Brüel & Kjær 4131 /32 User guide

One-inch Condenser Microphones
Microphone Cartridges Type 4131 /32
Cathode Followers Type 2612/2613, 2630
Accessories
Precision condenser microphones for measuremem
purpose. Each microphone is individually cali-
brated. Frequency range of calibration 20-
20ooo Hz. The 4132 fulfils the requirements of
the American Standard A.S.A. Z. 24.8-1949.
BRUEL& K. ... AC.R
N~RUM, DENMARK . Teleph.: 80 05 00. Cable: BRUKJA. Telex: 5316. COPENHAGEN
88 4131/ 32 • 0300045

One-inch
Condenser Microphones
Reprint July 1968

Contents
0. Introduction
Purpose of the One-inch Condenser Microphone . •. . . . . . . . . . . •. . . . 3
Principle of the Condenser Microphone . . . . . . . . . . . . . . . . . . . . . . . . •. 3
Definitions of Free-field and Pressure Response . . . . . . . . . •. . . . . . . . . . 5
Random Incidence Response (Diffuse field) . . . . . . . . . . . . • . . . . . . . . . . . 6
Response to be considered in ordinary noise control measurements •• 6
I. Description
General . . •. . . . . . . . . . . •• . ••. . . . . . ••••. . . . ••. . . . . . . . . . . . . . . . •. . . . . 8
Description of the Microphone Cartridges Type 4131-4132 . . . . . . . . 9
Silica Gel Cap UA 0135 • • • • • • . • • • • • • • • • • . . • • • • . • • • . . . . . • . • • . . . • . • 11
Cleansing of the Microphone Diaphragm . . . . •. . . . . . . . . . . . . •. ••. . •• 12
Description of the Cathode Followers Type 2612-2613 . . . . • • . . . . • • 12
General Characteristics . . . . . . . . . . . . . •••••. . . •. •. . ••. . •. . ••••. . . . •• 17
1.
Sensitivity .•••••••••••.•••••••••••••...•.•.............•.•••• 17
2. Frequency Response • • . . •• •• . • •••. •. •• • • • . . . . . . . . . . . . • •. . . . •• 18
3. Phase Characteristics ••••. . . . ••. . ••••••. •. . . •••• . . . . . ••• . • . . . 19
4. Free Field Corrections and Directional Patterns •. . . . . •. . . . . . . . . 20
5. Polarized Cartridge Capacity . . ••. •. . ••. . . . •. . . . . •. . . . . . . ••. . . . 22
6. Dynamic Range, Noise and Distortion ...........•.....••..... 24
7. Influence of the Polarization Voltage .......•.•............•.•. 26
8. Equivalent Air Volume . . . . . . . •••. . . . . . . . . . . . . . . . . . . . . . • . . ••• 27
9. Temperature Characteristics . . • . . . . . . . . . . . . . . . . . . . . . . . • • • • . . . . 29
10. Influence of Ambient Pressure ................................ 31
11. Sensitivity to Magnetic Fields •..............................• 33
12. Influence of Vibrations ....•.•..••...•.........•..••...•..••• 33
13. Influence of Humidity ...•••...........•••.................•. 34
2. Accessories
Random Incidence Corrector UA 0055 ...........•.......••....... 35
Wind Screen UA 0082 and Nose Cone UA 0051 ..............•.•..... 37
Microphone Stand UA 0049 •..••...•••.....•...............••...... 41
Extension Cables • ••. . •••••••. . . . •••••. . . . . •. . . . •. . . . . . . . • . ••. •. . 41
Adaptor UA 0030 •••.•.••••...••..•••••••.••...............•••.... 4:2
Flexible Extension Rod UA 0039 ..••• o • o o ••••••••••••••••••••••••• 42
Acoustic Couplers ....•••••••••......•• o •••••••• o •••••••••••••• o o • 43
Two Channel Microphone Selector Type 4408 . . . . . . . . . . . . • • . . . . . . . . 44
Microphone Power Supply Type 2801 ... o •••••••••• o ••••••••••••••• 44
Two-Channel Power Supply Type 2803 .... o • • • • • • • • • • • • • • • • • • • • • • • 45
Cathode Follower Type 2617 .......................•..•....••.... 0 46
Battery-driven Cathode Follower Type 2630 ..............• o •••••••• 47
Appendix
Microphone Calibration •.••....••.•• o • • • • • • • • • • • • • • • • • • • • • • • • • • • • 50
(Pistonphone 4220, Microphone Calibration Apparatus 4142,
Electrostatic Actuator UA 0023, Noise Source 4240, Insert Voltage
Junction Box ZH 0007)
Comparison Type 4132- Type WE 640 AA ........................ 55
The Range of B & K Condenser Microphones .••.•. 0 • • • • • • • • • • • • • • • 55
Specificatiooa . . . . . . . . . •. •. . . . . •. . . . . . . ••. . . . . . . . . . . . . •••. •. . . . . . . . . 61

