Generation of Ultrasound—Part I

Abstract

In the preceding chapter the generation of sound by means of a piston source, i.e. of a solid vibrating surface, was dealt with in some detail. In order to obtain a real sound source from such a piston, ways must be found to excite it into vibration. As is well known, in the audio range, i.e. with the loudspeaker, this is accomplished in most instances by an electrodynamical transducer which generates the vibrations and transfers them to a paper membrane which acts as a ‘piston’. In the ultrasonic range the vibrating piston is usually more compact. Even so, it is not sufficient to excite it just at one single point, otherwise it would be bent and thus not every portion would oscillate with equal phase and amplitude. Furthermore, at higher frequencies virtually none of the component elements are really rigid, instead, nearly all elements behave as transmission lines which exhibit certain resonances due to their limited extension. For this reason, principles of construction which have proven so successful in the audio range can be applied to the generation of ultrasound only to a very limited extent, if at all.