MIL-PRF-25670B
4.5.5 Harmonic distortion. (see 3.5.4) The earphone element shall be preconditioned as specified
in 4.5.5.1. The earphone element shall be mounted in the coupler as shown in figure 1 and tested as
follows. Power of 1 mW shall be applied to the earphone element, and distortion measured at
minimum 1/3-octave intervals over the specified frequency response range (see 3.1).
4.5.5.1 Preconditioning. A power of 100 mW shall be applied to the element continuously across
the frequency response range as specified (see 3.1), two times. NOTE: The voltage, v , to supply
100 mW shall be determined by first measuring the impedance, z, at 1 kHz and using the relation v =
sqrt (z X 0.1).
4.5.5.2 Distortion analyzer. The total harmonic distortion shall be determined by a distortion
analyzer having the following minimum requirements:
a. Frequency range
- Fundamental frequency from 20 to 20,000 Hz.
b. Frequency calibration
- ±2 percent from 20 to 20,000 Hz.
c. Harmonic measurement
- ±3 percent of full scale value for distortion levels as low as 0.5
percent.
4.5.6 Dielectric withstanding voltage. (see 3.5.5) The earphone element shall be tested in
accordance with method 301 of MIL-STD-202. The following details shall apply:
a.
Test voltage - 500 volts.
b.
Nature of potential - 60 Hz.
c.
Points of application - Between one of the terminals and the metallic case, or other
metallic surface which is not covered by a nonconducting coating.
4.5.7 Stray magnetic field. (see 3.5.6) Stray magnetic field measurements shall be made in a
shielded room or in an area free from appreciable magnetic disturbances. The earphone element
shall be placed with its geometric center at a distance as specified (see 3.1) from the pivot point of a
compass needle, and in the perpendicular bisector of the needle in the plane of rotation of the needle.
The earphone element shall be moved and rotated in a circular manner around the compass at the
distance specified, and the maximum deflection of the compass observed. The compass shall meet
the requirements of A-A-52024 Type II, or equivalent as approved by the qualifying activity.
4.5.8 Effect of stray magnetic field on the earphone element. (see 3.5.7) The earphone element
shall be firmly pressed against the ear, and the listener shall listen for 400 Hz pickup when the
earphone element is oriented for maximum signal 1 foot away from a 400 Hz power line carrying a
current of 30 amperes. The return line shall be at least 15 feet away from the line used for the test.
This test shall be conducted in an ambient acoustic noise level of not more than 65 dB with the
earphone element terminals open, and short-circuited, respectively.
4.5.9 Speech Intelligibility. (see 3.5.8) The intelligibility test shall be performed using the modified
rhyme test under the guidelines of the American National Standards Institute, Method for Measuring
Intelligibility of Speech Over Communications Systems, ANSI S3.2. Both the talker and the listener
are to be in an ambient noise level not exceeding 75 dB Overall Sound Pressure Level. The minimum
performance level (in percentage correct) shall be checked for compliance with the requirements of
3.5.8.
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