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

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

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Or. Betsy Ancker-Jofmeon, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

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 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.

Key Words: Calibration; ceramic; condenser; electret; humidity;

microphones; reciprocity; sensitivity; temperature.

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 environmental conditions could affect the sensitivity of the microphones. There

exist some published data 1 5 concerning the effects of environmental conditions 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 pressure 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 provide useful and meaningfully-comparable data.

Six commercially-avail able 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 measured (where physically possible) at the following combinations of frequency,

*By their very nature the small volumes in the electret and ceramic microphone 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.

1
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) relative 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 microphones was essentially duplicated, except that the 3-cm6 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 electrically isolate one of the microphones in the coupler, were required to compensate 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 preamplifiers 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 temperature 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 preamplifier 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

2
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 frequency measurement was repeated. By examining the first and final measurements 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 capacitance 8 of

microphone B, the absolute pressure response levels of microphones A, B,

and C were calculated. The pressure response of the remaining four microphones was determined using microphone B as the source, with the measurements on the electret microphones preceding those on the ceramic microphones. 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 measurements on all six microphones at a given temperature and humidity was

approximately 6 hours. The temperatures were changed in the following

3
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, respectively, from -20 to 50°C and for frequencies below 1000 Hz. At frequencies 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

4
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 expected 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 frequency, 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 instabilities they will exhibit similar behavior at standard conditions,

20 °C and 44% RH. As can be seen from Thbles 1-3 the condenser microphones 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.

5
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 4)

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 microphones 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 reasonable, however, to assume that if significant changes in temperature and

relative humidity occur between system calibrations, then the uncertainties in the measured sound levels could be at least of the magnitude

6
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 overall system sensitivity.

7
REFERENCES

1 G. M. Sessler and J. E. West, "Electret Transducers: A Review," J.

Acoust. Soc. Amer. .53, 1589-1600 (1973).

2 L. L. Beranek, Ed., Noise and Vibration Control (McGraw-Hill Book Co.,

New York, 1971) pp. 48-49.

3 G. Rassmussen, "Reliability of Measurement Microphones Under Outdoor

Conditions," Seventh International Congress on Acoustics, Budapest, 1971,

pp. 289-292.

4 Rassmussen, "Measurement Microphones," Inter-Noise 74 Proceedings,

International Conference on Noise Control Engineering, Washington, D.C.

(September 1974), pp. 55-60.

5 A.P.G. Peterson and E. E. Gross, Jr., Handbook of Noise Measurement , 7th

Edition (General Radio Co., Concord, Mass., 1972), pp. 172-173.

^"Method for the Calibration of Microphones," SI. 10-1966, American National

Standards Institute, New York, N. Y.

7 R. K. Cook, "Measurement of Electromotive Force of a Microphone," J. Acoust.

Soc. Amer. 19, 503 (1947).

8W. Koidan, "Method for Measurement of | E

^ /

1

" | in the Reciprocity Calibration

of Microphones," J. Acoust. Soc. Amer. 611 (1960).

USCOMM-NBS-DC 8
100

200

500

1000

2000

3000

4000

5000

100

200

500

1000

2000

3000

4000

5000

TABLE 1

Long-Term Stability of "l-inch" Condenser Microphones

at 20° C and 44% RH

Change in Sensitivity

(dB re value on May 21, 1975)

(Hz) June 17, 1975 July 22, 1975 September 9 , 1975

Cartridge #1

0.17 -0 . 11 -0.04

0.07 -0.12 -0.09

0.04 -0.15 -0.09

0.06 -0.12 -0.05

0.07 -0.03 +0.01

0.08 +0.11 +0.11

0.08 +0.17 +0.19

0.07 +0.21 +0.22

Cartridge #2

+0.18 +0.18 -0.11

+0.17 +0.12 -0.16

+0.18 +0.06 -0.15

+0.12 +0.06 -0.12

+0.01 +0.03 +0.01

-0.09 +0.05 +0.13

-0.15 +0.02 +0.18

-0.17 +0.03 +0.26

9
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)

Frequency (Hz) July 23, 1975 September 9

Cartridge #

1

100 -1.22 +1.53

200 -0.51 +1.33

500 -0.30 +1.25

1000 -0.34 +1.24

2000 -0.34 +1.17

3000 -0.38 +1.05

4000 -0.40 +0.97

5000 -0.32 +0.93

Cartridge #2

100 +0.26 +2.47

200 -0.02 +1.26

500 -0.11 +0.88

1000 -0.12 +0.88

2000 -0.12 +0.88

3000 -0.14 +0.84

4000 -0.16 +0.70

5000 -0.17 +0.59

10
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)

