Theatre Design & Technology - Winter 1981 - 19

Another operating guideline is the three-to-one distance rule.
This rule states that cancellation will not be noticed if the sound
source is three times closer to one of two microphones that
might be turned on to the same volume setting. The above
assumes that both microphones are receiving the same sound
level from the source initially. The whole point of the rule is to
make sure that at the point where the two microphone signals
are mixed together the level of one of them is at least ten
decibels below the other.
The three-to-one ratio is chosen because if a given sound
source is sampled by two microphones, and the second microphone is at least three times farther away than the first microphone, the signal level from the second will be ten decibels
below that of the first, assuming both microphones are in the
direct sound field and the mixer volume controls are set to the
same level. If the volume is turned up on the second microphone
10dB, the point where the two signals combine, the signals will
again be at the same level and cancellations will occur.
It might appear that I am belaboring this point, but the effect of
this incorrect microphone mixing technique can be disastrous to
the sound quality of the performance.

O-d-O
A-B

Stereo

Figure 3

Stereo Microphony
One consideration in selecting a stereo microphone location
is that of monaural compatibility. This is especially desirable in
theatrical applications since sound cues might be mixed to
mono for special effects or backstage monitors. Since a stereo
signal minimally requires two microphones, attempting to mix
the left and right signals together to produce mono can yield
frequency/phase problems similar to those described above.
Keeping this in mind, some typical microphone placements can
be considered.

A-B Stereo
Figure 3 shows an A-B stereo microphone setup position for a
group of performers, or an orchestra, combo, or vocalists. The
A-B microphone system uses two microphones, directional or
non-directional, suspended some distance apart, d, and some
distance from the ensemble. The distance between the microphones depends on the amount of stereo spread wanted in the
recording. If the microphones are placed too far apart the sound
becomes all left or all right with no blending or center image. If
the microphones are too close there will be little separation to
the signal. In any case, the microphones should not be farther
apart than about twenty-five feet, or echoes might be perceived
from too long a sound travel path between the microphones.
Too little separation is preferable to too much, because of the
resulting tighter center imaging of the signal.
The answer to the second part of the microphone placement
question-how far from the ensemble-is more artistic in nature
but does require some explanation of sound fields in real rooms.
In any likely recording room, there are essentially two separate
sound fields: the direct and the reverberant. The direct sound
field comprises the sound produced by the sound source that
travels directly to the microphone. This sound level decreases
with distance from the source by the inverse square law. The
sound, besides striking the microphone, travels outwards, reflecting off walls, ceiling, seats and people, where it creates a
diffuse or reverberant sound field. This sound field is fairly
constant in level throughout the room. When the direct sound
stops the reverberant field takes a time to die out-the reverberation time of the room. Moving outward from the source, the
direct sound diminishes thereby increasing the proportion of
reverberation in the sound as picked up by the microphone.
Assuming the room being used for recording has a good
"sound" this technique of moving the microphone closer or
farther from the source to adjust and control reverberation works
better in achieving naturalness than using many microphones
and adding artificial reverberation. This technique can be followed for any microphone style described in this article.
USITIlWinler, 1981

x-v

Stereo

Figure 4
An A-B stereo recording gives good spatial imaging but
requires care when mixing the two microphones together, to
avoid phase cancellation problems.

Coincident Techniques
A way around this problem is to use a coincident pair
technique. These are microphone placements that provide
stereo imaging but keep the microphone separation to a minimum. The two major types are X-V and M-S. X-V microphony
utilizes two directional (cardioid) microphones angled some
distance apart, usually somewhere between 90 and 135 with
their diaphragms as close together as possible to minimize any
differences in time arrival between the two microphones. The
stereo image is dependent more upon differences in intensity
between the two microphones than upon time arrival/phase
differences as with A-B. In X-V, when the two microphones are
mixed together, they produce a cardioid pattern that is the sum
of the individual cardioid patterns, with the most sensitive
direction towards the center of the ensemble (see Fig. 4).
Omnidirectional microphones should not be used for X-V stereo
since they will reproduce exactly the same signal left and right
resulting in two channel mono!
A second coincident technique is known as Middle-Side or MS. The M-S system uses two microphones, one is a cardioid
0

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Theatre Design & Technology

17



Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1981

Contents
Theatre Design & Technology - Winter 1981 - 1
Theatre Design & Technology - Winter 1981 - 2
Theatre Design & Technology - Winter 1981 - 3
Theatre Design & Technology - Winter 1981 - Contents
Theatre Design & Technology - Winter 1981 - 5
Theatre Design & Technology - Winter 1981 - 6
Theatre Design & Technology - Winter 1981 - 7
Theatre Design & Technology - Winter 1981 - 8
Theatre Design & Technology - Winter 1981 - 9
Theatre Design & Technology - Winter 1981 - 10
Theatre Design & Technology - Winter 1981 - 11
Theatre Design & Technology - Winter 1981 - 12
Theatre Design & Technology - Winter 1981 - 13
Theatre Design & Technology - Winter 1981 - 14
Theatre Design & Technology - Winter 1981 - 15
Theatre Design & Technology - Winter 1981 - 16
Theatre Design & Technology - Winter 1981 - 17
Theatre Design & Technology - Winter 1981 - 18
Theatre Design & Technology - Winter 1981 - 19
Theatre Design & Technology - Winter 1981 - 20
Theatre Design & Technology - Winter 1981 - 21
Theatre Design & Technology - Winter 1981 - 22
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Theatre Design & Technology - Winter 1981 - 25
Theatre Design & Technology - Winter 1981 - 26
Theatre Design & Technology - Winter 1981 - 27
Theatre Design & Technology - Winter 1981 - 28
Theatre Design & Technology - Winter 1981 - 29
Theatre Design & Technology - Winter 1981 - 30
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Theatre Design & Technology - Winter 1981 - 44
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