MIL-PRF-25670 Earphone Elements, General Specification ForThis specification covers the general requirements for earphone elements. These are products which have screw type terminals, and are used on communication headsets for applications requiring communication within high-noise conditions, at either ground-level or altitude. These parts meet established United States Air Force safety standards \(see 6.1\) for signal-generating equipment mounted in close-proximity to the human ear.

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MIL-PRF-25670B

4.5.3 Sensitivity. (see 3.5.2) The earphone element shall be mounted as shown in the test circuit

(see figure 1). Adjust the output from the audio oscillator to 1 mW at 1 kHz as measured by the

electronic voltmeter (voltmeter No. 1). The sensitivity shall be observed at 1 kHz at ground level and

when specified (see 3.1), at 25,000 feet. NOTE: The voltage, v , to supply 1 mW is determined by

first measuring the impedance, z, at 1 kHz and using the relation v = sqrt ( z X 0.001).

4.5.4 Impedance. (see 3.5.3) The earphone element shall be mounted in the coupler as shown in

figure 1. Power of 1 mW shall be applied across the terminals, and the impedance measured over

the frequency response range specified (see 3.1). The impedance shall be measured by either the

voltage method, or the resistance-substitution method. Recordings may be either the point-to-point or

the automatic recorder method.

4.5.4.1 Automatic recorder. If the impedance is recorded on an electro-mechanical graph

recorder, the minimum writing speed shall be 10 inches per second, the maximum chart speed shall

be 30 inches per minute, and the writing speed and chart speed shall be noted on the graph paper.

The output signal shall be recorded in the dB value above 0.0002 dyne/cm2.

4.5.4.2 Point-to-point. Measure and record the impedance in ohms at increments of 100 Hz from

100 to 1,000 Hz and at increments of 250 Hz from 1,000 to 3,000 Hz. The graph shall show the

impedance on the ordinate scale and the Hz value on the abscissa scale, recording from 100 to

10,000 Hz.

4.5.4.3 Voltage method. At the option of the manufacturer, the impedance may be calculated by

using a voltmeter. Using a circuit similar to that shown in figure 1, measure the voltage across the

300 ohm resistor and across the earphone element under

test. The earphone element impedance (Z) is equal to

300 X

VA - VB

4.5.4.4 Resistance-substitution method. The impedance shall be measured by substituting a

variable resistor in series with the element and adjusting for a 6 dB drop in acoustical output.

Measuring the value of the variable resistor provides the earphone impedance.