Abstract:
The three-transducer free-field reciprocity calibration technique has been extended to apply to ultrasonic transducers operating in air at frequencies above 100 kHz. The reciprocity method was extended to apply to this frequency range by correcting for attenuation, which is severe in air, and for diffraction. Validation of the extended reciprocity calibration technique was performed by comparing experimental measurements of receive and transmit sensitivity over the frequency range 100--500 kHz with predictions obtained from a mathematical model for the test transducers and calibration system. It is believed that these are the first measurements of absolute sensitivity to appear in the technical literature for transducers operating in this frequency range in air. Receive and transmit sensitivity measurements obtained with the extended reciprocity calibration technique agreed to within 2.5 dB of model predictions. The location of the resonance peaks in the sensitivity measurements had a maximum difference of 7% from those inferred from measurements of transducer admittance and those predicted by the transducer model. It was concluded that both the extended reciprocity calibration technique and the experimental model were accurate to within this level of discrepancy.