The Bruel & Kjaer 4230 Sound Level Calibrator is designed for the calibration of 1" and 1/2" diameter microphones. An adapter is placed over the opening for use with 1/2" microphones and is removed when 1" microphones are to be calibrated. The calibrator should be stored and operated in its leather case. The sound pressure level produced by the 4230 is 94 dB re 20 µPa. However, when calibrating a microphone which is to be used for free field measurements, a small correction is necessary to compensate for the difference between free field response at 0° incidence and the pressure response. The correction is -0.4 dB for 1" free field microphones and -0.2 dB for 1/2" free field microphones. This makes the effective calibration levels 93.6 dB and 93.8 dB respectively. No correction is required when calibrating pressure response microphones. Adapters are available so that the B & K 1/4" and 1/8" microphones can be calibrated using the 4230. For 1/4" microphones, the adapter DB 0310 is required. For 1/8" microphones both adapters DB 0310 and DB 0352 are required. In each case the extra adapters fit inside the 1/2" microphone adapter supplied (DB 0311).