CONCENTRATED EMISSION OF ULTRASONIC WAVES

For the purpose of concentrating the energy of an ultrasonic beam, Gruetzmacher ** used a quartz plate which, unlike ordinary plane-parallel plates, was cut in the form of a part of a spherical surface (inner radius 10 cm, outer 10.8 cm, length of the subtending chord 7 cm). Placing this plate in a vessel with oil, with the concave side upward, and exciting it from a 0.5 kW generator, the author observed the changes in the fountain formed on the surface of the oil as the thickness of the oil layer was varied. With thin layers the fountain did not differ from the usually observed fountains: it was several millimeters high and consisted of a row of little fountains distributed over the entire surface of the plate. As the thickness of the layer was increased, the area occupied by the little fountains decreased, but their intensity increased. Finally, at a layer whose thickness coincided with the radius of the plate, so that the focus of the ultrasonic beam was just on the surface of the oil, the area of the fountain sharply decreased, while its intensity greatly increased (the height of the fountain reached 40 cm). With a further increase in the thickness of the oil layer, the intensity of the fountain again decreased, while the area occupied by it increased. The author developed a manometric method that made it possible to compare the energy at the focus of the beam with the energy at separate points of it (obtained with a very thin layer of oil above the plate); by

** Gruetzmacher, Z. Physik, 96, 542, 1935.

According to the author’s measurements, the energy in the fountain increased 161-fold. Although this measurement method is somewhat disputable (especially from the quantitative standpoint), the very fact of the concentration of the beam is beyond doubt and deserves the attention of all workers who need powerful beams of ultrasonic waves.

N. Malov, Moscow