NBSIR 76-1090
EPA 550/9-76-009
Environmental Effects on
Microphones of Various
Constructions
Gale R. Hruska, Edward B. Magrab, William 8. Penzes
Institute for Basic Standards
National Bureau of Standards
Washington, D. C. 20234
July 1976
Final Report
Prepared for
U. S. Environmental Protection Agency
Office of Noise Abatement and Control
Washington, D. C. 20460
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NBSIR 76-1090
EPA 550/9-76-009
ENVIRONMENTAL EFFECTS ON
MICROPHONES OF VARIOUS
CONSTRUCTIONS
Gale R. Hruska, Edward B. Magrab, William B. Penzes
Institute for Basic Standards
National Bureau of Standards
Washington. D. C. 20234
July 1976
Final Report
Prepared for
U. S. Environmental Protection Agency
Office of Noise Abatement and Control
Washington, D. C. 20460
•t--. _ -* •*
•*•,»„••
U.S. DEPARTMENT OF COMMERCE. Elliot L Richardson. Secretary
Dr. Betay Ancfcer-Johnwn, AutsUutt Secretary tor Science end Technology
NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director
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ABSTRACT
The pressure sensitivities of two "1/2-inch" electret, two "1-inch"
ceramic, and two back-vented "1-inch" condenser microphones were measured
for numerous combinations of temperature, percentage relative humidity,
and frequency. The two condenser microphones were calibrated by the reci-
procity technique at each combination of temperature, relative humidity
and frequency. The condenser microphones were then used as calibrated
sources to determine the pressure sensitivities of the other microphones.
Insert voltage techniques were used to eliminate the environmental effects
on the electronics. It was found that the back-vented condenser micro-
phones are insensitive to changes in relative humidity. At frequencies
considerably below their resonance frequencies they exhibited only a very
small change in sensitivity with temperature. At frequencies closer to
the resonance frequency the temperature coefficient increases approximately
fourfold. The temperature and humidity coefficient for the electret and
ceramic microphones could not be determined due to the instability in their
sensitivities which produced changes that were larger than those induced
by the temperatures and humidities.
Key Words: Calibration; ceramic; condenser; electret; humidity;
microphones; reciprocity; sensitivity; temperature.
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INTRODUCTION
The introduction of federal regulations stipulating the permissible
noise levels in the environment has made it necessary for many acoustic
measurements to be performed over extended periods of time during which
the temperature and humidity vary markedly. These changes in the environ-
mental conditions could affect the sensitivity of the microphones. There
exist some published data1"5 concerning the effects of environmental con-
ditions on microphones of various construction. However these data appear
in abbreviated form, usually in terms of frequency-independent temperature
coefficients. Furthermore the experimental techniques employed to obtain
this information are rarely stipulated. It was felt therefore that a
modestly thorough investigation, which determined the changes in the pres-
sure sensitivity of those commercially available microphone constructions
most frequently found in noise-measuring and noise-monitoring systems and
which used a consistent and standardized measurement procedure would pro-
vide useful and meaningfully-comparable data.
Six commercially-available microphones were measured: two "1/2-inch"
electret, two "1-inch" ceramic, and two "1-inch" condenser microphones.
The condenser microphones had their pressure equalization port back-vented
through a dehumidifier containing silica gel.* The sensitivity was meas-
ured (where physically possible) at the following combinations of frequency,
*By their very nature the small volumes in the electret and ceramic micro-
phone cartirdges are unaffected by humidity in the same way that condenser
microphones are and therefore do not require a desiccant. However, certain
electret or ceramic configurations protect their electronics from the
effects of humidity with a desiccant.
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temperature, and relative humidity: frequency: 0.1, 0.2, 0.5, 1, 2, 3, 4,
and 5 kHz; temperature: every 10 °C from -20 to +50 °C; and (nominal) rela-
tive humidity: every 10% RH from 25 to 95% RH.
TEST METHOD AND PROCEDURE
The sensitivities of the microphones were measured using the procedures
specified in the U.S. Standard.6 The NBS facility used to calibrate micro-
phones was essentially duplicated, except that the 3-cm3 coupler, the
microphones and the microphone preamplifiers were placed in an environmental
chamber having wet and dry bulb temperature control and spatial uniformity
to within ± 0.2°C. So that the microphone could be inserted into the
coupler, several different size adapter rings, which are used to electri-
cally isolate one of the microphones in the coupler, were required to com-
pensate for thermal expansion and contraction over the temperature range.
