Extending the frequency range of the national physical laboratory primary standard laser interferometer for hydrophone calibrations to 60 MHz

A method for the primary calibration of hydrophones in the frequency range up to 60 MHz is described. The current National Physical Laboratory (NPL) p rimary standard method of calibrating ultrasonic hydrophones from 500 kHz t o 20 MHz is based on optical interferometry. The acoustic field produced by a transducer is detected by an acoustically transparent but optically refl ecting pellicle. Optical interferometric measurements of pellicle displacem ent at discrete frequencies in tone-burst fields are converted to acoustic pressure, and the hydrophone for calibration is substituted at the same poi nt, allowing sensitivity in volts per pascal to be obtained directly. For c alibrations up to 60 MHz, the interferometer is capable of measuring the di splacement of the pellicle as a function of frequency in a harmonically ric h nonlinear field up to and including the 12th harmonic of the shocked fiel d generated by a 5 MHz, focusing transducer, allowing hydrophones to be cal ibrated by substitution in the same field. Sources of uncertainty in the ne w method have been investigated. Best combined random and systematic uncert ainties at the 95% confidence level for the new method are 7% at 20 MHz, 11 % at 40 MHz, and 16% at 60 Mhz.