Handbook for Sound Engineers

Microphones 537

an on-axis sound originated at a distance of 20 ft (6 m),
a 90° to 180° off-axis sound occurring at the same
distance and intensity will be reduced by 20 dB or
more, providing none of the off-axis sound is reflected
into the front of the microphone by walls, ceiling, and
so on. On the other hand, should the off-axis sound orig-
inate at a distance of 2 ft (0.6 m) and at the same sound
pressure level as the sound at 20 ft (6 m) on axis, it will
be reproduced at the same level. The reason for this
behavior is that the microphone is still canceling the
unwanted sound as much as 20 dB, but due to the differ-
ence in the distances of the two sounds, the off-axis
sound is 20 dB louder than the on-axis sound at the
microphone. Therefore, they are reproduced at the same
level. For a pickup in an area where random noise and
reverberation are problems, the microphone should be
located with the back end to the source of unwanted
sound and as far from the disturbances as possible.
If the microphone is being used inside a truck or
enclosed area, and pointing out a rear door, poor pickup
may be experienced because all sounds, both wanted
and unwanted, arrive at the microphone on-axis. Since
the only entrance is through the truck door, no cancella-
tion occurs because the truck walls inhibit the sound
from entering the sides of the microphone. In this
instance, the microphone will be operating as an omni-
directional microphone. Due to the reflected sound from
the walls, the same condition will prevail in a room
where the microphone is pointed through a window or
when operating in a long hallway. For good pickup, the
microphone should be operated in the open and not in
closely confined quarters.
A shotgun interference tube microphone cannot be
compared to a zoom lens since the focus does not vary
nor does it reach out to gather in the sound. What the
narrow polar pattern and high rate of cancellation do are
to reduce pickup of the random sound energy and
permit the raising of the amplifier gain following the
microphone without seriously decreasing the SNR.
Difficulties may also be encountered using interfer-
ence tube microphones on stage and picking out a talker
in the audience, particularly where the voice is
75–100 ft (23–30 m) away and fed back through a rein-
forcement system for the audience to hear. Under these
circumstances, only about 30–50 ft (9–15 m) is possible
without acoustic feedback; even then, the system must
be balanced very carefully.

16.6.10 Rifle Microphones

The rifle microphone consists of a series of tubes of
varied length mounted in front of either a capacitor or

dynamic transducer diaphragm, Fig. 16-100. The trans-

ducer may be either a capacitor or dynamic type. The

tubes are cut in lengths from 2–60 inches (5–150 m) and

bound together. The bundling of the tubes in front of the

transducer diaphragm creates a distributed sound

entrance, and the omnidirectional transducer becomes

highly directional.

Sound originating on the axis of the tubes first enters

the longest tube and, as the wavefront advances, enters

successively shorter tubes in normal progression until

the diaphragm is reached. Sounds reaching the

diaphragm from the source travel the same distance,

regardless of the tube entered, so all sounds arriving

on-axis are in phase when they reach the diaphragm.

Sounds originating 90° off-axis enter all tubes simulta-

neously. A sound entering a longer tube may travel

18 inches (46 cm) to reach the diaphragm, while the

same sound traveling through the shortest tube will

travel only 3 inches (7.6 cm), with other differences for

the varied length of tubing causing an out-of-phase

signal at the diaphragm. Under these conditions, a large

portion of the sound originating at 90° is canceled, and

from 180° an even greater phase difference occurs, and

cancellation is increased considerably.

The RCA MI-100006A varidirectional microphone,

Fig. 16-100, consists of nineteen inches (0.8 cm)

plastic tubes, ranging from 3–18 inches (7.6–46 cm) in

length. The tubes are bundled and mounted in front of

an omnidirectional capacitor-microphone head. Rifle

microphones are not used very much today.

16.6.11 Parabolic Microphones

Parabolic microphones use a parabolic reflector with a

microphone to obtain a highly directional pickup

response. The microphone diaphragm is mounted at the

focal point of the reflector, Fig. 16-101. Any sound

Figure 16-100. RCA rifle microphone. Courtesy of Radio

Corporation of America.

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