Introduction to Acoustics

1040 Part H Engineering Acoustics
Part H 24.6
becomes
LC21 = (L2 + Lm1 + Ld + LWA)
− (L1 + Lm2 + Ld + LWB) . (24.48)
Subtracting (24.48) from (24.47), the difference in
level between the two microphones is
(L1 − L2) = 1/2(LC12 − LC21) . (24.49)
If either L1or L2 is the sensitivity level of a calibrated
reference microphone, then the sensitivity level
of the other microphone (the test microphone) can be deduced.
It is interesting to point out that with this method,
the level Ld of the driving sound need not have a flat
response or known levels.
The above relatively simple theoretical consideration
has assumed that the input capacitances of the two
microphones are identical, i. e. the same model of microphones,
and the same model of preamplifiers used for
both channels such that their input capacitances have the
same value.
If the input capacitances Cm of the preamplifiers
are the same but the two models of microphones are
different, then a correction Lcorr should be added to the
right-hand side of (24.49), where
Lcorr = 20 log[C2/(Cm + C2)]
− 20 log[C1/(Cm + C1)] . (24.50)
The two terms in (24.50) are the capacitance corrections
for the two microphones; and C1 and C2 are
the capacitances of microphone 1 and microphone 2,
respectively.
It is important to realize that the physical positions
of the reference microphone should be repeated accurately
before and after the microphones are interchanged
such that the sound field at the reference microphone
is unchanged. The difference in level readings shown
in (24.47) and(24.48) can be measured with the ratio
provision of a precision AC voltmeter.
Uncertainties
The uncertainty for the above method depends on the reference
microphone uncertainty that may be of the order
of 0.05 dB. With very sophisticated measuring methods
such as phase-lock amplifiers the uncertainty introduced
by this comparison method to be added to the uncertainty
of the reference microphone [24.57] is estimated to be
less than 0.1dB.
24.6.2 Comparison Method
with a Calibrator
In general, there are five components and three basic
steps in this comparison method [24.57]. The components
are the reference microphone, the test microphone,
a stable sound source, which is usually an acoustical calibrator
that generates a constant sound pressure level,
a microphone preamplifier and an acoustical level indicator
such as a measuring amplifier or a voltmeter.
Since the calibrator is used as a transfer standard, its
sound pressure level need not be known.
The three basic steps are as follows
1. The reference microphone which is connected to the
preamplifier is inserted into the acoustical calibrator
and a level reading Lr is noted.
2. The reference microphone is replaced with the test
microphone. With the same acoustical calibrator,
a second level reading Lt is noted.
3. The test microphone is removed, and again with
the reference microphone, step 1 is repeated to give
a third level reading.
The agreement between the first and third level readings
with the reference microphone is a confirmation
of the validity of the second level reading obtained
with the test microphone, that is, to assure that nothing
has changed after the first level reading is taken.
The difference between the level readings, (Lt − Lr) is
the difference in open-circuit sensitivities (expressed in
decibels) between the two microphones if the following
conditions are satisfied:
1. The equivalent volumes of the two microphones are
identical, or the equivalent volume of the sound calibrator
is much larger than that of the microphones,
such that any difference in the front volumes of the
microphones does not produce a significant change
in the sound pressure levels measured in steps 1 and
2 above.
2. The capacitances of the two microphones are identical.
Since the preamplifier has a finite input
impedance (a very high resistance in parallel with
a small known capacitance), the loading on the
microphone will be different if the microphone capacitances
were to be different, and this will produce
a small change in the overall combined sensitivity of
the microphone and preamplifier.
3. The environmental conditions remain constant during
the measurements.