Handbook for Sound Engineers

520 Chapter 16

Dr 2 is the reflected path from the surface to the micro-
phone in feet or meters,
Dd is the direct path from the source to the microphone

in feet or meters.

If the microphone is 10 ft from the source and both
are 5 ft above the floor, the canceled frequency is

(16-17)

If the microphone is moved to 2 ft above the floor,
the canceled frequency is 319.20 Hz. If the microphone
is 6 inches from the floor, the canceled frequency is
1266.6 Hz. If the microphone is 1 inch from the floor,
the canceled frequency is 7239.7 Hz.

16.5.1.3 Behind Objects

Sound, like light, does not go through solid or acousti-
cally opaque objects. It does, however, go through
objects of various density. The transmission loss or
ability of sound to go through this type of material is
frequency dependent; therefore, if an object of this type
is placed between the sound source and the microphone,
the pickup will be attenuated according to the transmis-
sion characteristics of the object.
Low-frequency sound bends around objects smaller
than their wavelength, which affects the frequency
response of the signal. The normal effect of placing the
microphone behind an object is an overall reduction of
level, a low-frequency boost, and a high-frequency
roll-off.

16.5.1.4 Above the Source

When the microphone is placed above or to the side of a
directional sound source (i.e., horn or trumpet), the
high-end frequency response will roll off because high
frequencies are more directional than low frequencies,
so less high-frequency SPL will reach the microphone
than low-frequency SPL.

16.5.1.5 Direct versus Reverberant Field

Micing in the reverberant field picks up the character-
istic of the room because the microphone is picking up
as much or more of the room, as it is the direct sound
from the source. When micing in the reverberant field,

only two microphones are required for stereo since

isolation of the individual sound sources is impossible.

When in the reverberant field, a directional microphone

will lose much of its directivity. Therefore, it is often

advantageous to use an omnidirectional microphone

that has smoother frequency response. To mic sources

individually, you must be in the direct field and usually

very close to the source to eliminate cross-feed.

16.5.2 Grounding

The grounding of microphones and their intercon-

necting cables is of extreme importance since any hum

or noise picked up by the cables will be amplified along

with the audio signal. Professional systems generally

use the method shown in Fig. 16-61. Here the signal is

passed through a two-conductor shielded cable to the

balanced input of a preamplifier. The cable shield is

connected to pin number 1, and the audio signal is

carried by the two conductors and pins 2 and 3 of the

XLR-type connector. The actual physical ground is

connected at the preamplifier chassis only and carried to

the microphone case. In no instance is a second ground

ever connected to the far end of the cable, because this

will cause the flow of ground currents between two

points of grounding.

In systems designed for semiprofessional and home

use, the method in Fig. 16-62 is often used. Note that

one side of the audio signal is carried over the cable

shield to a pin-type connector. The bodies of both the

male and female connector are grounded: the female to

the amplifier case and the male to the cable shield. The

microphone end is connected in a similar manner; here

again the physical ground is connected only at the

preamplifier chassis. Hum picked up on the shield and

not on the center conductor is added to the signal and

amplified through the system.

fc^1130 u0.5

7.07 7.07 10–+

=-------------------------------------

=136.47 Hz

Figure 16-61. Typical low-impedance microphone to

preamplifier wiring.

Microphone

3-Pin female

microphone

connector

2-conductor

shielded cable

3-pin male

microphone

connector

Preamplifier

3

2

1

+

Black

White/Red

Shield

(^32)

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