Abstract: A sonic transducer, having a drive electromechanical transducing element and a pickup electromechanical transducing element or a single transducing element electronically operating as both, is excited through suitable power amplifiers by a voltage controlled oscillator. A phase comparator connected to these transducing elements detects the phase difference between stress and strain in the transducer. Its output is connected to an integrator circuit means which integrates the phase difference signal and applies the integrated signal to control the voltage controlled oscillator. The maximum and minimum values of the voltage which is applied to control the oscillator are independently limited by a pair of diodes connected to sources of two different voltages. An adjustably fixable phase shift circuit is interposed between the pickup transducing element and the phase comparator to permit calibration of the system for optimal operation.