Abstract

The nonspecular reflection of acoustic beams at critical angles, especially at the Rayleigh angle, has been widely studied. An interesting detail, the focal shift of convergent ultrasonic beams, was recently predicted by Bertoni et al. [Trait. Sign. 2, 201–205 (1985)]. Their analysis shows that the axial displacement could be much higher than that of the better known lateral Schoch displacement. However, because the acoustic fields of well‐collimated beams change much slower along the axis of the beam than in the laterial direction, this effect is less pronounced. It is shown that by carefully choosing the acoustical and geometrical parameters of a convergent beam, this otherwise rather weak effect is readily detectable. The cylindrically focused beam of a 10‐MHz transducer of φ10‐mm diameter and 100‐mm focal length was reflected from a water‐aluminum interface. It is shown by Schlieren photography, that much below, exactly at, and much above the Rayleigh angle there is no focal shift, while slightly below and above the Rayleigh angle the focal shift was found to be a few centimeters in negative and positive directions, respectively, which is in good qualitative agreement with theoretical predictions.

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