G. This Headset-Microphone Kit is designed to fit both the regular and large size Combat Vehicle Crewman\'s Helmet. The electrical characteristics of the kit are compatible with Tactical Radio Systems, such as the VRC-12.

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MIL-DTL-55535A

4.7 Microphone tests.

4.7.1 Response test. The response of the microphone shall be calibrated and tested using the setup

in accordance with CECOM Drawing SC-D-10598 and measured by means of a suitable ac-electronic

voltmeter, having an input impedance of at least 0.5 (�) megohm. The response of this microphone shall

be determined using a constant sound pressure of 2.8 Pa (Pascals), at the point where the microphone is

to be placed. The driver unit used to obtain the desired sound pressure input, (Western Electric Co.,

Type No. 555W, or equal) shall have been previously calibrated by means of a condenser microphone

such as Western Electric Co. 640AA, or equal, (calibrated by "reciprocity method" in accordance with

ANSI S1.15/Part 2). The sample dynamic microphone shall be positioned with its face 0.25 (�) inch

directly in front of the driver unit with the diaphragm of the microphone parallel to the driver unit. The

voltage-frequency response of the microphone shall be measured across a non-inductive load resistance

of 150 ohms. The response of the microphone shall be tested across the frequency range of 300 to

3,500 Hz at approximately the following increments: 100 Hz increments from 300 to 1,000 Hz and 250

Hz increments from 1,000 to 3,500 Hz. The microphone shall meet the requirements of 3.6.1.

4.7.2 Distortion (see 3.6.2). Harmonic distortion shall be determined using the same equipment and

circuitry as used in 4.7.1 except that the constant sound pressure shall be 115 dB referenced at 20 �Pa

(micro-Pascals) and the output of the microphone terminated in 150 ohms shall be connected to a

Hewlett-Packard Distortion Analyzer Model 330C, or equal, for distortion measurements. The distortion

shall not be greater than 5 percent. Measurements will be made from 300 Hz to 1,000 Hz inclusive, at

100 Hz increments and from 1,000 to 3,500 Hz inclusive at 500 Hz increments.

4.7.3 Signal-to-noise ratio.

4.7.3.1 Noise spectrum. By means of apparatus specified on figure 3, and the test circuit on figure 4,

provision shall be made for the production of the noise spectrum, in table VI, at an rms sound pressure of

115 dB above a reference level of 20 �Pa, as measured at the microphone diaphragm. The "Noise"

source shall incorporate a loudspeaker capable of producing a sound pressure of 115 dB above a

reference level of 20 �Pa at the rear, as well as the front of the microphone. The loudspeaker shall be

located as far away from the microphone as practicable. The calibrating condenser microphone shall be

mounted in the test rig, in the box, 0.25 (�) inch from the mouth of the signal speaker, which shall be

short-circuited during adjustments of the noise spectrum.