User manual

27
Specifications
Free Field - Pressure Correction Factors
If a measurement microphone is held in a free-field environ-
ment, then the measurement microphone acts at high fre-
quencies like a reflector. The sound pressure increases in front
of the microphone capsule membrane. M2230, M2211 and
M2215 are free-field equalized measurement microphones,
they compensate for the increased pressure internally.
The calibrator offers no longer free-field conditions. Therefore,
the free-field equalization of the microphone must be compen-
sated. This needs to be considered prior the calibration. The
correction value needs to be added to the pressure response
of the microphone.
Example:
During the calibration, the XL2 measures the sound level in •
the calibrator. If the B&K4226 calibrator is used and it is set
to 16 kHz, then the XL2+M2230 reads just 86.7 dBA.
The free-field sound level is calculated by summing the XL2 •
measurement value and the correction value (= 86.7 dB +
7.3 dB = 94.0 dB).
The following corrections apply with the B&K4226 calibrator:
Nominal
Fre-
quency
[Hz]
M2230
Measurement
Microphone
[dB]
M2211
Measurement
Microphone
[dB]
M2215
Measurement
Microphone
[dB]
Measurement
Uncertainty U
[dB]
31.5
63
125
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.3
0.3
0.3
250
500
1000
0.0
0.0
0.0
0.0
0.1
0.1
0.0
0.0
0.0
0.3
0.3
0.3
2000
4000
8000
0.3
0.7
2.6
0.6
1.7
4.2
0.2
1.2
3.9
0.3
0.3
0.4
12500
16000
6.0
7.3
7.3
9.2
6.7
9.0
0.7
0.8
Correction values for other calibrators for M2230:
Type
Correction
Value
Calibration
Frequency
Calibration
Level
NTi Audio CAL200 0.1 1 kHz 114 dB
B&K 4231 0.2 1 kHz 114 dB
Norsonic Nor-1251 0.2 1 kHz 114 dB