BRUEL & KJAER 4138 User guide

Eighth-inch Condenser Microphone
Microphone Cartridge Type 4138
Cathode Follower Type 2615
with Adaptor UA 0036
Precision condenser microphone for special
measurement purpose . Each microphone is
individually calibrated. Frequency-range of
calibration : 30 Hz - 140 kHz. Dynamic range:
76-184 dB .
BRUEL& IK'-'AC.R
NA:.RUM, DENMARK . Teleph.: 80 05 00 . Cable: BRUKJA . Telex: 5316 . COPENHAGEN
B B 4138 - 0300011

Eighth-inch
CONDENSER MICROPHONES
September 1967

Contents
0. Introduction
Definitions of Free-field and Pressure Response . . . . . . . . . . . . . . . . . . . . . 3
Random
Incidence Response (Diffuse Field Response) . . . . . . . . . . . . . . . 4
1. Description
General Description of the Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Eighth-inch
Condenser Microphone Cartridges . . . . . . . . . . . .. . . . . . . . . . 6
Cleansing of the Microphone Diaphragm . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Associated Cathode
Follower Type 2615 + UA 0036 . . . . . . . . . . . . . . . . . . 8
Characteristics of the Eighth-inch Microphones ...................... 12
1. Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 12
2. Frequency Response
... ... ................... ....... ........ .. 12
3. Free Field Corrections ........................................ 14
4. Directional Characteristics ..................................... 14
5. Dynamic Range (Noise and Distortion) .......................... 15
6. Equivalent
Air Volume ......................................... 16
7. Influence of the Polarization Voltage ............................ 16
8. Temperature Characteristics ................................... 16
9. Influence of Ambient Pressure ...................... ..... ....... 16
10. Influence of Humidity .......................................... 17
11. Influence
of Vibrations ........................................ 18
12. Sensitivity to
Magnetic Fields .................................. 18
2. Accessories
Microphone Stand UA 0049 ........................................ 19
Extension Cables AO 0027-28-29, AR 0001 .......................... 19
Mechanical Adaptor DB 0900 ....................................... 20
Probe Microphone Kit UA 0040 .................................... 21
Two-Channel Selector 4408 ........................................ 22
Microphone Power Supply 2801 .................................... 23
Two-Channel Power Supply Type 2803 .............................. 24
Appendix
Principle of the Condenser Microphone .............................. 25
Microphone Calibration ............................................ 26
Pistonphone 4220 ................................................. 27
Electrostatic Actuator UA 0033 ..................................... 28
The Range of B & K Condenser Microphones ........................ 30
Specifications
Survey of Accessories ............................................. 33
Comparative specifications ........................................ 34

0. Introduction
Definitions.
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 the free field at the microphone
location with the microphone removed from the sound field .
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.
At higher frequencies the diffractions of the sound waves on the microphone
produce an appreciable change in the resulting sound pressure acting on the
microphone diaphragm as illustrated in Fig. 0.1. The difference P 1- 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 should be
added to the pressure frequency curve of the microphone in each particular
case.
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.
Free-Field Response= ~o
Pressure Response= ;,
vo~ts -------;rv L__M_icroph-one _r:::::I S::::::::--~ -~~
Wavelength
Sound
pressure p
0
-------~-
00incidence
f62f60
Fig. 0.1. Definitions of Free-field and Pressure response .
3

Random Incidence Response (Diffuse Field Response).
The random incidence response of a microphone for a given frequency is
the RMS 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 definite angles, with coefficients
proportional to the relative solid angles.*)
When the spectral distribution of the sound varies with the angle of incidence,
correct integration is only possible in the range where the microphone is both
linear and omnidirectional (practically up to 30 kHz with the eighth-inch
microphones).
Omnidirectional microphones are also necessary in the case of rapidly
moving sound sources (aeroplanes, motorcars, etc.).
*) So, S3 o, S 6o ---- S1so being the sensitivity of the microphone at angles of incidence of
oo, 30°, 60°, ---- 180°, the random incidence · (diffuse field) response S is given by the
formula :
S2 = 0. 018 (So2 + S 1so
2) + 0.
129 (S 3o2 + S 1so
2) +
0.224 (S 60
2 +
S1 20 2) + 0.258 S 902
4

1. Description
G·eneral Description of the Equipment.
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 by means of an adaptor making a small, rugged unit.
Fig. 1.1.
2615
~ --- -+69 . 3-- -- _.,14
UA0036 4138
Fig. 1.1. Eighth-inch microphone with outer dimensions in mm
(1 mm = 0.03937 inch).
The microphone 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 a Two-Channel Microphone Power
Supply Type 2803 or a Microphone Power Supply Type 2801 which provide
the necessary voltages in the same
conditions as the B & K measuring in-
struments (see Accessories).
For free-field and room-acoustic measurements, the
microphones should be
placed remote from the measuring instruments, since these would often
disturb the sound field because of their non-negligible dimensions. Different
types of microphone stands and extension cables are available for this
purpose, see Accessories. The length of cable, however, has a significant
influence on the response above 40 kHz as will be seen in the description of
the cathode follower.
5