The microphone preamplifiers, having insert voltage capabilities, provided
a means whereby the effects of the environmental conditions on the pre-
amplifiers were eliminated.
The coupler, the preamplifiers and the six microphones (plus an
additional "1-inch" condenser microphone required for the reciprocity
calibration) were placed in the environmental chamber. The desired tem-
perature and relative humidity were set and then two hours were allowed
for the chamber to reach equilibrium. Then the "source" microphone was
placed in the coupler and the "receiver" microphone connected to the pre-
amplifier but not placed in the top of the coupler. An additional fifteen
minutes were allowed for the chamber to return to equilibrium at which time
the receiver microphone was rapidly placed into the coupler. After
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equilibrium had again been established, the measurements were started by
first determining the resonance frequency of the lowest longitudinal mode
of the coupler volume. Then the voltage ratio measurements were made in
the following order: 5, 0.1, 0.2,...4, and 5 kHz. Then the resonance fre-
quency measurement was repeated. By examining the first and final measure-
ments at both 5 kHz and the two resonance frequencies, it was possible to
check that equilibrium conditions existed during the course of the
measurements.
The sequence of measurements was as follows: For a given temperature
and humidity the three condenser microphones, labeled A, B, and C, were
interchanged after each frequency run,as shown in the table below:
Frequency Run Source Receiver
1 A B
2 AC
3 B C
Runs 1 and 2 gave the ratio of the responses of the microphones B and C.
Using the results of Run 3 plus the measurement of the capacitance8 of
microphone B, the absolute pressure response levels of microphones A, B,
and C were calculated. The pressure response of the remaining four micro-
phones was determined using microphone B as the source, with the measure-
ments on the electret microphones preceding those on the ceramic micro-
phones. More specific details of the procedure are given in Appendix A
along with a discussion of the uncertainties in the measurement. The
total time, after warm-up, to perform the reciprocity and comparison meas-
urements on all six microphones at a given temperature and humidity was
approximately 6 hours. The temperatures were changed in the following
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order: 20, 30, 40, 10, 0, -10, -20, 50, and 20°C. At each temperature
the humidity was Incrementally increased from the lowest to the highest
obtainable humidity. The lowest humidity depended on the temperature,
with this minimum value increasing with decreasing temperature. Below
0°C the relative humidity could not be determined with good precision and
only one set of microphone calibrations was made at these temperatures.
The repeated calibration at 20°C was only performed at 44% RH.
DISCUSSION OF RESULTS
The changes in sensitivity of the six microphones normalized to
their respective pressure sensitivity at 1 kHz and 20°C are shown in
Figs. 1 through 8. Figures 1 and 3 show the change in sensitivity of
the "1-inch" condenser microphones as a function of frequency for the
various temperatures. These figures are replotted in Figs. 2 and 4,
respectively, as a function of temperature for selected frequencies.
The data points shown in these four figures are the mean values of the
sensitivity changes for the range of humidities tested. The spread of
the data at virtually every one of the points is less than ± 0.15 dB.
These figures show that the two "1-inch" condenser microphones exhibited
small temperature coefficients (change in microphone sensitivity per
change in temperature) of approximately -0.003 and +0.005 dB/°C, respec-
tively, from -20 to 50°C and for frequencies below 1000 Hz. At frequen-
cies above 1000 Hz the temperature coefficient increased, such that at
5000 Hz they become approximately -0.02 and -0.03 dB/°C, respectively.
This variation in the temperature coefficient as a function of frequency
is due to the relatively large amount of viscous damping introduced by
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the particular backplate design of these microphones, that is, by the
distance between the diaphragm and the backplate and the location, number,
and size of the holes in the backplate. Since the viscous losses greatly
affect the sensitivity near resonance, smaller microphones could be ex-
pected to show less sensitivity change at high frequencies.
