Bruel & Kjaer 4231 Sound Calibrator is a pocket-sized, battery operated sound source for quick and direct calibration of sound level meters and other sound measuring systems. It fits Brüel & Kjær 1" microphones and using the removable adapter, 1/2" microphones. With optional adapters, it can be used for 1/4" and 1/8" 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 mPa is equal to 1 Pa or 1 N/m2. The + 20 dB level step gives 114 dB SPL, which is convenient for calibration in noisy environments, or for checking linearity. The design of Type 4231 is based on a feed-back arrangement to ensure a highly stable sound pressure level and ease of use. The feed-back loop uses a condenser microphone (see Fig. 1), which is specially developed for this purpose. This microphone is optimized to have extremely high stability and independence of variations in static pressure and temperature around the 1 kHz calibration frequency. The result of this is a user-friendly calibrator where exact fitting of the microphone is non critical and the effects of changes in temperature and static pressure are negligible.