Theatre Design & Technology - Winter 1981 - 18

to the nature of sound waves. A sound wave is a series of
compressions (positive pressures) and rarefactions (negative
pressures) of air traveling outward from the source to the
listener. The distance between identical points on these consecutive compressions (or rarefactons) is known as the wavelength
of the sound wave. The frequency of a sound wave is defined as
how often consecutive compressions (or rarefactions) occur in a
given amount of time. Different frequency (or pitch) sound
waves have different wavelengths. In fact, the frequency and
wavelength of a sound wave are directly related by the formula:
1130 ft. per second (speed of sound) = Frequency
(in cycles per second or Hertz) x wavelength (in feet).
As the frequency of a sound wave increases, the wavelength
decreases. At higher frequencies, above about 200 Hz, the
wavelengths are measured in inches.
In the case of the footlight position, the microphone is
receiving the sound wave from the source, but the sound is
getting to the microphone via two different paths. The first path,
the more obvious one, is the straight line distance between the
source and the microphone. The second path is a reflection from
the stage floor. Because the path traveled by the reflection is
longer than the direct path, the reflected wave will arrive at the
microphone after the direct wave. Now, suppose the difference
in arrival path distance is one-half the wavelength of a particular
sound. This means the positive pressure wave (compression)
and the negative pressure wave (rarefaction) for a particular
frequency arrive at the microphone at the same time. When that
happens the positive and negative pressures cancel and at that
particular frequency the microphone produces no output. The
signals are said to arrive 1800 out of phase and the cancellation
of that particular sound wave is known as a phase cancellation.
At what frequencies these phase cancellations occur is dependent upon the distances between the microphone and nearby
reflecting surfaces.
It is interesting to note that for a given source/microphone/
floor relationship not just one frequency is affected, but rather
any frequency where the difference in signal path is one half or a
multiple of one half the wavelength. For example, if there is a
phase cancellation at 1000 Hz there will be another cancellation
at 2000 Hz, 4000 Hz, 8000 Hz, etc. This roller coaster-looking
response curve with a microphone at this position is known as a
comb filter because the deep notches in the response (sometimes 20-30 decibels deep) resemble the teeth of a comb.
The shorter the difference in arrival time (or distance), the
higher the first frequency that will be affected by phase cancellations. So where should the microphone be placed? Directly on
the floor is a much better location. Placing the microphone on
the floor accomplishes two things. First of all, by minimizing the
path length differences down to a fraction of an inch the
frequencies first affected will be above audibility. Secondly, with
the floor reflection arriving at the same time as the direct sound
there is twice the sound pressure at the microphone so the
microphone will have a hotter output. This means that the
microphone does not need to be as loud to get the same level
out of the P.A. system so the system will be less prone to
feedback!
Keep in mind that "ball" shaped microphones have internal
wind screens that keep the diaphragm from coming very close to
the floor. For this reason microphones without wind screens are
preferred for surface mounting applications.
Incidentally, if floor noise is encountered by placing the
microphone directly on the floor, the microphone could be
placed on a thin piece of foam to mechanically decouple the
microphone from the floor. The foam strips that package quartz
halogen lamps do well. Alternatively, there are commercial foam
mounts made especially for surface mounting microphones that

Theatre Design & Technology

do an excellent job of decoupling the microphone, holding it
within a quarter inch of the floor, and hiding the microphone in a
block of gray foam so that it is not as obtrusive on the stage.
Phase cancellations can occur not only when a single microphone is improperly placed near a reflecting surface, but any
time two microphones sampling the same sound field at different
distances from the source are mixed together. This occurs, for
example, when two microphones are mounted on a lectern but
separated by some distance, with a single person talking at the
lectern. As long as the talking remains an equal distance from
the microphones the sound arriving will be in phase and the
microphones will mix together satisfactorily. But if the person
drifts a bit off center and the path length to each microphone is
different, phase cancellations will occur. Many people assume
microphones work like spotlights in reverse. If one spotlight from
one side is good, two spotlights spaced apart striking the subject
from different directions are better. This is not true with sound.
Any time more than one microphone samples a sound field and
the microphone signals are mixed together, phase cancellations
can occur. A good rule of thumb is to never use two microphones when one will do.
The lectern problem can be solved in a number of ways. First,
have only one microphone on at a time, especially if the second
microphone is there just for system redundancy. Another solution is to place the microphones as close together as possible.
There are dual microphone clips available that hold two microphones within a fraction of an inch of each other that solve the
interference problem the same way placing the microphone on
the floor in the first example does. It minimizes the time arrival
differences for the two microphones so that they can both be on
at the same time and mixed together without causing phasing
problems, yet still permit the talker some freedom of movement.
Notice the dual microphone stand used at a presidential news
conference; the microphones are placed in that stand precisely
for the reason described above.
A typical microphone setup used in reinforcing a live performance involves three microphones placed in close proximity to
the floor-left, center, and right-on the downstage edge of the
stage. They are then plugged into the house sound system and
perhaps all turned on. If you understand the acoustic activity that
occurred in the previous two examples, you should be unwilling
at this point to turn on all three microphones and leave them on
for the run of the show. As the performers move about the stage
they are constantly changing their distances from the three
microphones causing severe phasing problem.
A simple experiment will demonstrate this point. Take three
identical microphones, place them on the stage floor about three
feet apart, plug them into a mixer and turn all three of them on,
mixing them together to mono at the same level. Take a sound
source--a transistor radio will do-and, staying a distance
upstage, perhaps ten feet, walk left and right as shown in Figure
2 at least as far apart as the microphones are spaced. Hook a
tape recorder to the mixer output and record a few minutes of
the radio program. When listening to the tape playback, notice
the swishing noise and the changing tonal quality of the music.
At one instant the bass is predominant, then the treble.
Try the experiment with the radio playing just hiss from tuning
between stations. The phenomenon will be even more pronounced. What you are hearing are the phasing effects causing
comb filters that are continually moving up and down the audio
spectrum as you shift the relative position of the radio, or
performer, and the footlight microphones.
Do not assume from the above example that footlight microphones are a bad idea. It is just that they should not all be on at
the same time. In the above experiment the three microphones
were all at the same volume setting. If the volume controls are
adjusted so that the signal level from secondary microphones is
at least ten decibels below that of the primary microphone the
second microphone's signal level will be too low to cause
phasing problems.

USITTlWinter, 1981

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