0. Introduction
Purpose of the "One-inch" Condenser Microphones.
The B & K One-inch Condenser Microphones are designed for precision sound
pressure measurements. Their range of application covers the whole of the
audible frequencies (20Hz to 18kHz*) and of pressure levels (15 dB to 146 dB).
Their most outstanding feature is excellent long-term stability under a great
range of environmental conditions and especially their insensitivity to
temperature variations. They are therefore well suited for field measure-
ments, though the accuracy of calibration matches laboratory standard require-
ments.
The One-inch microphones are of the omnidirective type, with a relatively
high sensitivity. They are not designed for permanent exposure outside
unless special precautions are taken.
An extensive series of accessories herein described provides a great measur-
ing versatility.
Prlndple of the Condenser Microphone.
A condenser microphone consists essentially of a thin metallic diaphragm
mounted in close proximity to a rigid back plate. Diaphragm and back plate
are electrically insulated from each other and constitute the electrodes of
a capacitor . See Fig. 0.1. Housing and insulator form with the diaphragm
D Insulator
Back
plate
Housing
Hole for
static preuure equalization
Fig. 0.1. Schematic construction of a condenser microphone cartridge.
*) Hz International.
cis Uaed in United Kingdom and U.S.A.
3

4131 . 4132 - INTRODUCTION
a closed chamber, which is only in communication with the outside for
slow static ambient pressure variations. When the microphone is exposed
to a sound pressure, the diaphragm is submitted to an alternating force
proportional to the pressure and the diaphragm area. The consequent move-
ment of the diaphragm varies the capacity, and these variations are trans-
duced into an AC voltage component if a constant charge is present between
the electrodes. The charge is obtained by means of a stabilized DC polari-
zation voltage, and it remains constant as long as the charging time constant
of the circuit is much longer than the period of the sound pressure variations.
It is possible by careful design to maintain the proportionality of the AC
output voltage to the sound pressure within a wide frequency range and an
extended dynamic range. The widest linear frequency range for the pressure
response is obtained if the resonance of the mechanical system (diaphragm)
is critically damped. This damping, which is due to the back and forth
movement of the air contained between diaphragm and back plate, is
determined by the shape of the back plate and the mechanical tension of
the diaphragm.
The low frequency limit of the linear range is set by the "cut-off" (or time
constant) of the microphone cartridge circuit. Referring to the schematic
Cartridge
Cathode Follower
Mechanical
Electrical
162f6f
C = Polarized cartridge capacity
b.C = Variation of capacity producing the signal
C5 =
Stray capacity of connection to cathode follower
Ri,Ci =Input impedance of cathode follower
Fig. 0.2. Simplified diagram of the Condenser Microphone. (The electrical
leakage
in the cartridge is neglected).
4

4131 · 4132- INTRODUCTION
diagram of Fig. 0.2 the cut-off frequency is equal to:
Rt R.
2 n (C + Cs + Ct) Rt + R.
Since the sensitivity of the microphone is determined by the relative variation
of capacity L1 C/(C + Cs + Ct), the total parallel capacity should be made
as small as possible. The first amplifier stage is therefore built in the same
housing as the microphone. A cathode follower stage giving high Rt is
employed, because the small parallel capacity requires high Rt and R. in
order to obtain a satisfactory low limiting frequency.*)
Definitions of Free-field and Pressure Response.
The Free-Field Response of a microphone is the ratio of the RMS output
voltage to the RMS sound pressure existing in a free field (plane sound
waves) at the microphone location with the microphone removed.
The Pressure Response of a microphone is the ratio of the RMS output
voltage to the RMS sound pressure, uniformly applied over the diaphragm.
The two definitions coincide for a microphone having negligible dimensions
with respect to the sound wavelength. In the case of the B&K one-inch
microphones this is practically fulfilled up to about 1400 Hz, where the
wavelength is equal to ten times the diameter of the microphone (i.e. 240 mm) .
The difference in response is then a small fraction of a decibel.**)
Free-Field Response=,;:; Pressure Response= 1,-
Wavelength
Sound pressure p
0
___ _ ___ .........,_
0° incidence
162160
Fig. 0.9. Definitions of Free-field and Pressure Response.
*) lt should be noted, however, that with the circuitry employed in the cathode follower
herein described (Type 2612113) the low-frequency cut-off Ia w may deviate somewhat
from a simple R-C law.
**) In the particular case of the 90° incidence (parallel to diaphragm) the difference in
response remains negligible up to 10 kHz. See Fig. 1.14.
5

