ULTRASONIC PROPAGATION, SCATTERING, AND DEFOCUSING IN SUSPENSIONS

Fine resolution narrow-band pressure amplitude distribution measuremen ts have been performed using a hydrophone in the field of a weakly foc used bowl transducer radiating into castor oil and 1 % and 2 % by volu me suspensions of 320-mum polystyrene beads in castor oil over the fre quency range 1.0 to 2.5 MHz at 20-degrees-C. The axial distributions p ermitted determination of the excess attenuation due to scattering in the suspensions as a function of frequency. The frequency range includ ed a resonance. The excess attenuation measured was compared with the theoretical predictions of Waterman and Truell [J. Math. Phys. 2, 512- 537 (1961)] and good agreement was obtained. The values of the excess attenuation obtained were used to predict axial pressure amplitude dis tributions in the suspension. Although the excess attenuation was dete rmined from far-field measurements, the agreement between the predicte d axial distributions and those measured experimentally extended well into the near field. Lateral distributions were measured at seven freq uencies at the position of the geometrical center of curvature of the source and at the position of the true focus in the 1% suspension. Com parison with similar scans in castor oil alone revealed no detectable defocusing due to the presence of the scatterers.