The condenser microphone is to-day accepted as the standard acoustical transducer
for all sound and noise measurement because of its very high degree of accuracy; an
accuracy which is higher than what is possible with any other acoustical transducer.
Not only is the condenser microphone an accurate laboratory tool used by standards
laboratories, it is also used for a broad range of field measurements under many
different and often severe environmental conditions.
The reason for the high degree of acceptance is that the condenser microphone has
the following properties which are essential for a standard transducer:
In order to attain the microphone’s high standard, considerable care is required in
both design and production. This includes advanced clean-room techniques, very tight
mechanical tolerances, a special diaphragm construction and artificial ageing
Microphones and Preamplifiers
The condenser microphone converts the acoustical pressure
electrical signal which thereafter is amplified in a preamplifier.
always be connected very close to the microphone since its
convert the very high impedance of the microphone into a low
permitting use of long cables and connection to instruments
impedance. The low impedance ensures very little pick up of
and this is especially important when using long cables.
Types of Microphones
Microphones are divided into 3 types according to their response in the sound field:
free field, pressure, and random incidence.
Free field microphones have uniform frequency response for the sound pressure that
existed before the microphone was introduced into the sound field. It is of importance
to note that any microphone will disturb the sound field, but the free field microphone
is designed to compensate for its own disturbing presence as discussed later.
The pressure microphone is designed to have a uniform frequency response to the
actual sound level present. When the pressure microphone is used for measurement
in a free sound field, it should be oriented at a 90° angle to the direction of the sound
propagation, so that the sound grazes the front of the microphone.
The random incidence microphone is designed to respond uniformly to signals arriving
simultaneously from all angles. When used in a free field it should be oriented at an
angle of 70° - 80° to the direction of propagation.
In the following we will have a closer look at the reason for the difference between the
microphones and when each type should be used.
Before measurements are undertaken, it is important to calibrate the microphone and
instrument together. This will check the function of the measurement system and
ensure that high accuracy can be obtained allowing comparison to be made between
measurements taken at different times. Calibration ought therefore to be made before
each series of measurements and it is recommended that the calibration is repeated
after a series of measurements as a double check.
Acoustic calibration is normally to be preferred, since the whole system from
microphone to indicating device will be checked. To carry out acoustic calibration, fit
the calibrator on the microphone, making sure it fits snugly. Switch on the calibrator
and adjust the read out on the indicating device to the sound level produced by the
calibrator being used. Two different calibrators are available for acoustic calibration: a
pistonphone and an acoustical calibrator.