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The Sound Level Calibrator Type 4231 is a pocket-sized, battery operated sound source for quick and direct calibration of sound level meters and other sound measuring systems. It fits Bruel & Kjaer 1" and 1/2" microphones and, with adaptors, it can be used for 1/4" microphones as well. The calibration frequency is 1000 Hz (the reference frequency for the standardized international weighting networks), so the same calibration value is obtained for all weighting networks (A, B, C, D and Linear). The calibration pressure of 94 +/- 0.2 dB re 20 uPa is equal to 1 Pa or 1N/m2. The +20 dB level step gives 114 dB SPL which is convenient for calibration in noisy environments.