MIL-DTL-25547C
4.6 Methods of examination and test.
4.6.1 Visual and mechanical examination. The loudspeaker shall be examined to verify that the materials, design,
construction, physical dimensions, marking, and workmanship are in accordance with the applicable requirements (see
3.2, 3.3, 3.24, and 3.25).
4.6.2 Voice coil impedance (see 3.4). The loudspeaker shall be hand held on the side opposite the cone. There
shall be no obstruction within 3 feet of the front of the cone. The voice coil terminals shall be connected in series with
a variable resistor across the output of a low-impedance 1,000 Hz signal source. The voltage drops across the voice
coil and the variable resistor shall be determined with an electronic voltmeter. The value of the variable resistor shall
be adjusted until the voltage drops are equal and the resistance of the variable resistor shall then be measured with a
resistance bridge or other suitable equipment. The voice coil impedance shall be considered as numerically equal to
the measured value of resistance of the variable resistor.
4.6.3 Voice coil polarity (see 3.5). A direct current voltage of 4.5 ± 1.5 volts shall be applied to the terminals of the
voice coil so that it causes the diaphragm of the loudspeaker to move in a direction away from the magnetic assembly.
The terminal connected to the positive voltage potential shall be marked as the positive terminal of the voice coil.
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4.6.4 Stray magnetic field (see 3.6). The stray magnetic field of the loudspeaker shall be determined, by using a
compass that meets the requirements of A-A-52024 or engineering approved equivalent. The loudspeaker shall be
placed with the geometric center of the magnet structure 20 inches ± 1 inch from the pivot point of the compass needle
and in the plane of rotation of the needle (see figure 3). The loudspeaker shall be oriented in all directions and the
maximum deflection of the compass observed. The test shall be made in a location substantially free from stray
magnetic disturbances. The maximum deflection of the compass shall be determined as specified in 4.6.4.1 or 4.6.4.2
at the option of the manufacturer.
4.6.4.1 Constant compass location (six permanently mounted compasses. With a compass permanently mounted
in each location indicated on figure 3, the readings shall be observed on each compass, as the loudspeaker is oriented
in all directions.
4.6.4.2 Changing compass location (one movable compass). In each direction that the loudspeaker is oriented, the
compass shall be relocated to each of the six compass locations and oriented so that the degree markings on the
compass line up (within ± 0.5 degrees) with the degree markings on the table as shown on figure 3. The reading on
the compass shall be observed in each location for each direction the loudspeaker is oriented.
4.6.5 Sensitivity.
4.6.5.1 At sea level (see 3.7.1). A sensitivity measurement shall be made with a constant signal input 3.94 ± .01
volts in a free field at a point on the principal axis 4 feet ± 1 inch from the front of the loudspeaker. The average
sensitivity for each frequency band may be obtained by measuring the area between the frequency response and any
reference line over the appropriate frequency band, provided this line was established from a basic reference level of
0.0002 dyne per square centimeter.
4.6.5.2 At altitude (see 3.7.2). The sensitivity test specified in 4.6.5.1 shall be repeated at a simulated altitude of
15,000 feet above sea level.
4.6.6 Frequency response (see 3.8). The loudspeaker shall be mounted and placed 4 feet ± 1 inch from a calibrated
Western Electric Type 640AA (or equal) condenser microphone and amplifier. The microphone shall be placed on the
axis of the loudspeaker. A constant voltage having an rms value of 3.94 ± .05 volts shall be applied to the voice coil
terminals and the frequency varied continuously from 200 to 7,000 Hz. The acoustic output shall be recorded on a
direct-writing strip graph chart using an automatic plotter or curve tracer with a minimum writing speed of 10 inches per
second and a maximum chart speed of 30 inches per minute; or, at the option of the manufacturer, point-to-point
measurements may be made every 50 Hz from 200 to 600 Hz; every 100 Hz from 600 to 1,500 Hz; every 250 Hz from
1,500 Hz to 2,000 Hz; every 100 Hz from 2,000 to 2,500 Hz; every 250 Hz from 2,500 to 3,000 Hz; and every 500 Hz
from 3,000 to 7,000 Hz with the data recorded and a graph drawn.
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