The changes in sensitivity for the electret microphones are shown in
Figs. 5 and 6 and for the ceramic microphones in Figs. 7 and 8. In these
figures the vertical bars indicate the range of the data as a function of
humidity. The absence of the vertical bars indicates that the data were
only taken at a single relative humidity. The data were presented in
this form because no meaningful relation could be established for the
change in sensitivity of these microphones as a function of either fre-
quency, temperature or humidity. This type of behavior can be explained
if the changes due to temperature and humidity are much smaller than the
inherent instability of the microphone itself. Consequently it is not
possible to obtain from these measurements the temperature coefficients
of the electret and ceramic microphones.
If the electret and ceramic microphones are subject to these in-
stabilities they will exhibit similar behavior at standard conditions,
20 °C and 44% RH. As can be seen from Tkbles 1-3 the condenser micro-
phones are within ±0.2 dB of their original sensitivities whereas the
electret and ceramic microphones show relatively large variations. These
tables give an indication of the moderately long-term stability of these
types of microphone constructions.
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CONCLUSIONS
From the data presented the following conclusions are reached:
1. The back-vented condenser microphones with a dehumidifier were
insensitive to relative humidity.
2. At frequencies far below the condenser microphone's resonance
frequency the temperature coefficient was extremely small (^ ± 0.005 dB/°C),
3. Condenser microphones containing a large amount of viscous
damping would be expected to have a greatly increased (by a factor of A)
temperature coefficient in the vicinity of the microphone's resonance
frequency.
4. The electret microphones which were tested exhibited short-term
sensitivity instabilities of the order of ± 0.5 dB. The ceramic micro-
phones examined showed short-term instabilities of the order of ± 1 dB
or larger, at some frequencies. The magnitudes of these instabilities
made it impossible to determine the changes in sensitivity as functions
of temperature or relative humidity.
5. The condenser microphones exhibited long-term instabilities
(i.e., over a period of about sixteen weeks) of the order of ± 0.2 dB.
The electret and ceramic microphones showed long-term instabilities of
up to ±1.5 dB, with larger values at a few frequencies.
From the data which were taken, it is not possible to predict with
assurance what uncertainties would occur if the ceramic and electret
microphones were used for outdoor noise measurements. It seems reason-
able, however, to assume that if significant changes in temperature and
relative humidity occur between system calibrations, then the uncer-
tainties in the measured sound levels could be at least of the magnitude
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of the short-term instabilities already mentioned. Furthermore, in actual
sound measuring systems, changes in temperature and relative humidity
could affect the electronics thus introducing additional changes in over-
all system sensitivity.
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REFERENCES
1G. M. Sessler and J. E. West, "Electret Transducers: A Review," J.
Acoust. Soc. Amer. 5_3, 1589-1600 (1973).
2L. L. Beranek, Ed., Noise and Vibration Control (McGraw-Hill Book Co.,
New York, 1971) pp. 48-49.
3G. Rassmussen, "Reliability of Measurement Microphones Under Outdoor
Conditions," Seventh International Congress on Acoustics, Budapest, 1971,
pp. 289-292.
^Rassmussen, "Measurement Microphones," Inter-Noise 74 Proceedings,
International Conference on Noise Control Engineering, Washington, D.C.
(September 1974), pp. 55-60.
5A.P.G. Peterson and E. E. Gross, Jr., Handbook of Noise Measurement, 7th
Edition (General Radio Co., Concord, Mass., 1972), pp. 172-173.
6"Method for the Calibration of Microphones," SI.10-1966, American National
Standards Institute, New York, N. Y.
7R. K. Cook, "Measurement of Electromotive Force of a Microphone," J. Acoust.
Soc. Amer. 19_, 503 (1947).
8W. Koidan, "Method for Measurement of |E'/I'| in the Reciprocity Calibration
of Microphones," J. Acoust. Soc. Amer. 32, 611 (1960).
