Rion NA-42, Instruction Manual

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85
Technical Reference
Microphone
There are various ways to convert sound pressure changes into electrical
signals. For acoustic measurements, the condenser (electrostatic) microphone
principle is widely used, because it offers high stability, fl at frequency re-
sponse, high sensitivity and allows compact dimensions. As an example, this
manual describes the microphone UC-34.
The construction princi-
ple of a condenser micro-
phone is shown at right.
The microphone uses a
movable electrode (dia-
phragm) and a fi xed elec-
trode (backplate), placed
at a distance of several
tens of microns from each
other. This results in a
capacitor (condenser) with
a capacitance of several
tens of picofarads. Sound
pressure changes cause
the diaphragm to vibrate
which results in a change in capacitance. To convert this change into an
electrical signal, a DC voltage (DC bias) of normally 200 V is supplied to
the backplate via a high-value resistor, and the voltage difference between
the two electrodes is extracted as a signal.
Using stiffness control, the condenser microphone is designed so that dia-
phragm displacement is proportional to the air pressure change, regardless of
frequency. The effectiveness of this design depends on various factors, such as
microphone sensitivity and the resonance frequency determined by diaphragm
tension and rear air chamber volume (this chamber is the near-hermetically
sealed air-fi lled cavity inside the microphone). In order to increase sensitiv-
ity, the resonance frequency must be lowered, but this means that the upper
frequency limit of the microphone also becomes lower. Reversely, extending
Diaphragm
Backplate
Rear air
chamber
Insulator
Terminal
Condenser microphone construction principle