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ENGLISH
11
For a complex sound source, such as an orchestra, it may be necessary for the engineer
to use a combination of one of the above techniques with some ‘spot’ mics to bring
distant sounds ‘closer’ in the recording.
Coincident microphones
The coincident pair technique is the most commonly-used stereo microphone
technique used in radio, TV and commercial stereo recordings – due to the fact that
this technique generally recreates the most faithful stereo image.
Coincident microphones do not record ‘time-of-arrival’ differences because the two
microphones are mounted in such close proximity to one another that sound reaches
both simultaneously. The diagram below shows how an orchestra might be recorded
using this technique, where the microphones marked ‘A’ are in a coincident pair
formation.
In this situation the two directional microphones are mounted on a stereo bar and face
outwards so that the angle between them is 90 degrees. Using a stereo bar means that
the mics aren’t in exactly the same horizontal position, but they are close enough for
any time-of-arrival differences to be inaudible. The microphone pointing to the left of
the orchestra (at the first violins) would be panned hard left and the microphone
pointing to the right (at the cellos) would be panned hard right.
The polar pattern of the directional microphones being used will affect the perspective
of the recording, and therefore the positioning needs to be considered accordingly. In
this example crossed cardioids would probably need to be placed above the conductor's
head to pick up a full stereo image; a crossed figure-of-eight pair would have to be
placed further back in order to cover the whole orchestra. As a result of this
positioning, cardioids will pick up less of the room ambience and give a much ‘closer’
sound whilst the figure-of-eights would produce a more ‘distant’ sound and pick up
more of the room ambience.
Spaced omnidirectional microphones
Omnidirectional mics cannot be used in coincident pairs because their directional
response means they do not record level differences proportional to the angle of the
incident sound. As a result, the only way to use two omnis is to space them apart and
record the time-of-arrival differences. Spaced microphone techniques often do not
create as accurate a stereo image as coincident techniques. This is mainly due to the
fact that when these recordings are replayed, there is second set of time-of-arrival
differences caused by the distance between the listener and the left and right
loudspeakers. This often has the result that the stereo image becomes ‘blurred’, and can
cause sounds to be perceived as grouped around the loudspeakers rather than spread
uniformly between the speakers to create a full stereo image.
So, why might you choose to use a spaced microphone technique over a coincident
pair? The main reason engineers choose this technique is so they can use high quality
omni-directional microphones, whose low frequency performance is generally superior
to that of directional microphones. It should also be noted that the quality of the stereo
image captured by a carefully set up spaced omni technique is not a problem in many
situations. The sound is still perceived as having width and the recordings often sound
more spacious than those made using coincident techniques.
The microphones marked B in the diagram above are spaced omnidirectional mics.
The mic to the left of the orchestra should be panned fully left and the mic on the
right should be panned fully right. The distance they are set apart depends on the
sound source being recorded but should generally be between a third and half the
width of the sound stage. How far the microphones are placed from the orchestra
would depend on the acoustics of the room and how much perspective was desired by
the engineer.
There are no hard and fast rules as to which is the best microphone technique for
stereo recording, and both techniques are capable of producing excellent results.
Whichever technique you choose for a particular situation, experimentation and
careful microphone placement are the keys to a successful stereo recording.