BRUEL & KJAER 2204 - Sound Level Calibrator 4230

S =peak amplitude of motion of piston from mean position.
V == volume of cavity with the pistons in the mean position + equi·
valent volume of the microphone.
The sound pressure level in dB produced at the microphone diaphragm is
consequently
S.P.L. == 20 log _ P_
P•
where po is the reference pressure = o.0002 ~tbar.
The Pistonphone used with a 1" microphone produces a constant sound
pressure level of 124 dB at 250 Hz. From the above formula can be seen
that the only variable quantity is the ambient atmospheric pressure. A baro-
meter is therefore supplied with the pistonphone, calibrated directly in dB to
be added or subtracted from the value indicated on the pistonphone. The
accuracy of calibration is ± o.2 dB and distortion less than 3 %.
Fig. 7.4. Calibration of the Precision Sound Level Meter Tupe 220!.
To calibrate proceed as follows:
1. Place the Pistonphone on the Sound Level Meter as shown in Fig. 7.4 and
switch to position "Measure". Make sure that the Pistonphone fits tightly
to the microphone.
2. On the Sound Level Meter set KNOB 1 to "Lin." "Fast", KNOB 3 fully
clockwise and adjust KNOB 2 until the figure 120 appears in the red
circle. The meter should now indicate 4 dB (or 3.9 dB if the Pistonphone
is calibrated to give 123.9 dB). If it does not, adjust the ADJ. potentio-
meter until correct deflection is obtained.
The Sound Level Calibrator Type 4230.
This is also sufficiently accurate to calibrate the 2203 to IEC 179 standards.
It operates in a reverse way to a piezoelectric microphone. An alternating
potential difference is applied across the piezoelectric ceramic which causes
a bending moment to be produced in it. The "bender" then causes the
diaphragm to vibrate and produce sound waves.
The Calibrator calibrates the meter at 1000 Hz ( ± 2 %) and thus is in-
dependent of the weighting networks. The pressure produced is 94 ± o.3 dB
( = 10 flbar or 1 N/m2).
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