Microphone Technology 64522.2.3 Moving Coil
The moving coil microphone and the ribbon microphone are collectively referred to as
being dynamic microphones. Much discussion has been given previously with regard
to some of the features of the moving coil microphone. The mechanical resonance of
the moving coil structure is usually made to occur at the geometric mean of the low
frequency and high frequency limits describing the microphone’s pass band. In a typical
case this resonance occurs at about 630 Hz. In the pressure responsive version of such a
microphone the back chamber to the rear of the diaphragm contains an acoustic resistance
that highly damps the diaphragm mechanical resonance. This damping greatly broadens
the resonance, forcing the response to be uniform except at the frequency extremes.
Oftentimes a small resonant tube tuned to a low frequency and vented to the outside is
incorporated in the rear cavity. In addition to extending the response at low frequencies,
this tube allows the static air pressure in the rear chamber to track slow changes in
atmospheric pressure. Even in microphone structures featuring an otherwise sealed rear
cavity, a slow leak must always be provided for static pressure equalization. A small air
chamber that is resonant at a high frequency may also be located in the rear cavity in
order to enhance the response at high frequencies. Moving coil microphone structures are
usually quite rugged.
22.2.4 Ribbon
The ribbon microphone employs a conductor in a magnetic fi eld, as does a moving coil
microphone. Unlike the moving coil, which is located in a radially directed magnetic fi eld,
the conductor in a ribbon microphone is a narrow, corrugated metal ribbon located in a
linearly directed magnetic fi eld that is perpendicular to the length of the ribbon. The ribbon
itself constitutes the diaphragm, both faces of which are exposed to external sound fi elds.
The driving force on the ribbon is directly proportional to the pressure difference acting
on the two faces of the ribbon and hence is proportional to the space rate of change of
acoustic pressure. The space rate of change of pressure is called the pressure gradient.
The ribbon responds to the acoustic particle velocity with maximum response occurring
when the incident sound is normal to a face of the ribbon. This microphone is inherently
directional with a fi gure eight polar pattern. Although featuring excellent performance
over a wide frequency range, the structure is inherently fragile and is not suitable for
exterior use under windy conditions.