Handbook for Sound Engineers

516 Chapter 16

The operation of the Shure SM-81 microphone is
explained in Section 16.2.3.4.

16.4 Microphone Sensitivity^7

Microphone sensitivity is the measure of the electrical
output of a microphone with respect to the acoustic
sound pressure level input.

Sensitivity is measured in one of three methods:

The common sound pressure levels used for
measuring microphone sensitivity are:

94 dB SPL is recommended since 74 dB SPL is too
close to typical noise levels.

16.4.1 Open-Circuit Voltage Sensitivity

There are several good reasons for measuring the
open-circuit voltage:

• If the open-circuit voltage and the microphone
  impedance are known, the microphone performance
  can be calculated for any condition of loading.


• It corresponds to an effective condition of use. A
  microphone should be connected to a high impedance
  to yield maximum SNR. A 150–250: microphone
  should be connected to 2 k: or greater.


• When the microphone is connected to a high imped-
  ance compared to its own, variations in microphone
  impedance do not cause variations in response.

The open-circuit voltage sensitivity (Sv) can be

calculated by exposing the microphone to a known SPL,

measuring the voltage output, and using the following

equation:

(16-4)

where,

Sv is the open-circuit voltage sensitivity in decibels re

1 V for a 10 dyn/cm^2 SPL (94 dB SPL) acoustic input

to the microphone,

Eo is the output of the microphone in volts,

dBSPL is the level of the actual acoustic input.

The microphone measurement system can be setup as

shown in Fig. 16-59. The setup requires a random-noise

generator, a microvoltmeter, a high-pass and a low-pass

filter set, a power amplifier, a test-loudspeaker, and a

sound level meter (SLM). The SLM is placed a specific

measuring distance (about 5–6 ft or 1.5–2 m) in front of

the loudspeaker. The system is adjusted until the SLM

reads 94 dB SPL (a band of pink noise from 250 to

5000 Hz is excellent for this purpose). The microphone

to be tested is now substituted for the SLM.

It is often necessary to know the voltage output of

the microphone for various SPLs to determine whether

the microphone will overload the preamplifier circuit or

the SNR will be inadequate. To determine this, use

(16-5)

where,

Eo is the voltage output of microphone,

Sv is the open-circuit voltage sensitivity,

dBSPL is the sound pressure level at the microphone.

16.4.2 Maximum Power Output Sensitivity^7

The maximum power output sensitivity form of specifi-

cation gives the maximum power output in decibels

available from the microphone for a given sound pres-

sure and power reference. Such a specification can be

Figure 16-58. Shure SM81 electret capacitor microphone.
Courtesy Shure Incorporated.

Open-circuit voltage 0 dB = 1 V/μbar

Maximum power output 0 dB = 1 mW/10 μbar

= 1 mW/Pa

Electronic Industries

Association (EIA) sensitivity

0 dB = EIA standard

SE-105

94 dB SPL 10 dyn/cm^2 SPL 10 μbar or 1 Pa

74 dB SPL 1 dyn/cm^2 SPL 1 μbar or 0.1 Pa

0 dB SPL 0.0002 dyn/cm^2 SPL 0.0002 Pa or

20 μPa—threshold

of hearing

Sv= 20 logEo dBSPL 94+–

Eo 10

Sv dBSPL 94–+

20

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