USCOMM-NBS-DC
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TABLE 1
Long-Term Stability of "1-inch" Condenser Microphones
at 20°C and 44% RH
Change in Sensitivity
(dB re value on May 21, 1975)
lency (Hz)
100
200
500
1000
2000
3000
4000
5000
100
200
500
1000
2000
3000
4000
5000
June 17,
0.17
0.07
0.04
0.06
0.07
0.08
0.08
0.07
+0.18
+0.17
+0.18
+0.12
+0.01
-0.09
-0.15
-0.17
1975 July 22, 1975
Cartridge //I
-0.11
-0.12
-0.15
-0.12
-0.03
+0.11
+0.17
+0.21
Cartridge #2
+0.18
+0.12
+0.06
+0.06
+0.03
+0.05
+0.02
+0.03
September 9, 1975
-0.04
-0.09
-0.09
-0.05
+0.01
+0.11
+0.19
+0.22
-0.11
-0.16
-0.15
-0.12
+0.01
+0.13
+0.18
+0.26
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TABLE 2
Long-term Stability of "1/2-inch" Electret Microphones
at 20°C and 44% RH
Change in Sensitivity
(dB re value on May 21, 1975)
luency (Hz)
100
200
500
1000
2000
3000
4000
5000
100
200
500
1000
2000
3000
4000
5000
July 23, 1975
Cartridge //I
-1.22
-0.51
-0.30
-0.34
-0.34
-0.38
-0.40
-0.32
Cartridge #2
+0.26
-0.02
-0.11
-0.12
-0.12
-0.14
-0.16
-0.17
September 9, .
+1.53
+1.33
+1.25
+1.24
+1.17
+1.05
+0.97
+0.93
+2.47
+1.26
+0.88
+0.88
+0.88
+0.84
+0.70
+0.59
10
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TABLE 3
Long-Term Stability of "1-inch" Ceramic Microphones
at 20°C and 44% RH
Change in Sensitivity
(dB re value on May 21, 1975)
luency (Hz)
100
200
500
1000
2000
3000
4000
5000
100
200
500
1000
2000
3000
4000
5000
July 23, 1975 September 9, :
Cartridge #1
-1.51
-1.47
-1.44
-1.45
-1.40
-1.40
-1.56
-3.90
Cartridge #2
0.23
0.24
0.26
0.30
0.49
0.78
0.52
0.07
0.19
0.04
0.04
0.04
0.31
0.81
1.16
0.70
0.73
0.12
0.00
0.05
0.57
1.08
1.25
0.36
11
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40°C
_
i i iiiiii-i- T T'i
1 1 1 J- 1 1 1 IT _L J. -r 1
-
-
FREQUENCY (kHz)
1 1 1 1 1 1 1 1 1 1 III
1 1 1 1 1 1 1 1 r 1
0,1 0.2 0.5 1 25
2
1
0
1
-2
^™
50°C
. i i i i i 1 1 1 i iii
I x J I Iji
-
FIGURE SB. CHANGE IN SENSITIVITY OF "I-INCH" CERAMIC MICROPHONE NO, 2
AS A FUNCTION OF FREQUENCY AND TEMPERATURE, THE VERTICAL BARS
INDICATE THE RANGE OF THE DATA AS A FUNCTION OF HUMIDITY,
-------
APPENDIX A
MICROPHONE CALIBRATION METHOD
Introduction
The microphone calibration procedure is performed in two stages.
The first stage employs a reciprocity calibration of the two condenser
microphones. The second stage uses one of these microphones as a source
from which the response of the electret and ceramic microphones are ob-
tained. The reciprocity method is used to calibrate the reference
(source) microphone at a particular temperature, humidity, and frequency.
The reciprocity technique requires three microphones. One of these micro-
phones must be reversible, that is, it must perform according to certain
relationships between the acoustic pressure and velocity on the surface
of the microphone diaphragm and the current and voltage produced by the
microphone. A third microphone is used as a source. After the reference
microphone has been calibrated it is used as a source to generate a known
sound pressure from which the other microphone responses are determined.
The details and theory of how one performs these calibrations are presented
in Ref. 6. The purpose of this appendix is to describe how HBS performed
these measurements.
Reciprocity Method
Two different types of measurements are made in the NBS reciprocity
method of calibration: voltage ratio and capacitance. The voltage ratio
measurement is performed as shown in Fig. A-l. With the switch in position
1 the oscillator excites the source microphone. The receiving microphone,
acoustically coupled to the source through a 3-cm3 plane-wave coupler,
A-l
-------
converts the acoustic signal into a voltage, which is filtered and ampli-
fied. The magnitude of the signal isiread on the meter and is denoted A.
