Sonic and ultrasonic surface waves in visco-elastic materials

Ultrasonics International 87 abstracts proteins has been studied as a function of frequency in the range 200 to 1200 kHz using spherical resonators of small volume. While most of the solutions produced the expected well-behaved data typical of relaxation processes, some produced anomalous results in which the absorption peaked strongly in the frequency domain and clearly was not arising from relaxation processes. It is shown that the effect is not a resonator-borne artifact but depends directly on the conditions in the solution such as temperature, pH, dissolved gas and concentration of specific additives. The sharpened frequency-dependence of the absorption resembles that of a resonance rather than a relaxation process. The theory of resonance absorption was discussed in a general way by Litovitz in 1959, but the experimental observation of the effect has been reported in the literature only rarely specifically for the simple organic liquid toluene and for a much more complex system consisting of an aqueous suspension of spirogyra. There has not been an attempt to quantify the effects or to explain them on a physical or molecular basis. We shall present results showing these "resonance' for several solutions but notably for aqueous solutions of glutamic acid and ammonium sulphate and of glycine and guanidine hydrochloride. In the case of the glutamic acid-ammonium sulphate mixtures, glutamic acid alone (30 mM) at pH 3.5 shows a broad relaxation absorption centred on a frequency of 400 kHz whereas ammonium sulphate (1.5 M) has almost negligible loss in the neighbourhood of this frequency. The mixture of the two, on the other hand causes the loss to peak at about 400 kHz and to fall to zero at 370 and 520 kHz on either side. This result cannot be explained in terms of classical absorption or scattering theory or in terms of altered solvation of the amino acid brought about by the ammonium sulphate. Even more unusual results are found for the glycineguanidine hydrochloride mixtures. Addition of the protondonating guanidine ion not only reduces the absorption of the mixture to negligible values at frequencies in the neighbourhood of 430 kHz for low ultrasound transducer power, but for higher powers actually reduces it to the point where it represents an effective acoustic gain over the system filled with pure water. (In this frequency range water has one of the lowest acoustic losses). The material specificity of the process seems to rule out anomalous resonator behaviour. The glycine-guanidine mixtures were quite viscous in contrast to water near its boiling point which is the only known case where the classical Stokes value of minimum loss is approached. It is suggested that stimulated sound scattering has been observed. The theory of such a process, which is akin to Raman scattering at optical frequencies, has been developed in recent years by Bunkin, Lyakhov and coworkers. The exact nature of the scattering centres is not yet clear. While they may be associated with micro-phases present in the solution, we consider that coherent behaviour related to protontransfer reactions, which are known to occur in these biological systems, is very likely. The observation offers new possibilities in the fields of molecular and bio-acoustics for stimulated scattering and the selective control of ultrasound absorption.

Sonic and ultrasonic surface waves in visco-elastic materials

H. W. Jones, H. W. Kwan and E. Yeatman, Dalhousie University, Nova Scotia, Canada This paper describes the properties of Lamb and Rayleigh waves in visco-elastic fluids. In such fluids the shear modulus is frequency dependent in both its real and imaginary parts.

This leads to unusual propagation effects in the acoustical waves which depend on such properties. A short discussion of some limited experimental studies of mode conversion experiments designed to launch such waves will be given.

Some results on wavefield backpropagation

T.J. Teo and J.M. Reid, Drexel University, USA Wavefield backpropagation can be accomplished using angular spectrum decomposition. If evanescent waves are ignored, a reciprocity can be established which provides symmetry between the forward propagation and backpropagation. This result, which was originally established in optics, can also be applied to ultrasound ~'2. We extend this result to the case of nonmonochromatic propagation in this work. Wideband wavefield backpropagation is especially useful when finite apertures are used. Both lateral and axial resolution will deteriorate when only finite apertures are available in field measurement. We will show how axial resolution deteriorates more rapidly than does lateral resolution with decreasing aperture. Wideband backpropagation can substantially improve the axial resolution. The theoretical predictions on the improvement in axial resolution with bandwidth agree fairly well with results from computer simulations 2. References 1. Shewell, J.R. and Wolf, E. Inverse diffraction and a new reciprocity theorem J Optical Soc America (1968) 15961603 2. Teo, T.J. and Reid, J.M. Multifrequency holography using backpropagation Ultrasonic Imaging (in press)

Acousto-optics Experimental testing of truncated Raman-Nath system solutions

A. Defebvre, R.A. Mertens, W. Hereman, J.P. Ottoy, Instituut voor Theoretische Mechanica, Gent, Belgium The aim of this paper is to test experimentally some theoretical solutions of the Nth order approximation (NOA) method, applied to the diffraction of light by high frequency ultrasonic waves, having both large amplitude and beamwidth. I. Theory. We summarize some recent methods of solution of a/Vth order truncated system of Raman-Nath equations 1: 2d~>p/d~ -- (q~p- 1 -- ~p+l ) = ip2p~p p = 0 , 1 . . . . . N (¢N+1 --~0) with boundary conditions Cp(0) = &p0, with Cp = amplitude of the diffracted lightwave of order p, ~ = 2~zAn/~. (v = ~z= L), p = ~.2/noA2An (~ and A: light and ultrasonic wavelengths respectively, L = beamwidth, An = peak modulation index). (1) The modified" Laplace transform' method2,3: the solution of the truncated system is assumed to be convenient for the complete one and is written as

~p(~) =tiP ~ aPJp+t(fl~) t=0

Jn(z) being the Bessel function of order n, fl (with 1 < fl < 3) an accelerating convergence coefficient, the a P are derived from recurrent relations and expressed as numerical coefficients independent from p and ;. (2) The eigenvalue method 4, where the intensities of the successive diffraction order are given by

In(v) = &no - 4(1 + &.0)x

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