Abstract: First and second ultrasonic sensors 1A and 2A are arranged facing each other, and these ultrasonic sensors 1A and 2A are provided with first and second reflectors 1 and 2 on their surface, respectively, and thereby, a propagation time t.sub.t of ultrasonic wave in a distance L within a medium subject to measurement is determined from the following equation t.sub.t =(t.sub.4 -t.sub.3 +t.sub.2 -t.sub.1)/2.

Abstract: A portable ultrasonic underwater sensor 15 has a piezoelectric element 21 disposed in a case 22 constructed as a cylinder 24 and having a truncated cone 25 both being integrally and axisymmetrically formed so as to constitute a vibrator 22A. A piezoelectric element 21 and the vibrator 22A are bonded together with a center of the piezoelectric element 21 matches the center of a reverse face of the truncated cone 25 of vibrator 22A, whereby the vibrator 22A is entirely resonant in a vibration mode in which flexional vibration at the center of the truncated cone 25 is a maximum amplitude, and wherein means for holding the vibrator 22A is located along a vibration nodal line on an external side face of the vibrator 22A.

Abstract: A method and apparatus for ultrasonic wave measurement wherein the time interval between like order echoes selected with respect to adjacent transmitted waves is controlled to be equal to an integral multiple of a period (t.sub.3) of a continuous oscillation wave. In addition, the time interval between like order echoes adjacent to each other that are selected with respect to the same transmitted wave is controlled to be equal to the period (t.sub.3) of the continuous oscillation wave. The period (t.sub.3) of the continuous oscillation wave that is obtainable under the above controlled state, is measured, and the result of the measurement is used as ultrasonic wave propagation time for obtaining the absolute sound velocity.