Handbook for Sound Engineers

Microphones 519

TN is the thermal noise relative to 1 V,
bw is the bandwidth in hertz,
Z is the microphone impedance in ohms.

Thermal noise relative to 1 V can be converted to
equivalent input noise (EIN) by

(16-14)

Since the EIN is in dBm and dBm is referenced to
600 : , the impedance Z is 600:.

16.5 Microphone Practices

16.5.1 Placement

Microphones are placed in various relationships to the
sound source to obtain various sounds. Whatever posi-
tion gives the desired effect that is wanted is the correct
position. There are no exact rules that must be followed,
however, certain recommendations should be followed
to assure a good sound.

16.5.1.1 Microphone-to-Source Distance

Microphones are normally used in the direct field.
Under this condition, inverse square law attenuation
prevails, meaning that each time the distance is doubled,
the microphone output is reduced 6 dB. For instance,
moving from a microphone-to-source distance of 2.5 to
5 cm (1 to 2 in) has the same effect as moving from
15 to 30 cm (6 to 12 in), 1 to 2 ft (30 to 60 cm), or 5 to
10 ft (1.5 to 3 m).
Distance has many effects on the system. In a rein-
forcement system, doubling the distance reduces gain

before feedback 6 dB; in all systems, it reduces the

effect of microphone-to-source variations.

Using the inverse-square-law equation for attenuation,

(47-15)

it can be seen, at a microphone-to-source distance of

2.5 cm (1 in), moving the microphone only 1.25 cm

(½ in) closer will increase the signal 6 dB and 1.25 cm

(½ in) farther away will decrease the signal 3.5 dB for a

total signal variation of 9.5 dB for only 2.5 cm (1 in) of

total movement! At a source-to-microphone distance of

30 cm (12 in), a movement of 2.5 cm (1 in) will cause a

signal variation of only 0.72 dB. Both conditions can be

used advantageously; for instance, close micing is

useful in feedback-prone areas, high noise level areas

(rock groups), or where the talent wants to use the

source to microphone variations to create an effect.

The farther distances are most useful where lecterns

and table microphones are used or where the talker

wants movement without level change.

The microphone-to-source distance also has an effect

on the sound of a microphone, particularly one with a

cardioid pattern. As the distance decreases, the prox-

imity effect increases creating a bassy sound (see

Section 16.2.3.1). Closing in on the microphone also

increases breath noise and pop noise.

16.5.1.2 Distance from Large Surfaces

When a microphone is placed next to a large surface

such as the floor, 6 dB of gain can be realized, which

can be a help when far micing.

As the microphone is moved away from the large

surface but still in proximity of it, cancellation of some

specific frequencies will occur, creating a notch of up to

30 dB, Fig. 16-65. The notch is created by the cancella-

tion of a frequency that, after reflecting off the surface,

reaches the microphone diaphragm 180° out of polarity

with the direct sound.

The frequency of cancellation, fc, can be calculated

from the equation

(16-16)

where,

c is the speed of sound, 1130 feet per second or 344

meters per second,

0.5 is the out-of-polarity frequency ratio,

Dr 1 is the reflected path from the source to the surface in

feet or meters,

Table 16-1. Sensitivities of Various Types of
Microphones

Type of Microphone Sp Sv

Carbon-button 60 to50 dB

Crystal 50 to 40 dB

Ceramic 50 to 40 dB

Dynamic (moving coil) 60 to 52 dB 85 to 70 dB

Capacitor 60 to 37 dB 85 to 45 dB

Ribbon-velocity 60 to 50 dB 85 to 70 dB

Transistor 60 to 40 dB

Sound power 32 to 20 dB

Line level 40 to 0 dB 20 to 0 dB

Wireless 60 to 0 dB 85 to 0 dB

EINdBm –10198 dB log bw

10 logZ 6 20–– log0.775 V.

+

+

=

attenuationdB 20

D 1

D 2

= log------

fc 0.5c

Dr 1 Dr 2 –+ Dd

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