Sensitive Electrodynamic Balance for Measurement of Absorption of Ultrasonic Waves in Liquids

Abstract

In this paper a new method for measurement of absorption of ultrasonic waves in liquids is developed. Its main feature is a low frequency (L.F.) modulated ultrasonic transducer actuating a thin circular disk which is placed at some distance above it within the ultrasonic cell. The disk is suspended by a glass rod from a metallic reed which forms one plate of a small air condenser. The vibrations of the suspended system, which are resonant with the L.F. modulating component, are detected by a crystal controlled oscillator, working at the steep portion of its crevasse. The output of the oscillator is applied to a wave analyzer whose output gives directly a measure of the ultrasonic pressure on the disk. In a further development in the detecting end, a null method is used in which the reed is brought to rest by a L.F. balancing voltage supplied by the same modulator and the zero reading is adjusted by the aid of a cathode-ray oscillograph. Two liquids are chosen for testing the apparatus, namely, benzene and water representing both high and low absorptive liquids, respectively. The values obtained are, 2α/v2 = 16.2×10−16 sec2 cm−1 for benzene, and 2α/v2 = 52×10−17 sec2 cm−1 for water, which compare favorably with the values obtained by the more elaborate pulse method. Moreover, the high sensitivity of the method eliminates all the complications which usually accompany high-power ultrasonic beams.