GB837274A - Improvements in or relating to method and apparatus for measuring wave propagation time - Google Patents

Abstract

837,274. Acoustic delay lines. ELECTROCIRCUITS Inc. April 20, 1956, No. 12227/56. Class 40 (8). [Also in Group XL (c)] A variable ultrasonic liquid delay line as used in a frequency-modulated radar or ultrasonic testing device (see Group XL (c)) comprises separate transmitting and receiving piezo-electric transducers 55, 57, Figs. 3 and 4 and Fig. 2 (not shown) spaced apart in a liquid medium, e.g. water or mercury, on the axis of an annular metal or plastic member 59 whose internal surface has a right-angled groove, Fig. 4, and comprises a plurality of small arcuate sections having progressively increasing radii of curvature in the horizontal plane so that the contour of the aperture in the horizontal plane is substantially a single-turn spiral. A rightangle reflecting prism 63 is mounted on the axis of the member 59 such that the ultrasonic beam from transducer 55 is reflected outwards by the upper surface of the prism 63 to the upper surface of the groove in the member 59, then downwards to the lower surface of the groove, back to the lower surface of the prism 63 and thence to the receiving transducer 57 so that the . length of the transmission path changes discontinuously when the prism 63 or the member 59 is rotated about the axis. The discontinuous change in path length prevents unwanted Doppler effects; for slow rates of rotation the Doppler effect is negligible and the inner surface of member 59 may then be a smooth spiral. As used in Fig. 2 the test transducer 43 is arranged so that the path length between it and the nearest surface of the sample 49 is equal to the unvarying portion of the transmission path in the delay line. In an alternative form, Fig. 5 (not shown) in which an electrical input is applied through a hybrid network to a common transmitting and receiving crystal transducer, the ultrasonic waves transmitted by the transducer are reflected back thereto by the edge of a spiral cam which is rotated to vary the length of the transmission path.