Hydrophone calibrator
±0.6 dB | Type 4229 Bruel and Kjaer

Hydrophone Calibrator Type 4229 is a precision, high sound pressure source. It provides a quick and easy method to calibrate sound measuring systems employing Brüel & Kjær Hydrophones, in air. If required, sound pressure levels produced in the couplers can be monitored by a 1/2-inch microphone, giving a calibration accuracy of within 0.3 dB. 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 is compact and battery operated, suitable for both field and laboratory use.
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 resulting tone, of 251.2Hz, is electronically controlled to within ±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.