BRUEL & KJAER 2672, Руководство пользователя

Страница: 24 / 32

Chapter 4 — Using Type 2672
Gain
Sound Quality Conditioning Amplifier Type 2672
User Manual
Brüel & Kjær
4
−
4
For a given calibration signal Vi, the output Vo of this arrangement will change
measurably, even for small changes in the microphone’s capacitance Cm. The CIC
technique is about 100 times more sensitive than the insert-voltage calibration ar-
rangement shown in the lower diagram of Fig.4.1.
In the extreme case where there is significant leakage between the microphone’s
diaphragm and its back plate (Cm becomes very large), the signal output will
change by tens of decibels compared with only tenths of a decibel using the insert-
voltage method.
Another important CIC feature is that, unlike the insert-voltage technique, it is far
less sensitive to external electrical fields.
4.4.2 CIC using Type 2672
CIC is available via the Preamp. input and can be used with HATS Type 4100 (not
Type 4100 D) or when using Microphone Preamplifier Type 2669 L or 2670.
Once you have recorded a calibration signal using a pistonphone to make an abso-
lute calibration of the system, you can then apply the 40 Hz and 1 kHz signals from
the CIC generator and note the output level from Type 2672. Later, you can check
the calibration simply by measuring these output levels again and comparing them
with the original values. One of the advantages of this is that you do not need to
remove the ears from the HATS every time you want to calibrate.
Using two frequencies allows you to obtain diagnostic information about faults in
the system if the measured signal is different to the original values. If the change
in the 1 kHz tone level is greater than the change at 40 Hz, the fault is due to a
change in capacitiance in the microphone and/or preamplifier input. If the level
changes are identical at both frequencies, the fault is likely to be found in the
cables or connectors.
For calibration at other frequencies, you can use the external input.
4.5 Gain
4.5.1 High/Low
Type 2672 has a wide range of gain settings (–20 to + 50 dB) to allow the recording
of a wide range of sounds. When you record a sound, you first record a calibration
signal of, for example, 94 dB. If the sound you are recording is at a much lower
level, you may want to increase the gain setting to obtain a better recording. You
can do this by adjusting Gain High/Low, or by adjusting the gain of your DAT
recorder.
Note: Always remember to make a note of any gain adjustments for later reference.