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LI0196 – 19 July 2017
Weighting Network
The stated levels for calibrator 42AG are all unweighted levels for the two calibration frequencies,
250 Hz and 1 kHz.
All sound level meters shall display at least the A-weighted sound pressure level. The
A-weighting is a standardized, frequency dependent weighting which originally was developed to
mimic our auditory organ and is specified in the standards for a sound level meter. The nominal
attenuation of an A-weighting network is 0 dB at 1 kHz (reference frequency) and 8,6 dB at 250
Hz.
The nominal A-weighted sound pressure levels for the calibrator will therefore at 250 Hz be 85,4
dB and 105,4 dB, respectively. The attenuations of the C-weighting are for both frequencies 0,0
dB and C-weighted levels therefore correspond to the unweighted levels.
Free-field Microphones
A free-field microphone is designed to compensate for the pressure build-up that takes place
when it is used in a free-field. When calibrated with a sound calibrator at 1 kHz, the output
from the microphone will be lower compared to the output at free field conditions because the
calibrator performs a pressure calibration of the microphone. Therefore, the result of a calibration
of a free-field microphone must be corrected, for a 1/2" microphone (type WS2F) the pressure
increase at free field conditions will typically be 0.15 dB at 1 kHz.
Most sound level meters are designed to show the correct free field sound pressure level. This
means that the indication shall correspond to the level in a free, progressive field before the
sound level meter was placed in the field. The size of the microphone but also the sound level
meter body will modify the pressure just in front of the microphone - especially at higher frequen-
cies. Your sound level meter should therefore be adjusted to 93,8 dB for a 94 dB, 1 kHz calibra-
tion signal.
