From the B&K document "Measurement Microphones", 1994

What is a measuring microphone?

A 'measuring' microphone is a calibrated microphone designed
for use with systems that quantify sound, for example sound
level meters. Measuring microphones are the most accurate
and reliable class of microphone available.

Of course, the high performance of these microphones requires
extremely high standards of engineering. The specifications of
measuring microphones are certainly in a different league from
other classes of microphone - but no microphone can ever be
perfect. This is why measuring microphones are calibrated, because
then their characteristics are well-defined and their behaviour
is predictable.

Throughout the microphone's working life its accompanying
calibration chart is a most important reference document. To
ensure that the calibration chart remains valid over a long period
of time, a measuring microphone must have exceptionally
stable characteristics. Amongst manufacturers, Bruel & Kjeer
stands alone in producing measuring microphones of the necessary
quality - time has proven it.

Construction

All Bruel&Kjrer measuring microphones are condenser (or capacitor)
designs which feature a tensioned-metal diaphragm
supported close to a rigid metal backplate. 

The microphone's output-voltage signal appears on a gold-plated
terminal mounted on the backplate which is isolated from
the microphone casing (or cartridge) by an insulator. The cartridge
Internal cavity is exposed to atmospheric pressure by a
small vent and the construction of the microphone is completed
with the addition of the distinctive Bruel&Kjrer diaphragm
protection grid.

Principle of Operation

The diaphragm and backplate form the parallel plates of a simple
alr-capacltor which is polarized by a charge on the backplate.
When the diaphragm vibrates in a sound field, the
capacitance of the capacitor varies and an output voltage is
generated.

The voltage signal replicates the sound-field pressure variations
as long as the charge on the microphone backplate is
kept fixed.

Preamplifiers and power supplies

Measuring microphones are always used in conjunction with a
microphone preamplifler. The preamplifier converts the microphone's
high output impedance to a low impedance suitable
for feeding into the input of accessory equipment. This impedance
conversion next to the microphone serves to minimize the
pick-up of noise in the signal cable to the accessory equipment.
Surprisingly, considering their name, most microphone preamplifiers
actually attenuate the signal - but only by a fraction of
a decibel.