Severe, shift in the curve can be shown to result from a 5% reduction in the value of the longitudinal mode velocity (Fig. In both cases, shear velocity = 3252 m s -1. Dotted line: longitudinal wave sound velocity = 5647.7 m s -1. Solid line: longitudinal wave sound velocity = 5949 m s -1. Variation of Yp with ka for a steel sphere in water.
In both cases, longitudinal velocity= 5949 m s -t. Dotted line: shear wave sound velocity in steel=3089.4ms -t. Solid line: shear wave sound velocity in steel=3252ms -1. Variation of the radiation force function Yp with ka for a steel sphere in water. 1, which is the result of re-running the computation for conditions identical apart from a reduction of 5% in the value assumed for shear mode velocity. The effect of such possible errors is illustrated by the dotted curve in Fig. Hill, 1975), it is desirable to work in conditions where ka > 5 and here it is important to note the influence of uncertainties in the knowledge of the longitudinal and shear sound velocities. For certain applications, particularly those where the use of a sphere radiometer constitutes a primary standard (e.g. Computations of values of Yp have been carried out by Hasegawa and Yosioka (1969) in terms of the variation with ka (k = wave number of the acoustic wave in the target medium and a = radius of the sphere) and their values for steel (extended by us for the range of ka values between 8 and 26) are shown by the solid line in Figs. The basis of the practical application of the sphere radiometer is that there is a factor Yp which can he calculated from theory and which expresses the ratio between the forward force experienced by a spherical target and the acoustic energy density in the beam which it intercepts. The use of this type of device is valuable in biomedical work (Dunn and Fry, 1973) and it is the purpose of this note to point out some of the problems that may arise in such applications. In the design of a radiation force method for absolute measurement of the average acoustic power associated with a medical ultrasonic beam, the choice of spherical geometry for the target is desirable because it is only in this case that calculation of the inevitable diffraction effects is theoretically tractable. Key Words: Ultrasonics, Acoustics, Radiometry. Without a knowledge of these acoustic parameters of the target, uncertainties may therefore occur in the computation of Y~. (Received 18 September 1975) Abstract-Making use of the theoretical work of Hasegawa and Yosioka, it is demonstrated that errors associated with either of the sound velocities in the spherical target can significantly shift the resonance dips in the variation of the radiation force function Y, with ka.
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