Rion NA-42, Instruction Manual

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86
Technical Reference
the upper frequency limit means that sensitivity is decreased.
The tuning of microphone frequency response is achieved by opening a
number of holes in the backplate in order to adjust the viscous resistance
(damping characteristics) of the air that moves between the diaphragm and
the backplate.
Since the UC-34 was specially designed to achieve low noise performance, it
is suitable for measurement of low sound levels. Noise in a microphone can
be divided into two types: electrical noise which arises due to the capacitance
of the microphone and the input impedance of the connected preamplifi er,
and internal noise in the microphone itself. Normally, electrical noise forms
the largest share of the overall noise components, and internal noise does not
pose a problem. But in combination with the high-impedance preamplifi er
NH-34, electrical noise in the UC-34 is greatly reduced, and internal noise of
the microphone therefore forms the major part of the noise components.
Internal noise in a microphone is generated mostly by viscous resistance
of the air between the diaphragm and the backplate, leading to mechanical
thermal noise. In order to decrease this noise, it is desirable to reduce the
viscous resistance. But this will in turn cause the microphone to lose fl at
frequency response and exhibit a sharp peak such as shown in the fi gure
below. The preamplifi er therefore incorporates a fi lter with exactly opposite
characteristics, thus restoring fl at frequency response.
Frequency response of Microphone UC-34
100 1 k 10 k 50 500 5 k 40 k 20 200 2 k 20 k
+20
+10
0
-10
-20
Response
(dB)
Frequency (Hz)