The switch is then placed in position 2 disconnecting the oscillator from
the source microphone and connecting it to an attenuator calibrated in
hundredths of a decibel. The output of the attenuator is connected across
the resistor R , which is in series with the receiving microphone. The
o
attenuator is varied until the meter indicates a value equal to A. The
resulting attenuation reading, denoted A (dB), gives the logarithm of
3
the ratio between the open circuit voltage of the oscillator driving the
source microphone to the open circuit voltage of the receiving microphone.
Consequently the voltage ratio V., is given by V = 10 a
K R.
Measurements of the capacitance of a microphone are made according
to the method developed by Koidan8 and is shown in Fig. A-2. The source
and receiving microphone are placed in the coupler in the same manner as
in the voltage ratio measurement in order that the source microphone be
subjected to the same acoustic impedance. When the two switches are in
position 1 a voltage is placed across the source microphone, which is in
series with a resistance R. The reading of the meter, denoted Mj, that
is connected across the resistance R is recorded. The two switches are
now placed in position 2 and the attenuator, which is calibrated in
hundredths of a decibel, is adjusted so that the meter again reads MI.
If the microphone is assumed to be purely capacitive with an impedance
at the given frequency that is much larger than R, the capacitance of
the microphone, C, is given by
2irfRV3
A-2
(A-l)
-------
where f is the frequency of the oscillator (Hz), V e 10 c and
A is the attenuator reading in dB.
The reciprocity calibration method used three microphones, denoted
//O, #1, and //2, and the two types of measurements discussed above to de-
termine the pressure response of two of them (//I and //2). Microphone //O,
which will not be calibrated, is used as a source. With microphone //O
as the source and microphone //I as the receiver the voltage ratio V is
obtained. With microphone #0 as the source and microphone #2 as the
receiver a voltage ration V is obtained. Lastly, with microphone //I
used as a source and microphone #2 as the receiver a voltage ratio Vg is
obtained. Using Eq. (A-l) this yields the capacitance of the microphone.
These three sets of measurements and some correction factors described
below, will yield the pressure response of microphones //I and //2.
From Section 4 of Ref. 6 the expressions for the pressure response
of microphones //I and #2, denoted r and r2, respectively, are
where
r = K/G(f)^— V/Pa (A-2)
*v
r2 = K^GCO\— V/Pa (A-3)
n 2
G(f) = ^ A2(f ,fQ) (A-4)
s
sindrf/f )
*(f.f0) - (rf/ ' (A-5)
A-3
-------
In Eqs. (A-2) to (A-4) f and R are defined as in Eq. (A-l), P is
s
the ambient barometric pressure, f is the first longitudinal natural
frequency of the coupler, and K is a constant. For the setup used in
this experiment the value of f was the frequency at which the first
maximum value of the output of the receiving microphone was obtained
when the frequency was varied with a constant voltage applied to the
source microphone. The quantity K is a function of the volume of the
coupler and the equivalent volumes of the microphones, adjustments for
the heat conduction effects at the walls of the cavity and the presence
of the capillary tubes in the coupler, and the ratio of the specific
heats of air. The latter is virtually independent of temperature over
the range considered, and while the volumes and capillary correction
change very slightly with temperature, they have been assumed to remain
constant. The constant K has been eliminated from the final results by
referencing the r to their respective values at a given frequency and
temperature.
Comparison Method
Comparison calibrations used microphone #1 as the source and the
electret and ceramic microphones as the receiving ones. The pressure
response of these microphones is then determined from the expression
K
— TV - V/Pa
s a
where fQ is the first longitudinal natural frequency of the coupler with
the microphone a as the receiver, V is the voltage ratio,, K is assumed
a a
constant,
A-4
-------
6 = ^ (A-7)
and r is given by Eq. (A-2) and C by Eq. (A-l). The constant K is
eliminated by referring the pressure response to its response at a given
frequency and temperature.
In arriving at Eqs. (A-l) through (A-7) several assumptions were made
regarding the effects of temperature on some of the constants in Eqs. (A-2),
(A-3) and (A-6). Furthermore, there may be inaccuracies in the measure-
ments themselves. The estimated maximum value of the errors introduced by
these two factors is tabulated in Table A-l which indicates that the total
maximum absolute error that exists in the results presented is probably
less than 0.2 dB.