Frequency (Hz) July 23, 1975 September 9, 1975

Cartridge #1

100 -1.51 0.19

200 -1.47 0.04

500 -1.44 0.04

1000 -1.45 0.04

2000 -1.40 0.31

3000 -1.40 0.81

4000 -1.56 1.16

5000 -3.90 0.70

100

Cartridge #2

0.23 0.73

200 0.24 0.12

500 0.26 0.00

1000 0.30 0.05

2000 0.49 0.57

3000 0.78 1.08

4000 0.52 1.25

5000 0.07 0.36

11
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FUNCTION

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AND

TEMPERATURE.

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VERTICAL

BARS

INDICATE

THE

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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 obtained. 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 microphones 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 NBS 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-cm 3 plane-wave coupler.

A-l
converts the acoustic signal into a voltage, which is filtered and amplified. The magnitude of the signal is read 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

attenuator is varied until the meter indicates a value equal to A. The

resulting attenuation reading, denoted A (dB), gives the logarithm of

cL 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 VD = 10 ^ ^a /20).

K. K

Measurements of the capacitance of a microphone are made according

Q to the method developed by Koidan 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 , 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 .

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

2TrfRV

3 (A-l)

A-2
where and f is the frequency of the oscillator (Hz), V, = xo*- Ac^ 20 ^

is the attenuator reading in dB.

The reciprocity calibration method use'd three microphones, denoted

//0, //l, and # 2 , and the two types of measurements discussed above to determine the pressure response of two of them (//l and #2). Microphone //0,

which will not be calibrated, is used as a source. With microphone #0

as the source and microphone //I as the receiver the voltage ratio is

obtained. With microphone #0 as the source and microphone #2 as the

receiver a voltage ration V

2 is obtained. Lastly, with microphone #1

used as a source and microphone #2 as the receiver a voltage ratio V Q 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 #1 and //2.

From Section 4 of Ref. 6 the expressions for the pressure response

of microphones #1 and #2, denoted r

1 and r„ , respectively, are

V/Pa (A- 2)

V/Pa (A-3)

G(f) - A

2 (f,f ) (A-4)

s

1

r 0 = K^(f)\|~

where

and A( f .£0

) sin<Trf/f ) o

(tt f / f ) o

(A-5)

A-3
In Eqs. (A-2) to (A-4) f and R are defined as in Eq. (A-l) , P

g is

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

a

eA < f - f a>

P V

s a V/Pa (A- 6)

where f 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

A-4

constant

,
K

(A- 7)

and r is given by Eq. (A-2) and C by Eq. (A-l) . The constant K is

1 a 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 measurements 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.

A5
TABLE A-l

Estimated Magnitudes and Sources of Errors

in the Microphone Calibration Data

Estimated

Maximum Error

Source of Error (dB)

Assumptions that the following are independent of

temperature

:

1 . Volume of coupler 0.02

2. Coupler capillary corrections' 0.02

3. Heat conduction correction 0.06 dB @ 100 Hz

(less at higher freq

4. Equivalent volumes of microphones Unknown

Accuracy of measurement

6. Resonance frequency 0.02

7. Attenuator readings (combined) 0.04

0.05

(reciprocity)

(comparison)

8. Barometric pressure 0.01

A6
CAPILLARY

TUBE (3 Z

Z °

uu

A7

Figure

A-l.

Schematic

of

equipment

required

for

the

voltage

ratio

MEASUREMENTS,
SOURCE

MICROPHONE PC

UJ

Q.

Q- Z

O O

PC O

o C^3

CD

A8

Figure

A-2,

Schematic

of

equipment

required

for

the

measurement

of

the

CAPACITANCE

OF

THE

SOURCE

MICROPHONE,
NBS-114A (REV. 7-73)

U.S. DEPT. 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

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2003110

11. Contract/Grant No.

12. 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

16. ABSTRACT (A 200-word or 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.

17 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 ; ceramics condenser; electret; humidity; microphones; reciprocity;

sensitivity; temperature.

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21. NO. OF PAGES

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22. Price

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