4131 • 4132- INTRODUCTION
At higher frequencies the diffractions of the sound waves on the microphone
produce an appreciable change in the resulting sound pressure actjng on the
microphone diaphragms as illustrated on Fig. 0.3. The difference Pl -po, called
free-field correction, depends on the orientation of the microphone with
respect to the direction of propagation of the sound and on the external
dimensions of the microphone (in particular those of the front and of
fitted protective grids or "nose cones").
The free-field behaviour of a microphone is thus described by means of a
set of free-field correction curves for various incidences, which have been
measured on a model of the microphone (see B&K Technical Review Nos.
1 & 2-1959),
and which should be added to the pressure frequency curve of
the microphone in each particular case. See Fig. 1.16-1.19.
For microphones intended for free-field work it is possible to give the
diaphragm resonance such a damping that the normal incidence free-field
corrections are compensated for up to frequencies well above the resonance
frequency, in order to obtain the flattest possible frequency response.
Random Incidence Response (Diffuse Field Response).
The random incidence response of a microphone for a given frequency is
the root-mean-square value of the free field sensitivity for all angles of
incidence of the sound wave. It corresponds to the diffuse field sensitivity
of the microphone, the diffuse field being a sound field in which the sound
energy density is uniform and the mean acoustic power per unit area is
the same in all directions. The International Electrotechnical Commission
(publication no. 123, § 8.2) has given a practical rule for the calculation
of the random incidence sensitivity from the free-field sensitivities at de-
finite angles, with coefficients proportional to the relative solid angles.
See page 21.
Figs. 1.14, 2.3
and 2. 7 show the random incidence frequency response of the
different B & K one-inch microphone combinations. These curves should be
taken into consideration 1n the case of measurements in highly reverberant
rooms giving rather diffuse field conditions, where no predominant sound
source is present near the microphone. However, when the spectral distribu-
tion of the sound varies with the angle of incidence, correct integration is
only possible with a microphone which is both linear and omnidirectional in
the whole frequency range of interest.
Response to be considered in ordinary noise control measurements.
In practice most measurements are carried out in sound fields whose
characteristics are in between the definitions of the free field corresponding
to a perfectly "dead" room or 100 % open space, and of the diffuse field
corresponding to a perfectly reverberant room.
Consider the common example of sound level measurements in a large
typing office at various locations. There the sound field is approximately
6

4131 . 4132- INTRODUCTION
free (predominant direction of the acoustic energy flow) near a particular
typewriter, and diffuse (if no predominant reflected wave) near the
boundaries of the room.
The frequency response of the microphone is thus changing from one
position to the other. This change is only important at frequencies above
3 kHz for the one-inch microphones, as seen in Fig . 1.14, but these are especi-
ally important in the case of "noiseness" (Noys or PN dB) measurements.
In intermediate positions, it is practically impossible to predict the nature
of the field, or which incidences should be considered. A microphone which
is omnidirectional at least up to 10 kHz is then obviously advantageous.
Then the random incidence response is of course identical to the free-
field response which is the same at all incidences. Omnidirectivity is re-
quired in the standards for sound level meters (e.g.: IEC 123, § 5.1) but it
is
rather difficult to achieve without reducing the dimensions and thereby
the sensitivity of the microphone. Thorough research carried out by B & K
has culminated in the successful design of a specially shaped device which,
when mounted in place of the protecting grid on the one-inch microphone
ensures satisfactory omnidirectivity . (Type UA 0055, see Accessories).
Omnidirectional microphones are also necessary in the case of rapidly
moving sound sources (aeroplanes, motorcars, etc.).
7

1. Description
General.
A complete microphone consists of a microphone cartridge and a cathode
follower for impedance conversion, allowing long cables and relatively low
input impedance amplifiers to be used between the microphone and the
measuring instrument. The microphone cartridge is screwed onto the housing
of the cathode follower making a small, rugged unit. Fig. 1.1.
"
131
.~ ~~r-~.8-5 .• 5 --F~f;;;;;;;2~!S;,3;;;;;;;,;
1"'~ I t §5t
f 2m cable
16'1795
Fig. 1.1. Complete Microphone, consisting of Microphone Cartridge Type 4131
or 4192 and Cathode Follower Type 2612 or 2613.
The microphones may be directly connected to the different B~K measuring
instruments which are provided with a CONDENSER MICROPHONE input
socket fitting the microphone connecting plug. Stabilized plate and heater
voltages for the cathode follower and polarization voltage for the cartridge
are available on this seven-pin socket. The microphones may also be used
with other equipment when operated from Microphone Power Supplies which
provide the necessary voltages in the same conditions as the measuring
instruments, or with the Battery-driven Cathode Follower Type 2630 (see
Accessories).
For free-field and room-acoustic measurements, the micro-
phones should be placed apart from the measuring instruments, since
8