Microphone Ampl.
4138 2615
Level Recorder
UA0036
2804
Fig. 1.2. Typical B &K measuring equipment. Both amplifier and level recorder
are linear up to 200 kHz. A filter set (Type 1612) is also available tor automatic
plotting of spectrograms in the range 22 Hz-45 kH z.
Eighth-inch Condenser Microphone Cartridges.
This B & K microphone cartridge is specially designed for prec1s1on sound
pressure measurements in
the frequency range of 30 Hz to 140 kHz. This very
wide frequency range is obtained by reducing the mechanical dimensions to
a minimum. For
this very reason, however , the sensitivity is quite low and the
microphone is not well suited for measuring sound pressure levels below
76 dB re. 0.0002 ,u bar. On the other hand a dynamic range up to 184 dB re.
0.0002 ,u bar and an exce-llent response to sharp pulses, because of the wide
frequency range, makes the cartridge well suited for measuring all kinds of
noise at high pressure levels (e. g. missile exhaust noise etc.).
Excellent long term stability under a great range of environmental conditions
and especially insensitivity to temperature variations ensures that even for
field measurements the accuracy of calibration matches laboratory standard
requirements.
-70
dB
re.1V/)Jbor
- 8
0
- 9
0
-
10
0
- 11
0
-1 20,0
001
20
0.1
Type 4138 + UA 0036+2615
0.
01
50 100 200
0.1 500 1000 2
000 5000 10000 20 000 Hz 50000 100000 200000 0.001
1 10 kHz 100
167 0 4 2
Fig. 1.3. Most significant pressure response of the eighth-inch microphone .
6

The characteristic of the Microphone Cartridge Type 4138 is illustrated in
Fig.
1.3 . It is based on an extremely accurate mechanical construction. Fig. 1.4
shows a
cross section of the microphone cartridge. The thickness of the
diaphragm (approximately 2 fl ), the shape of the back plate and the distance
(12 - 13 fl) between membrane and back plate is determined so that a critical
damping of the diapragm resonance is achieved. In this way the widest
possible flat frequency range of the pressure response is obtained .
The pressure increase with the
protection grid mounted is appreciably higher
and at high frequencies it is rather dependent on how tightly the grid is
screwed onto the cartridge. Measurements involving
ultrasonic frequencies
should therefore be carried out with the protection grid removed.
Towards
the low frequencies the response of the cartridges is only affected
by the
influence of the pressure equalizing arrangement. This arrangement
Oiaphrogm
eac\1- p\.ate
167132.
Fig. 1.4. Exploded view of a 1/8" microphone cartridge.
J. Smm
Fig. 1.5. Dimensions of the 1/ 8" microphone cartridge Type 4138.
7

consists of a capillary leakage hole through which the equalization of the
static air pressure on both sides of the d·iaphragm is obtained at a suitable
rate. The
influence of ambient pressure (or altitude) variations on the micro-
phone sensitivity has been practically eliminated by proper design of the
pressure equalization hole. The time constant of the pressure equalization of
the eighth-inch cartridges corresponds to a - 3 dB cut-off frequency of 30 Hz
and
will not influence the measurements, since the - 3 dB low frequency
cut-off of the complete microphone is >5 Hz.
The choice of the materials and the design of the suspensions of the different
parts are made principally in order to obtain the best possible long-term
stability and temperature independency of the sensitivity.
In particular , the diaphragm is not clamped but electro-deposited onto the
tensioning ring,
forming one mechanical piece. The tension of the diaphragm
is thereby
unaffected by temperature variations. The cartridges are also
subjected to a complete artificial aging process during manufacture consisting
of controlled temperature variations over at least two weeks. The basic parts
are made
of high nickel alloy (K-Monel). The insulator used is silicone treated
quartz and for guaranteeing a disturbance -free connection to the cathode
fo·llower, gold-plated contacts are used.
The diaphagm is mounted
at a distance of 12-13 fl from the back plate. The
polarized cartridge capacity is of the order of 3.5 pF. The insulation is verified
as higher than 105 MQ.
Cleansing of the Microphone Diaphragm.
CAUTION. On no account should the diaphragm come into contact with
fingers
or other objects. The protecting grid should only be removed when
absolutely necessary.
Should the diaphragm become contaminated by liquids
or dust it may be dried off with cottonwool or a very soft paint-brush, using
great care, and if necessary a proper solvent. The diaphragm will not normally
corrode , but any appreciable added mass will change the frequency response.
Associated Cathode Follower Type 2615 + UA 0036.
The eighth-inch microphone cartridges Type 4138 are intended to be used
with the cathode follower Type 2615 provided with an adaptor Type UA 0036.
The "goose-neck" cathode follower Type 2614 may be used instead of the
2615 but it is then necessary in order to obtain identical results above 40 kHz
to load the output of the 2614 with a capacitance of 600 pF (e.g. the 10 m long
cable Type AO 0028).
Note: On no account should it be attempted to bend the Adaptor UA 0036.
A photograph of the 2615 is seen in Fig. 1.6 and the basic diagram is given
in Fig. 1.
7. The electrical components have been divided into two groups.
Those belonging to the
high impedance input circuit are mounted in the
main housing
containing the vacuum tube, while all parts on the low im-
pedance side are located in the connecting plug. A two-meter long multicore
cable is inserted between housing and plug. The dimensions of the housing
8