PURPOSE: To improve sensitivity by receiving acoustic wave transmitting an output signal and separating it into induced desired energy component and undesirable energy component such as electromagnetic disturbance from a source coupled to an output from an input without receiving acoustic conversion.

CONSTITUTION: A burst switch 36 and a gate switch 38 modulate an excitation signal and acoustic component of a sensor 22 is gated. The excitation signal is converted into a series of (r), (f), bursts and undesirable component is gated to be excluded by the switch 38. The switch 36 is driven by a pulse a little shorter than acoustic delay of the sensor 22 by a pulse source 40 and percentage of time emitting acoustic wave of the sensor 22 is made maximum. The pulse from the pulse source 40 is made shorter than length of the acoustic delay, and simultaneous reach of the electromagnetic disturbance and the acoustic wave energy is prevented. The pulse from a signal source 42 is designated as timing for closing the switch 38 while the acoustic wave energy is in an output converter of the sensor 22. Period for calculating presence of the electromagnetic disturbance in the output of the sensor 22 is opened.