Use of reciprocity to characterize ultrasonic transducers in air above 100 kHz

The three-transducer free-held 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 determinations of receive and transmit sensitivity with predictions obtained from a mathematical model for the test transducers and calibration system. It is believed that these are the first comprehensive, broadband experimental determinations of absolute transmit and receive sensitivity to appear in the technical literature for transducers operating in air over the frequency range 100-500 kHz. 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 these inferred from measurements of transducer admittance and these predicted by the transducer model. It was concluded that both the extended reciprocity calibration technique and the transducer model were accurate to within this level of discrepancy. (C) 1998 Acoustical Society of America.