A-5
-------
TABLE A-l
Estimated Magnitudes and Sources of Errors
in the Microphone Calibration Data
Source of Error
Assumptions that the following are independent of
temperature:
1. Volume of coupler
2. Coupler capillary corrections
3. Heat conduction correction
4. Equivalent volumes of microphones
Accuracy of measurement
6. Resonance frequency
7. Attenuator readings (combined)
8. Barometric pressure
Estimated
Maximum Error
(dB)
0.02
0.02
0.06 dB @ 100 Hz
(less at higher freq.)
Unknown
0.02
0.04 (reciprocity)
0.05 (comparison)
0.01
A-6
-------
OSCILLATOR
COUPLER
6 1
O 0
0 2
PRECISION
ATTENUATOR
SOURCE MICROPHONE
CAPILLARY TUBE
RECEIVING
MICROPHONE
MICROPHONE
PREAMPLIFIER
FILTER
METER
FIGURE A-l. SCHEMATIC OF EQUIPMENT REQUIRED FOR THE VOLTAGE RATIO
MEASUREMENTS,
-------
00
ncpll 1 ATCI Q
UoU 1 LLA 1 U n
<
O. .,_
C
i 1
.A
* R
> 2
^M
PRECISION
ATTENUATOR
<
^
V
j
^M
•=
^
>
> \
— 4
i
rl^s
1*r
^
•• ^
\,
(ELE(
> 1
> 2
SOURCE MICROPHONE
-COUPLER
RECEIVING MICROPHONE
(ELECTRICALLY DISCONNECTED)
METER
FIGURE A-2, SCHEMATIC OF EQUIPMENT REQUIRED FOR THE MEASUREMENT OF THE
CAPACITANCE OF THE SOURCE MICROPHONE,
-------
NBS-1UA (REV. 7-73)
U.S. OEPT. OF COMM.
BIBLIOGRAPHIC DATA
SHEET
1. PUBLICATION OR REPORT NO.
NBSIR 76-1090
2. Gov't Accession
No.
3. Recipient's Accession No.
4. TITLE AND SUBTITLE
EPA 550/9-76-009
Environmental Effects on Microphones of Various
Constructions
5. Publication Date
July 1975
6. Performing Organization Code
200.03
7. AUTHOR(S)
Gale R. Hruska. Edward B. Magrab. William B. Penzes
9. PERFORMING ORGANIZATION NAME AND ADDRESS
8. Performing Organ. Report No.
NATIONAL BUREAU OF STANDARDS
DEPARTMENT OF COMMERCE
WASHINGTON, D.C. 20234
10. Project/Task/Work Unit No.
2003110
11. Contract/Grant No.
2. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)
Co-sponsored by NBS and Office of Noise Abatement and Control
U.S. Environmental Protection Agency
Washington, D.C. 20460
13. Type of Report & Period
Covered
final, March-June, 1975
14. Sponsoring Agency Code
15. SUPPLEMENTARY NOTES
6. ABSTRACT (A 200-word ot less factual summary of most significant information. If document includes a significant
bibliography or literature survey, mention it here.)
The pressure^sensitivities of two "1/2-inch" electret, two "1-inch" ceramic, and two
back-vented 1-inch" condenser microphones were measured for numerous combinations of
temperature, percentage relative humidity, and frequency. The two condenser
microphones were calibrated by the reciprocity technique at each combination of
temperature, relative humidity and frequency. The condenser microphones were then
used as calibrated sources to determine the pressure sensitivities of the other
microphones. Insert voltage techniques were used to eliminate the environmental
effects on the electronics. It was found that the back-vented condenser microphones
are insensitive to changes in relative humidity. At frequencies considerably below
their resonance frequencies they exhibited only a very small change in sensitivity
with temperature. At frequencies closer to the resonance frequency the temperature
coefficient increases approximately fourfold. The temperature and humidity coefficient
for the electret and ceramic microphones could not be determined due to the instability
in their sensitivities which produced changes that were larger than those induced by
the temperatures and humidities.
7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper
name; separated by semicolons)
Calibration; ceramic; condenser; electret; humidity; microphones; reciprocity
sensitivity; temperature. y'
8. AVAILABILITY [£ Unlimited
I ' For Official Distribution. Do Not Release to NTIS
I ' Order From Sup. of Doc., U.S. Government Printing Office
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| Order From National Technical Information Service (NTIS)
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