Microphone Technology 657The three-dimensional directional response of such a microphone is obtained by
revolving the directional polar pattern about the cylindrical axis of the microphone.
Ribbon microphones, however, don’t follow the aforementioned rules, as their
diaphragms do not possess cylindrical symmetry. Such microphones are usually designed
to be addressed from the side, as illustrated in Figure 22.6.
The directional response in the horizontal plane of the depicted ribbon microphone is a
fi gure eight. Revolving this pattern about the principal axis generates two spheres that
describe the microphone’s response in three dimensions. The polar directional patterns
listed in Table 22.2 are displayed in Figure 22.7A , while the three-dimensional directional
response is sketched in Figure 22.7B.
The polar patterns of Figure 22.7A are theoretical ideals and have a linear radial axis
consistent with the form of the describing equations. Real microphones fall short of the
theoretical ideal in two ways. They never display complete nulls in response and the polar
response curves are frequency dependent. Compare the measured polar response curves
of a cardioid microphone presented in Figure 22.8 with its counterpart in Figure 22.7A.
Manufacturer’s polar response data are usually presented employing a logarithmic polar
axis while excluding a small region in the vicinity of the origin. Such a presentation for
yet again a different cardioid microphone is given in Figure 22.9.
In examining Figure 22.9 , note that the reference axis has a different orientation and that
the radial coordinate represents attenuation expressed in decibels relative to the on-axis
value.
22.4 Wireless Microphones ...........................................................................................
Modern wireless microphones allowing untethered motion of the user have proven
themselves to be indispensable in concerts, religious services, dramatic arts, and motion
picture or video production.
Wireless microphones for use in the performing arts and sound reinforcement fi rst
made their appearance about 1960. The fi rst transmitter units were designed to operate
in the broadcast FM band between 88 and 108 MHz. The receivers were conventional
FM broadcast units. The transmitters did not have to be licensed as the low radiated
powers involved complied with Part 15 of the FCC rules. Frequency modulation was
accomplished in the transmitter by allowing the audio voltage signal to vary the junction