5
LI0196 – 19 July 2017
Principles of Operation
The 42AG consists of a sound source – a small loudspeaker – a reference microphone, and an
electronic control circuit. The special reference microphone is extremely stable and independent of
variations in barometric pressure, temperature, and humidity. Together, the reference microphone
and control circuit maintain a constant sound pressure level inside the calibration coupler, making
it possible to calibrate without separately accounting for the ambient calibration conditions and
changes to the load volume.
42AG can produce a sinusoidal signal of 250 Hz or 1 kHz at 94 dB or 114 dB. The environmen-
tal conditions: air pressure, temperature and humidity can be displayed.
The calibrator complies with IEC 60942 (2003), Class 1 and has been designed to serve 1" and
smaller microphones and sound level meters equipped with such microphones.
42AG is designed for use in the field. The accuracy that can be achieved is within ±0.2 dB which
is more than adequate for field checks.
For proper laboratory calibration, we recommend the use of a pistonphone, or sending the micro-
phone to a calibration laboratory.
Sinus generator
Amplifier
Feed-back and
Control Circuit
Coupler
Microphone
to be
calibrated
Venting
Sinus Generator
Amplifier
Feed-back and
Control Circuit
Coupler
Microphone
to be
calibrated
Venting
Fig. 1.
Simplified block diagram illustrating 42AG's principles of operation.
Caution.
The loudspeaker inside the calibration coupler is not protected. Therefore, do not touch
the diaphragm and also take care to prevent any hard objects from entering the coupler as this
may deform or damage the diaphragm.
