of the rear face of the crystal 60 each of the electrodes 61 64 and 69 may be formed by conductive coatings formed on the faces of the crystal 60 when the crystal is vibrating a feedback signal generated across the feedback and ground electrodes 69 and 61 is carried to the power supply unit 40 by a feedback conductor 68 and the ground conductor 67 the feedback electrode 68 is also included in the cable 42 fig3 is a view of the rear face of the crystal 60 and shows the annular high voltage excitation electrode 64 and the circular feedback electrode 69 referring now to fig4 there is illustrated a block diagram of a circuit for generating the ultrasonic energy to be applied to the patient via the applicator 41 an oscillator 70 is arranged to generate an electrical signal having a frequency appropriate for driving the crystal 60 through an amplifier or driver circuit 72 the crystal ac feedback signal on lead 68 which is representative of the frequency and magnitude of the vibrational energy transmitted by the crystal to the load such as the tissue of the patient shown in fig1 is applied to an automatic gain control circuit agc 74 the ground lead 67 is not shown in fig2 the ac feedback signal is also applied to an ac dc converter 78 the output of the agc circuit 74 is applied to a frequency control input 76 of the oscillator 70 via a high and low pass filter 75 the feedback signal on lead 68 includes a component having a frequency equal to the resonant frequency of the crystal 60 and components having frequencies equal.