Pistonphone Type 4228

The calibration frequency, nominally 250Hz, is 251.2Hz ±0.1% as defined by ISO 266. The sound level produced by the pistonphone is nominally 124 ± 0.2 dB re 20 mPa at the reference conditions. Each pistonphone, however, is individually calibrated with an uncertainty of less than 0.09 dB and delivered with a calibration chart. When loaded with a microphone of one of the specified types, the uncertainty is less than 0.12 dB. The high sound level from the pistonphone allows correct calibration to be made in the field, even in very noisy surroundings.

The piston arrangement, based on an original Brüel & Kjær design, consists of two pistons mounted on opposite sides of a cam disc. The rotation of the cam disc forces the pistons to move, in phase, in and out of the coupler cavity. The design reduces cam disc eccentricity and harmonic distortion, and ensures maximum level stability.

The pistonphone is very simple to operate with only one control switch. It can be held in one hand in any position, while, with the free hand, the sensitivity of the sound measuring equipment is adjusted until a reading corresponding to the sound pressure level produced is obtained.

The pistonphone fits Brüel & Kjær 1²-inch, 1/2-inch, 1/4-inch and 1/8-inch microphones, and microphones having the same standard diameter (for example, Types WE-640AA, MR 103). Fig. 2 shows its use with 1-inch and 1/2-inch microphones. A barometer supplied with the pistonphone gives the ambient pressure correction in dB, in the range 650 to 1080 hPa, provided it is checked annually to maintain its accuracy.