Handbook for Sound Engineers

Microphones 517

calculated from the internal impedance and the

open-circuit voltage of the microphone. This specifica-

tion also indicates the ability of a microphone to convert

sound energy into electrical power. The equation is

(16-6)

where,
Sp is the power level microphone sensitivity in decibels,
Vo is the open-circuit voltage produced by a 1μbar
(0.1 Pa) sound pressure,
Ro is the internal impedance of the microphone.

The form of this specification is similar to the voltage
specification except that a power as opposed to a voltage
reference is given with the sound pressure reference. A
1 mW power reference and a 10μbar (1 Pa) pressure
reference are commonly used (as for the previous case).
This form of microphone specification is quite mean-
ingful because it takes into account both the voltage
output and the internal impedance of the microphone.
Sp can also be calculated easily from the open-
circuit voltage sensitivity

(16-7)

where,
Sp is the decibel rating for an acoustical input of
94 dBSPL (10 dyn/cm^2 ) or 1 Pa,
Z is the measured impedance of the microphone (the
specifications of most manufacturers use the rated
impedance).

The output level can also be determined directly

from the open-circuit voltage

(16-8)

where,

Eo is the open-circuit voltage,

Z is the microphone impedance.

Because the quantity 10 log(E^2 /0.001Z) treats the

open-circuit voltage as if it appears across a load, it is

necessary to subtract 6 dB. (The reading is 6 dB higher

than it would have been had a load been present.)

16.4.3 Electronic Industries Association (EIA)

Output Sensitivity

The Electronic Industries Association (EIA) Standard

SE-105, August 1949, defines the system rating (GM) as

the ratio of the maximum electrical output from the

microphone to the square of the undisturbed sound field

pressure in a plane progressive wave at the microphone

in decibels relative to 1 mW/0.0002 dyn/cm^2. Expressed

mathematically,

(16-9)

where,

Eo is the open-circuit voltage of the microphone,

P is the undisturbed sound field pressure in dyn/cm^2 ,

Zo is the microphone-rated output impedance in ohms.

For all practical purposes, the output level of the

microphone can be obtained by adding the sound pres-

sure level relative to 0.0002 dyn/cm^2 to GM.

Because GM, SV, and SP are compatible, GM can also

be calculated

(16-10)

where,

GM is the EIA rating,

RMR is the EIA center value of the nominal impedance

range shown below.

The EIA rating can also be determined from the

chart in Fig. 16-60.

Figure 16-59. Method of determining open-circuit voltage

sensitivity of a microphone. (From Reference 7.)

Random

noise

generator

Filter

set

Power

amplifier

Test

loudspeaker

Micro-

voltmeter

Sound

level

meter

94 dB SPL

Microphone

to be tested

Sp 10

Vo^2

Ro

= log----- -+44 dB

Sp=Sv–44 dB 10 logZ+

Sp 10

Eo^2

0.001Z

= log-----------------–6 dB

Ranges (ohms) Values Used (ohms)

20 – 80 = 38

80 – 300 = 150

300 – 1250 = 600

1250 – 4500 = 2400

4500 – 20,000 = 9600

20,000–70,000 = 40,000

GM 20

Eo

P

= log----- -–50 dB 10 logZo–

GM=Sv–5 10 logRMR– 0 dB