Free-field reciprocity calibration of low-frequency ultrasonic transducers

Abstract

Techniques developed for the calibration of electroacoustic transducers provide many practical applications of the principles studied in advanced undergraduate and introductory graduate courses. This communication describes an implementation of the standard method of reciprocity calibration as presented in the popular text, Fundamentals of Acoustics by Kinsler et al. The measurements were performed on inexpensive off-the-shelf transducers designed to operate near 25 kHz and 40 kHz. The anechoic environment consisted of a 2-m3 box lined with absorbing material whose absorption coefficient was measured by the pulse-echo technique. The air density and the speed of sound were calculated using formulas available in the literature. The resulting values of transducer sensitivity (calibration factor) compared well, within 5 -10 %, with the values specified by the vendor. No accuracy statement was available for the transducers under test, not only because of their price, but mainly because no primary calibration services are available in the low ultrasonic range of frequencies, in contrast to the higher, MHz, range. A recent dissertation by N. Bouaoua, discovered in the course of this research, could fill this void should its procedures be adopted by a standards laboratory.