How The Calibrator Works - BRUEL & KJAER 4226 User Manual

2 Multifunction Acoustic Calibrator Type 4226 – User Manual
1.2

How the Calibrator Works

Fig.1.2 shows a block diagram of Type 4226. A crystal-locked sine generator produces signals of
very stable frequency. These signals pass through a variable gain amplifier and are sent to the
transducers located in the Type 4226's acoustic coupler. A reference microphone that is
symmetrical to the microphone to be calibrated is situated in the coupler, both are exposed to the
actual SPL in the coupling volume.
Fig.1.2
Simplified block
diagram of Type 4226
The reference microphone drives a compression-loop, which in turn controls the variable gain
amplifier of the calibrator.
This method of signal generation has many advantages: over a wide frequency range it gives a
very accurate and stable sound pressure level (SPL). The SPL obtained depends essentially on the
reference microphone, which is itself very stable with respect to time and temperature variation.
The SPL produced is practically independent of atmospheric pressure changes. The calibrator
gives you a signal when the constant SPL in the coupler is correct.
The response obtained from a microphone of any type, free-field or pressure, will always be its
pressure response. However, the main interest for sound level meters is the free-field response.
Because the difference between the pressure response and the free-field response is known with
Brüel & Kjær microphones, you can correct the SPL in the coupler as a function of frequency to
obtain a flat response from an ideal free-field microphone. This SPL correction is automatically
applied by the calibrator when you specify which Brüel & Kjær microphone is to be calibrated.
SPL dB Sound
Mic. Field
114 Lin.
a Press.
104
b Falcon
94
Inv. A
94
Tradi-
c
tional
Acoustic
Coupler
Input Amp. Detector
+ att. + A +
Weight Corrector
Variable
Gain
Amplifier
Cal.
Uncal.
Level
Shift
AC Output
SPL
Set
Ext. Gen.
Crest
Frequency Fast
Reference
Display
Slow
Test
Cal.
Error
Comparator
Dectector
C
o
Frequency
n
t
r
o
Sine
l
Generator
l
e
Function
r
Level
Zero
Decector
Up On
Down Off
Frequency Timer
892146/1