Norsonic nor1256, Instruction Manual

Page: 8 / 16

4 Nor1256
Sound Calibrator
Recalibration
At regular intervals – at least once per year – proper
operation of the sound calibrator should be verified,
preferably by an accredited acoustic calibration labo-
ratory or the Norsonic factory. The calibration should
be performed as specified for periodic verification
in the International standard for sound calibrators
IEC 60942 (2003), Annex B.
The level should preferably be measured with a labo-
ratory standard microphone type LS2, according to
IEC 61094-1, where the pressure sensitivities at the
different frequencies are known with a sufficiently high
accuracy.
Various types and makes of microphones may have
different effective load volume. Due to its working
principle, the Nor1256 has a large effective coupler
volume. The variations in sound pressure level due
to variations in effective load volume among micro-
phones are therefore for most applications insignifi-
cant. An equivalent microphone volume of 250 mm³ is
used as reference load volume for ½” microphones.
This corresponds to most ½” measuring microphones
with protecting grid mounted like the Norsonic types
1220, 1225, 1226, 1227 and 1230. If no adaptor ring is
used, the nominal reference front volume is 1333 mm³.
This corresponds to most 1” measuring microphones
with the protection grid mounted.
The sound pressure levels generated by the calibrator
may be adjusted by the following procedure (NOTE: it
will not be possible to switch the calibrator OFF during
calibration mode):
The nominal attenuation of the A-weighting 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 correspond therefore with the un-
weighted levels.
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
– and eventually the sound level meter body – will
modify the pressure just in front of the microphone –
especially at higher frequencies. For a ½”microphone
(type WS2F) the pressure increase will typically be
0,1 dB at 1 kHz. It is therefore a common recom-
mendation to adjust the sound level meters applying
such microphones to display 93,9 dB and 113,9 dB,
respectively at 1 kHz.
If microphones other than WS2P are used, the read-
ing from the sound level meter should be corrected
accordingly. See the list below for some microphones
in common use