Norsonic nor1251, User Documentation

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The Sound Calibrator Nor-1251 principles of operation is shown in
the below block diagram and is as follows:
• The microphone to be calibrated is placed in the coupler of the
calibrator where the sound pressure signal is generated by a
miniature loudspeaker.
• The electric signal—driving the loudspeaker—is generated by
an electronic oscillator.
• The sound pressure generated is measured with a pressure
sensitive silicon sensor 1 . This signal is used to adjust the level of
the oscillator signal. The rear side of the silicon sensor is located
in a separate vented cavity to prevent pick-up of extraneous
noise signals. Noise pick-up is also limited by a bandpass filter
in the feedback path.
• Because of the high stability of the silicon sensor and the elec-
tronic controller, the acoustic signal generated, is virtually inde-
pendent of battery voltage and ambient conditions such as tem-
perature, humidity and atmospheric pressure. The feedback
principle automatically compensates for variation in the equiva-
lent volume of the microphones. Hence, it creates an effective
coupler volume many times the volume given by the mechani-
cal dimensions of the coupler. The system also compensates for
drift in the loudspeaker.
• The acoustic coupler is vented to the inside of the calibrator,
which in turn is vented to the outside for equalization to the
atmospheric pressure. A separate channel vents the rear side of
the reference transducer to the outside of the calibrator.
• A Light Emitting Diode (LED) is illuminated whenever the level
control is in balance. When no microphone is placed in the
coupler, the loudspeaker will generally not be able to generate
and maintain the correct pressure in the coupler. This situation
will be indicated by extinguished LED.
• An electronic circuit inside the calibrator will switch off the power
at approximately five minutes after the ON button was oper-
ated. If you need that the calibrator stays on for a longer time,
keep the ON button depressed—either by depressing it manu-
ally, or by use of adhesive tape.
• If the calibrator fails to establish the correct sound pressure level
(an extinguished LED), or if the battery voltage is too low, the
automatic switch-off circuitry will be activated. Hence the cali-
brator will switch off a few seconds after the microphone has
been removed.
Loudspeaker
Microphone
Vented
cavity
Coupler
Oscillator
Level
controller
Detector BP filter
Amplifier
Pressure
sensor
LED
Operating principles and Characteristics
1) Patent DE 4294875 CE
A simplified block diagram of the Nor-1251