Abstract: For accurate measurements of the sound velocity in liquids, for example water, the propagation delay is measured between an ultrasonic wave which is emitted by a transducer and the wave which returns to the transducer upon reflection. By making the reflector spherical and disposing it at a distance from the transducer which is within 11/2 times the length of the Fresnel region of the emitted acoustic wave, a favorable ratio between the 1 .tau. and the 2 .tau. signal is obtained, which ratio can be further improved when the central portion of the reflector is slightly flattened.

Abstract: A voltage sensitive trigger circuit with a threshold which must be temperature and supply-voltage independent. To this end, a preamplifier is employed which in itself is not only provided with negative feedback but which also receives a current which is derived from that trigger transistor, which in the quiescent condition is conducting.

Abstract: Apparatus for the detection of an acoustic image by means of an election tube including a face plate and an electronically scanned transducer made of a thin foil of polymer material whose acoustic wave velocity of propagation is less than 1.8 that of a liquid imaging medium. The face plate also is made of a polymer material having a velocity of propagation less than 1.8 that of said liquid. Preferably, the transducer foil is much thinner than the face plate and has a thickness approximately 0.5 times the acoustic wavelength.