BRUEL & KJAER 2204 - 7 Methods of Calibration; Noise Source Type

7. Methods of Calibration
From time to time it may be necessary to make an acoustical calibration of
the Sound Level Meter in addition to the built-in electrical calibration of the
amplifiers and meter circuit. Briiel & Kjrer produce three different acoustical
calibrators for use with the Sound Level Meter. They are easily portable and
can be used in the field.
The Noise Source Type 4240.
The Noise Source is a small mechanical-acoustical device producing an approx-
imately white noise spectrum of gaussian amplitude distribution. It is easily
mounted on the microphone as shown in Fig. 7.1, and gives a sound level of
approximately 108 dB at the microphone diaphragm. The actual value is
written on each Noise Source as they are individually calibrated at the factory.
Accuracy of calibration using the Noise Source: ± 1.5 dB.
-~~~~~~_e _ _____ --
a.
Fig. 7.1. Sketch showing the Noise Source correctlu placed on the Microphone.
The Pistonphone Type 4220.
In order to meet the IEC specification for precision sound level meters, an
overall measuring accuracy of ± 1 dB is required. It is therefore necessary,
when precision measurements are carried out, to calibrate the whole
apparatus including the microphone under the conditions of actual measure-
ment. All errors due to temperature, pressure and humidity, tolerances on
microphone sensitivity, cable attenuation etc. are then automatically reduced
to zero.