MIL-DTL-9415B
4.5.2.1 Loudspeaker (see 3.4).
4.5.2.1.1 Voice coil polarity. A direct current potential of 1.5 to 6 volts shall be applied to the voice coil terminals
with the positive terminal connected to the red terminal of the voice coil. The direction of motion of the loudspeaker
cone shall be observed (see 3.4.2.2.1).
4.5.2.1.2 Voice coil impedance. The impedance of the voice coil shall be determined over the frequency range of
200 Hz to 7,000 Hz with the loudspeaker mounted in the housing with the amplifier. Impedance measurements shall
be made with a constant signal input of 2.65 volts. In individual tests, the voice coil impedance shall be measured at
the frequency above fundamental resonance which gives the lowest impedance (see 3.4.2.2.2).
4.5.2.1.3 Sensitivity. Sensitivity measurements shall be made with a constant signal input of 2.65 volts in a free
field at a point on the principal axis 3 feet away from the loudspeaker. In individual tests, sensitivity measurements
shall be made for the frequency bands of 320-500 Hz, 500-800 Hz, 800-1,250 Hz, 1,250-2,000 Hz, 2,000-3,200 Hz,
3,200-5,000 Hz, and 5,000-6,000 Hz; the sensitivity measurements for the 320-1,250 Hz, 1,250-6,000 Hz, and 320-
6,000 Hz bands may be omitted. The average sensitivity for each frequency band may be obtained by measuring the
area between the response-frequency characteristic and any reference line provided that this line was established
from a basic reference level of 20 µPa (micropascals) (see 3.4.2.3.1).
4.5.2.1.4 Response-frequency characteristic. The response-frequency characteristic of the loudspeaker shall be
measured as a continuous function of frequency over the range of 200 Hz to 7,000 Hz in a free field at a point on the
principal axis 3 feet away from the loudspeaker. Response measurements shall be made with a constant signal input
of 2.65 volts. In performing individual tests it is not necessary that the response-frequency characteristic of the
loudspeaker be obtained in a manner that will provide a permanent record. For first article and sampling tests, the
response-frequency characteristic shall be recorded and plotted on a semi-logarithmic coordinate paper with
response in dB on the linear ordinate scale and frequency in Hz on the logarithmic abscissa scale. The length of a
10-to-1 frequency interval on the abscissa scale shall equal the length of 30 dB on the ordinate scale (see 3.4.2.3.3).
4.5.2.1.5 Acoustic quality test. A discrete constant voltage having an rms value of 3.0 volts and a frequency
between 200 Hz and 1000 Hz shall be applied to the input terminals of the loudspeaker. The acoustic output of the
loudspeaker shall be checked for any buzzes, rattles, or other spurious sounds (see 3.4.2.3.8).
4.5.2.1.6 Ruggedness. The loudspeaker shall be operated at 400 Hz for 50 hours with a signal input of 4.5 volts.
The operation may be continuous or in 8-hour intervals. The sensitivity, response-frequency characteristic, and
distortion-frequency characteristic of the loudspeaker shall be measured before and after the test (see 3.4.2.3.9).
4.5.2.1.7 Directional characteristics. The off-axis sound pressure shall be measured in a free field at a point 3 feet
away from the loudspeaker at angles of 15 degrees and 30 degrees with respect to the principal axis. The vertices of
the angles shall be the intersection point of the loudspeaker principal axis and the plane containing the mounting
surface of the loudspeaker (see 3.4.2.3.5).
4.5.2.1.8 Linearity. The acoustic output of the loudspeaker in dB relative to 20 µPa (micropascals) shall be
measured by using a warble signal, having a frequency range from 750 Hz to 1,250 Hz and a rate of sweep of 5.5
times per second. Input voltages shall be as follows: 0.112, 0.200, 0.355, 0.632, 1.12, 2.00, 3.55, 6.32 volts. The
acoustic output shall be plotted as a linear function of the input voltage (see 3.4.2.3.6).
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