FEATURES: ❍ Conforms to IEC 942 (1988) Class 1 and ANSI S1.40–1984 (R 1997) ❍ Robust pocket-size design with highly stable level and frequency ❍ Calibration accuracy ± 0.2 dB ❍ 94 dB SPL, or 114 dB SPL for calibration in noisy environments ❍ Extremely small influence of static pressure ❍ Sound pressure independent of microphone equivalent volume ❍ 1 kHz calibration frequency for correct calibration level independent of weighting networks ❍ Fits Brüel & Kjær 1′′ and 1 / 2 ′′ microphones ( 1 / 4 ′′ and 1 / 8 ′′ microphones with adaptor) ❍ Switches off automatically when removed from the microphone The Sound Level Calibrator Type 4231 is a handy, portable sound source for calibration of sound level meters and other sound measurement equip- ment. The calibrator is very robust and stable and conforms to IEC 942 Class 1 and ANSI S1.40-1984. The Sound Level Calibrator Type 4231 is a pocket-sized, battery oper- ated sound source for quick and di- rect calibration of sound level meters and other sound measuring systems. It fits Brüel & Kjær 1′′ and 1 / 2 ′′ micro- phones and, with adaptors, 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 cal- ibration value is obtained for all weighting networks (A, B, C, D and Linear). The calibration pressure of 94 ±0.2 dB re 20 μPa 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. The design of the 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 optimised to have extremely high sta- bility and independence of variations in static pressure and temperature around the 1 kHz calibration frequen- cy. The result of this is a user-friendly calibrator where exact fitting of the microphone is non critical and the ef- fects of changes in temperature and static pressure are negligible. The calibrator gives a continuous sound pressure level when fitted on a microphone (see Fig. 2) and activat- ed. The sensitivity of the sound meas- uring equipment can then be adjusted until it indicates the correct sound pressure level. The calibrator is automatically switched off when removed from the microphone. A leather protecting case, which does not need to be removed to use the calibrator, is supplied.