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Using the Sound Calibrator Nor-1251
T
he Sound Calibrator Nor-1251 is a small, battery operated
sound source for the calibration of microphones and sound
measuring instruments.
The microphone is placed in an acoustic coupler where the
calibrator produces a regulated sinusoidal sound pressure sig-
nal. The calibrator is supplied with an output of 114dB SPL @
1000Hz.
Due to the principle of operation 1 , the calibration level is
virtually independent of ambient conditions like temperature,
atmospheric pressure and humidity within the specified range
of operation. The calibrator complies with IEC60942 Class1 and
has been designed to serve one-inch and smaller microphones
and sound level meters equipped with such microphones.
One-inch microphones fit directly in the calibrator coupler,
while half-inch and smaller microphones are served by applica-
tion of suitable adaptor rings. Adaptor for half-inch microphones
are included, while adaptors for other sizes must be ordered
separately.
To activate the sound calibrator:
• Press the ON button to turn on the calibrator.
The calibrator shuts off by itself after about five minutes. How-
ever, if a microphone has not been inserted or if it is removed,
the unit will switch off after a few seconds.
Your sound measuring device may or may not use a spectral
weighting network (such as the A-weighting network). For sound
calibrators with an output frequency of 1000Hz, no compensa-
tion needs to be made for this.
Operation
Using the Sound Calibrator Nor-1251 is easy and uncompli-
cated.
To calibrate with the Nor-1251, do as follows:
1. Insert the microphone to be calibrated in the coupler of the
calibrator. Use a suitable adaptor, if necessary.
2. If the background noise is high, watch the noise level and
press the ON button to switch on the calibration signal.
Verify that the calibration signal is at least 20dB above the
background noise floor.
3. Adjust the sensitivity of the microphone or measuring instru-
ment to display an excitation sound pressure level correspond-
ing to the level indicated on the calibrator itself.
When the ON button has been operated, and a microphone
inserted, the sound calibrator will stay
on for approximately five
minutes. The on -time will be reduced as the battery-voltage de-
creases.
Proper operation of the calibrator is indicated by the green
LED indicator – see Fig. to the right for details.
If the battery-voltage is too low to guarantee operation within
the specifications, the calibrator will switch off as soon as the
If you Have a Free-field Microphone
Most ½” free-field microphones (as defined by
IEC 61672-1) have a pressure response which gets lower
as the frequency of the input sound signal is increased.
The reason for this is that free-field microphones have
been designed to compensate for their own presence in
the sound field.
For example; when using the Norsonic microphone car-
tridge 1225, at 1000 Hz the sensitivity of your sound meas-
uring instrument should be adjusted to a value 0.15dB
lower than the nominal calibrator output level. This differ-
ence corresponds to the difference between the pres-
sure and the free-field response of the microphone – cf.
the table starting at page 15.
1) Patent DE 4294875 CE