Using a dummy hydrophone, the microphone itself can be calibrated. All Brüel & Kjær Hydrophone Calibrators undergo individual factory calibration making them traceable to NIST.
The hydrophone calibrator works like a pistonphone. A sound pressure is produced in the coupler cavity by oscillating pistons. This arrangement is an original Brüel & Kjær design, and consists of four pistons mounted symmetrically around a cam disc. Rotating the cam disc moves the pistons in and out of the coupler cavity in phase.
The design reduces the effect of cam disc eccentricity and harmonic distortion and ensures maximum sound pressure level stability.
The tone, of 251.2Hz, is electronically controlled for ±0.1%.
At this frequency, the sensitivity of Brüel & Kjær Hydrophones in air and in water is almost identical, so they can be calibrated in air without any significant loss of accuracy.
The sound pressure levels produced in the coupler volumes are typically 156.5, 166, 162 and 151.5 re 1 mPa for Hydrophones Types 8101, 8103, 8100/04 and 8105 respectively. Accurate values for these levels are given on the calibration chart supplied with each calibrator.
Three couplers are provided for calibrating Brüel & Kjær Hydrophones. A coupler (WA-0658, typical sound pressure level 152 dB) for calibrating Hydrophone Type 8106 is available as an accessory. Such high levels mean that hydrophones can be calibrated even in very noisy areas.
The calibration is done by using the dummy hydrophone (DO-0077) and coupler UA-0548. When calibrated, this microphone can be used to monitor the actual sound pressure level inside any coupler volume during hydrophone calibrations.
By comparing the sound pressure levels measured by the microphone and the hydrophone, a more accurate hydrophone calibration can be made. Variations in atmospheric pressure in the range 650 hPa to 1080 hPa can be corrected for, in dB, using the barometer supplied.
The hydrophone calibrator can operate in the temperature range –10°C to +50°C (14°F to122°F) without significant loss of accuracy.
