MIL-PRF-26542F
4.5.5.1.2 Frequency-response and sensitivity to a "distant" sound source for noise cancellation
characteristics measurements. The frequency-response characteristics and sensitivity to a "distant"
sound source shall be determined by measuring the voltage output of the microphone developed across a
resistive load (see 3.1) when the microphone is placed in a calibrated sound field at least 1 meter
minimum from a sound source in an anechoic chamber.
4.5.5.1.2.1 Calibration of a sound field. The sound field shall be calibrated at a point not less than 1
meter from the sound source by using a Western Electric 640-AA condenser microphone or an approved
equivalent using the free field calibration of the calibrating microphone. The calibration of the condenser
microphone 640-AA shall be in accordance with ANSI S1.15. The point where the calibration microphone
is placed shall be known as the "calibration point".
4.5.5.1.2.2 Sensitivity to a "distant" sound source. The sensitivity of the microphone to a distant sound
source at the calibration point shall be established by measuring the voltage developed by the
microphone across a resistive load (see 3.1) and referred to a sound pressure of 74 dB relative to 20 µPa
at 1,000 Hz. The sensitivity measurement shall be made twice, in each case with the cylindrical axis of
the microphone element normal to the axis of the sound source. In the first measurement, called
"sensitivity A", the talking port of the microphone shall point away from the sound source. In the second
measurement, called "sensitivity B", the talking port shall point toward the sound source. In each
measurement, the microphone shall be adjusted about the cylindrical axis of the microphone element to
obtain maximum outputs.
4.5.5.1.2.3 Frequency-response to a "distant" sound source. The frequency-response of the
microphone at the calibration point shall be established by measuring the voltage developed by the
microphone across a resistive load (see 3.1) as a continuous function of frequency from 200 Hz to 6,000
Hz, referred to a sound field of constant sound pressure as determined with the standard condenser
microphone. Two frequency-response curves shall be made; in each case with the cylindrical axis of the
microphone element normal to the axis of the sound source. The frequency response obtained with the
talking port of the microphone pointing away from the sound source shall be known as a "distant
response A" and that obtained with the talking port pointing toward the sound source shall be known as
"distant response B".
4.5.5.1.3 Formation of noise cancellation characteristics.
4.5.5.1.3.1 Plotting of data. The close frequency-response characteristics obtained herein shall be
plotted on semi-logarithmic coordinate paper with the length of a 10 to 1 frequency interval along the
abscissa equal to the length of 30 dB along the ordinate. The "distant response A" shall be plotted on the
same coordinates with the point on the curve at 1,000 Hz located below the point on the close response
curve at 1,000 Hz by the number of dB equivalent to the ratio of the close sensitivity in volts at 1,000 Hz
to the "distant sensitivity" in volts at 1,000 Hz, i.e.,
Close sensitivity at 1,000 Hz
Number of dB = 20 LOG10
Distant sensitivity at 1,000 Hz
The same procedure shall be followed with "distant response B" and "distant sensitivity B". A fourth curve
shall be obtained by averaging the ordinates of "distant response A" and "distant response B" at each
point between 200 Hz and 6,000 Hz and called the "average distant response curve".
4.5.5.1.3.2 Construction of "difference curves". The three curves "distant response A", "distant
response B", and "average distant response", shall be individually subtracted from the close response
curve, and the differences plotted. These shall be called, respectively, "difference curve A", "difference
curve B", and "average difference curve".
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