Microphones 583track tape machine. Sound reinforcement practice
usually requires good isolation of the various sound
sources. In either case, the end result is a composite of a
number of monaural sources, which are often placed in
the stereo image with pan pots. This practice is not the
same as true stereo recording, which can provide a
sense of depth and realism unachievable with panned
mono sources. It requires greater effort for superior
results; a good acoustic environment is essential.
There are a number of stereophonic recording tech-
niques available to the engineer. The simplest requires
two microphones, often omnidirectional types, spaced
apart by a distance ranging from several feet to more
than 30 ft (9 m), Fig. 16-175. The spacing depends on
the size of the sound source, the size of the room, and
the effect desired. A broad source like an orchestra will
require a wider spacing than a small source such as a
single voice or instrument. If the microphones are too
far apart, a hole in the middle of the stereo image will
result, since the sound produced in the center of the
stage will be too far from either microphone. When
placed too closely together, a mono result will be
obtained. When the spacing is comparable to the wave-
length of the sound, phase cancellations may result
(comb filters), which will destroy the monaural compat-
ibility of the recording. The best spacing seems to be
from 10–40 ft (3–12 m). Experimentation is necessary
since every situation will be different. Needless to say,
good monitoring is required; stereo headphones will not
generally reveal defects evident on good monitor loud-
speakers. A method of summing the two channels to
mono is essential for testing compatibility.
Variations on the spaced microphone technique
involve using bidirectional or unidirectional micro-
phones, which may be helpful when the room character-
istics are not perfect for the material being performed.
Adding a microphone in the center, fed to both left and
right channels (fill microphones), and combinations of
spaced micing and other techniques might be required.16.13.2 Microphone ChoiceEvery microphone type has certain characteristics.
These characteristics must be taken into account when
choosing a microphone for a specific application. Some
of the factors to be considered are general type
(condenser, moving coil dynamic, ribbon); directional
pattern (omni-, bi-, or unidirectional); and specific
microphone traits (bright, bassy, dull, presence peak,
and so on).
Also, the susceptibility of the microphone to over-
load or its tendency to overload the associated preampli-
fier must be considered. The off-axis frequency
response can have a large effect on the sound of a
microphone in a particular application. Certain micro-
phones may exhibit unusual traits that may make them
more, or less, suitable for a certain application. For
example, the design of the grille may have a major
effect on the sound of a microphone when recording
closely micing vocals.
Some of these characteristics can be inferred from
the microphone specifications (i.e., frequency response,
overload point, directional pattern—both on-and
off-axis). Other characteristics are not as easy to
measure or visualize, and experience and experimenta-
tion are necessary to make an intelligent choice.16.13.3 Microphone CharacteristicsThere are many criteria used to judge the suitability of a
microphone for a particular application—some are quite
subjective. Frequency response is one obvious charac-
teristic, distortion is another. The ability of a micro-
phone to accurately translate waveforms into electrical
signals is vital for good reproduction. Generally, the less
massive the internal parts that must be moved by the
sound pressure, the more accurate the reproduction,
especially the reproduction of waveforms with steep
leading edges and/or rapid level changes (e.g., percus-
sive sounds). The condenser microphone has the lowest
mass (only a thin plastic diaphragm with a very thin
coating of metal must be moved by the sound pressure).
The diaphragm and coil in the dynamic microphone
have considerably more mass than the condenser
diaphragm. The ribbon in a ribbon microphone has rela-
tively low mass and is somewhere between the
condenser and the dynamic microphone.Figure 16-175. Spaced omnidirectional microphones for
stereo recording.
Left
10' to 40'Right20' or more