When an object vibrates in the presence of air, the air
molecules at the surface will begin to vibrate,
which in turn vibrates the adjacent molecules next to them. This vibration will
travel through the air as oscillating pressure at frequencies and amplitudes
determined by the original sound source.
The human eardrum transfers these pressure oscillations, or sound, into electrical signals that are interpreted by our
brains . Microphones are designed, like the human ear, to transform
pressure oscillations into electrical signals, which can be recorded and
analyzed to tell us information about the original source of vibration .
Dynamic microphones depend on the principle that moving a
wire within a magnetic field generates a current in the wire. In the case of moving-coil dynamic
microphones, the wire is a coil of very fine diameter wire, situated within a
magnetic field and attached to a diaphragm in contact with the air. As the pressure varies, the diaphragm moves
in response to the changing force applied by the moving air. The coil produces a small voltage as it moves
in the fixed magnetic field. This
voltage is fed, usually through a transformer, to an external amplifier optimized for low input impedance
and high gain.
The mass of a coil of wire is not negligible, so the construction of
the element requires special care to make sure the element can move easily
enough to allow the small air pressure variations to produce a measurable voltage at all audible frequencies.
In order to calculate the maximum output for a microphone,
using a specific preamplifier and its corresponding peak voltage, we first need
to calculate the pressure in Pascals that the microphone can accept. The amount
of pressure can be calculated by using the following formula:
Where P = Pascal’s (Pa) & Voltage is the preamps output
peak voltage.
Once the maximum pressure level that the microphone can
sense at its peak voltage is determined, this can then be converted to decibels
(dB), using the following logarithmic scale:
Where: P = Pressure in Pascal’s
Po = Reference Pascal’s (Constant = 0.00002 Pa)
The above formula will provide the maximum rating that a
microphone (when combined with a specific preamplifier) can be capable of
measuring.
Temperature will have an effect on the microphones
performance. Sensitivity levels can be directly affected by extreme
environmental conditions. As the
temperature approaches the maximum specifications of the microphone, its
sensitivity specification will decreaseOne should be aware of not only the operating temperature,
but also the storage temperature of the microphones. If operated and/or stored
in extreme conditions, the microphone can be adversely affected and also will require to be calibrated .