An Ultrasonic Shutter

Abstract

The nearly total transmission of a sound wave through a plate having a thickness of an even number of quarter-wavelengths, and the nearly total reflection from the plate when the thickness is an odd number of quarter-waves, was established mathematically by Rayleigh nearly a century ago. This basic theory becomes complex when applied to some types of piezoelectric plates with their combined electrical and mechanical resonant and antiresonant frequencies. A simplified electromechanical analogy of this type plate is presented to explain how the introduction of a variable electrical load can control the frequency of optimum sound transmission through the plate by varying its effective mechanical impedance. The use of a plate of barium titanate (resonant at 1100 kilocycles) in a water channel demonstrates how the intensity of a sound beam may be controlled over a 40-decibel range by the use of a variable inductance across the plate electrodes. A carbon microphone wired directly to the plate electrodes and affording only passive control permits amplitude modulation of the sound carrier passing through the plate.