FEATURES: ■ Well defined sound source ■ High calibration level, 124dB SPL at microphone diaphragm * Accurate calibration within ± 0,2 dB ■ Transistorized frequency control * Individually calibrated B Battery operated ■ Fits 1", 1/2", 1/4" and 1/8” microphones USES: ■ Direct calibration of sound measuring equipment * Field and laboratory use The Pistonphone Type 4220 is a small, battery operated, high level precision sound source, which provides quick and accurate direct calibration of sound measuring equipment, tape recordings of sound etc. with an error of less than ± 0,2 dB. It is always ready for use and can be utilized in the field under severe environmental conditions while still maintaining high accuracy. It is also extremely useful as a laboratory standard sound pressure level. The Pistonphone fulfils the recommendations of the IEC on the calibration of precision sound level meters. The calibration frequency, which is 250 Hz with the self-contained batteries, is controlled within ±1% by means of a transistor circuit. The piston arrangement, an original B & K design, consists of two pistons moving in opposite direction and ensures maximum stability and low non-linear distortion (see Fig.1). The operation procedure is simple: Fit the microphone into the coupler of the Pistonphone and push the control switch to the "On" position and the Pistonphone will now produce a constant sound pressure lev- ei on the diaphragm of the microphone. The Pistonphone 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 the B & K 1 ", 1/2", 1/4" and 1/8" microphones and microphones having the same standard diameter, such as the types WE 640 AA, MR 103, etc. Fig.2 shows its use with 1" and 1/2" microphones. The Pistonphone can also be adapted to other types of microphones by means of special adaptors. The sound level produced by the Pistonphone, when fitted to a B & K microphone is 1 24 ± 0,2 dB re 2 x 10-5 Pa. This high level allows correct calibration to be made even in very noisy surroundings. Each pistonphone is individually calibrated at normal atmospheric pressure. A barometer supplied with the Pistonphone gives the ambient pressure correction in dB, in the range 790 to 1060 mbar.