, respectively, than during normal afternoon conditions. Fluctuations of 35 dB in the sound intensities at microphones 1730 and 2180 m from the source have been witnessed as the location of the focusing region changed. Ray tracing analysis for meteorolgical data acquired predicts excess attenuations of sound consistent with those recorded. The use of a monostatic acoustic sounder proved useful in correlating anomalous sound levels with the appearance of temperature inversions.

ISSN:0001-4966    

DOI:10.1121/1.391777

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Burgers’ equation, an equation for plane waves of finite amplitude in thermoviscous fluids, is generalized by replacing the thermoviscous termAut’t’(Ais the thermoviscous coefficient,uparticle velocity, andt’retarded time) with an operatorL(u). This operator represents the effect of attenuation and dispersion, even if known only empirically. Specific forms ofL(u) are given for thermoviscous fluids, relaxing fluids, and fluids for which viscous and thermal boundary layers are important. A method for specifyingL(u) when the attenuation and dispersion properties are known only empirically is described. A perturbation solution of the generalized Burgers equation is carried out to third order. An example is discussed for the case α2=2α1, where α1and α2are the small‐signal attenuation coefficients at the fundamental and second‐harmonic frequencies, respectively. The growth/decay curve of the second harmonic component is given both with and without the inclusion of dispersion. Dispersion causes a small reduction of the component. The extension of the generalized Burgers equation to cover nonplanar one‐dimensional waves is given.

ISSN:0001-4966    

DOI:10.1121/1.391778

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP