Handbook for Sound Engineers

518 Chapter 16

16.4.4 Various Microphone Sensitivities

Microphones are subjected to sound pressure levels
anywhere from 40 dB SPL when distant micing to
150 dB SPL when extremely close micing (i.e., ¼ inch
from the rock singer’s mouth or inside a drum or horn).

Various types of microphones have different sensi-
tivities, which is important to know if different types of
microphones are intermixed since gain settings, SNR,
and preamplifier overload will vary. Table 16-1 gives
the sensitivities of a variety of different types of
microphones.

16.4.5 Microphone Thermal Noise

Since a microphone has an impedance, it generates
thermal noise. Even without an acoustic signal, the
microphone will still produce a minute output voltage.
The thermal noise voltage, En, produced by the elec-
trical resistance of a sound source is dependent on the
frequency bandwidth under consideration, the magni-

tude of the resistance, and the temperature existing at

the time of the measurement. This voltage is

(16-11)

where,

k is the Boltzmann’s constant, 1.38 × 10–23 J/K,

t is the absolute temperature, 273° + room temperature,

both in °C,

R is the resistance in ohms,

bw is the bandwidth in hertz.

To change this to dBv use

(16-12)

The thermal noise relative to 1 V is 198 dB for a

1 Hz bandwidth and 1: impedance. Therefore,

(16-13)

where,

Figure 16-60. Microphone sensitivity conversion chart.

SPL

620

100

10

1.0

0.1

0.01

0.001

0.002

110

100

90

80

70

60

50

40

30

20

0.01

0.1

1.0

10.0

100.0

90

80

70

60

50

40

30

Threshold of pain

$ 0.1% atmosphere

Peak rms sound with lips at

microphone; riveter at 35 ft.

Reference sound field

Peak rms sound pressure at

1 ft from man's mouth,

conversational speech

Average rms sound

Ordinary conversation at 3 ft

Average medium office

Decibels

Theater with audience

Country house

Rustle of leaves

in gengle breeze

Threshold of hearing

Sound pressure level

Dynes/square centimeter

SV Millivolts

Open-circuit voltage response

SV = 20 LOG10 E/P dB

E = Open circuit voltage

P = Sound pressure in dB referred to 1 MBAR

Open Circuit power response

SP = SV LOG R + 44 dB

R = Nominal impedance

Sensitivity response

GM = SV 10 LOG 10 RMR 50 dB

RMR = Center value of nominal

impedance range

Impedance RMR

40,000

9,600

2,400

600

150

38

Nominal

impedance

in ohms

100,000

80,000

60,000

40,000

20,000

10,000

8,000

6,000

4,000

2,000

1,000

800

600

400

200

100

80

60

40

20

10

8

6

4

2

1

Gm



190

180

170

160

150

140

130

120

130

120

110

100

90

80

70

60

50

40

30

20

10

0

En= 4 ktR bw

EINdBv 20

En

0.775

= log-------------

TN

1 V

---------=–10198 dB++log bw 10 logZ