CHAPTER 1
Introduction
3
Each reference sound source is individually calibrated and, as standard, a calibration certificate
is supplied showing the sound power output from 100 Hz to 20 kHz at 230 V/50 Hz and
115 V/60 Hz operating voltages. Temperature, barometric pressure, humidity and rotational
speed are all given on the calibration certificate. The maximum interval between successive
calibration is 24 months. If there are signs of physical damage to Type 4204, then a
recalibration is recommended.
In the field, the sound power produced may differ from the calibrated value. Changes in mains
supply line voltage and frequency, ambient pressure, and temperature all affect the rotational
speed of Type 4204. In addition, changes in ambient pressure and temperature alter the specific
acoustic impedance of the air, also changing the sound power. Once the values are measured,
however, simple formulae can be used to calculate the correct sound power levels from the
values given on the calibration certificate.
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