Handbook for Sound Engineers

538 Chapter 16

arriving from an angle other than straight on will be
scattered and therefore will not focus on the pickup. The
microphone is focused by moving the diaphragm in or
out from the reflector for maximum pickup. This type
concentrator is often used to pick up a horse race or a
group of people in a crowd.

The greatest gain in sound pressure is obtained when
the reflector is large compared to the wavelength of the
incident sound. With the microphone in focus, the gain
is the greatest at the mid-frequency range. The loss of
high frequencies may be improved somewhat by defo-
cusing the microphone a slight amount, which also
tends to broaden the sharp directional characteristics at
the higher frequencies. A bowl 3 ft (0.91 m) in diameter
is practically nondirectional below 200 Hz but is very
sharp at 8000 Hz, Fig. 16-102. For a diameter of 3 ft,
the gain over the microphone without the bowl is about
10 dB and, for a 6 ft (1.8 m) diameter bowl, approxi-
mately 16 dB.

16.6.12 Zoom Microphones

A zoom microphone,^10 or variable-directivity micro-
phone, is one that operates like and in conjunction with
a zoom lens. This type of microphone is useful with
television and motion-picture operations.

The optical perception of distance to the object is
simply determined by the shot angle of the picture. On
the other hand, a sound image is perceived by:

• Loudness.


• Reverberation (ratio of direct sound to reflected
  sound).


• Acquired response to sound.
  
  
  • Level and arriving time difference between the two
    ears.
  
  
  
  

If the sound is recorded in monophonic, the

following factors can be skillfully combined to repro-

duce a natural sound image with respect to the

perceived distance:

• Loudness: Perceived loudness can be controlled by
  varying microphone sensitivity.


• Reverberation: The representation of the distance is
  made by changing the microphone directivity or the
  ratio between direct and reverberant sound. In a
  normal environment, we hear a combination of direct
  sound and its reflections. The nearer a listening point
  is to the source, the larger the ratio of direct to rever-
  berant sound. The farther the listening point is from
  the source, the smaller the ratio; therefore, use of a
  high-directivity microphone to keep direct sound
  greater than reflected sound permits the microphone
  to get apparently closer to the source by reducing
  reverberant sound pickup. For outdoor environ-
  ments, use of directional microphones allows the
  ambient noise level to be changed for natural repre-
  sentation of distances.


• Acquired human response to sound: Normally we can
  tell approximately how far a familiar object as a car
  or a person is by the sound generated by the objects
  because we acquire the response to sound through
  our daily experiences.

Figure 16-101. A parabolic bowl concentrator for direc-
tional microphone pickup.

Parabolic

reflector

Microphone

Focus

Source of sound

Figure 16-102. Polar pattern for a parabolic concentrator.

200 Hz

600 Hz

1000 Hz

4000 Hz

8000 Hz

0 o

45 o

o

o

o

o

o

o^ 5 dB







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