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
4131
•
4132
-
DESCRIPTION
these
would
often
disturb
the
sound
field
because
of
their
non-negligible
dimensions.
A
Microphone
Stand
and
extension
cables
are
available
for
this
purpose,
see
Accessories.
An
example
of
measuring
equipment
is
shown
in
Fig. 1.2.
2603
Microphone stand
Fig. 1.2.
Typical
measuring
arrangement:
one-inch
microphone
equipped
witll
Cathode
Follower
Type
2613
connected
to
a B & K
Microphone
Amplifier.
Description
of
the
Microphone
Cartridges
Type
4131
and
4132.
Two
slightly
different
one-inch
microphone
cartridges
are
available:
(1)
the
Microphone
Cartridge
Type
4131
designed
for
free-field
measure-
ments
and
featuring
a
normal
incidence
free-field
frequency
characteristic
which
is
flat
up
to
18
kHz
(with
protecting
grid
mounted).
(2)
the
Microphone
Cartridge
Type
4132
presenting
a
pressure
frequency
characteristic
which
is
flat
up
to
7
kHz.
The
difference
in
characteristics
are
summarized
in
Fig.
1.14.
The
two
cartridges
have
basically
the
same
mechanical
construction
and
differ
only
in
the
damping
of
the
diaphragm
resonance.
The
resonance
is
over-
damped
in
the
case
of
Type
4131,
in
such
a
way
that
the
normal
incidence
free-field
pressure
increase
is
compensated
by
the
decrease
of
pressure
sen-
sitivity
up
to
18
kHz.
In
the
case
of
Type
4132
the
resonance
is
approximately
Fig. 1.3.
Exploded
view
of
a
one-inch
microphone
cartridge.
g
fl31 . 4132-
DESCRIPTION
critically
damped
in
order
to
obtain
the
flattest
possible
pressure
frequency
response
(Fig. 1.14).
The
resonance
frequency
is
approximately
10
kHz
for
the
4131
and
8
kHz
for
the
4132.
The
damping
is
controlled
by
means
of
appropriate
holes
in
the
back
plate
and
by
adjusting
the
tension
of
the
diaphragm.
Towards
the
low
frequencies
the
response
of
the
cartridges
is
only
affected
by
the
influence
of
the
pressure
equalizing
arrangement.
This
arrangement
consists
of
a
capillary
leakage
hole
through
which
the
equalization
of
the
static
air
pressure
on
both
sides
of
the
diaphragm
is
obtained
at
a
suitable
rate.
The
influence
of
ambient
pressure
(or
altitude)
variations
on
the
microphone
sensitivity
has
been
minimized
by
proper
design
of
the
pressure
equalization
hole.
(For
adjustment
of
the
pressure
equalization
see
item
10
of
General
Characteristics).
The
equalization
hole
is
situated
in
front
of
the
grid-
and
coupler
mounting-
thread.
The
pressure
equalization
is
then
obtained
also
in
the
case
of
closed
cavity
measurements.
In
addition,
the
temperature
inside
the
microphone
is a
few
degrees
higher
than
the
temperature
at
the
equalization
opening
due
to
the
heat
generated
by
the
cathode
follower,
this
prevents
moisture
condensation
taking
place
inside
the
cartridge.
The
time
constant
of
the
pressure
equalization
of
Types
4131-4132 is o.05
second.
This
corresponds
to
a
-3
dB
cut-off
frequency
of
3 Hz
approximately,
and
will
not
influence
the
measurements,
since
the
- 3 dB
low
frequency
cut-off
of
the
complete
micro-
phone
is 10 Hz.
Microphone
cartridges
with
lower
or
higher
time
constants
may
be
obtained
from
B & K
if
specified
when
ordering.
The
mechanical
construction
of
the
cartridges
is
shown
in
Fig. 1.3.
The
choice
of
the
materials
and
the
design
of
the
mountings
of
the
different
parts
are
made
principally
with
a
view
to
obtain
the
best
possible
long-term
stability
and
temperature
independence
of
sensitivity.
The
cartridges
are
also
2311mm60NS 2
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Fig. 1.4.
Dimensions
of
the
"one-inch"
microphone
cartridges
Type
4131-4132.
10
4131
•
4132
-
DESCRIPTION
subjected
to
a
complete
artificial
aging
process
during
manufacture.
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
follower,
gold-plated
contacts
are
used.
The
diaphragm
is
made
of
a 5
11-
thick
nickel
plate
and
is
mounted
at
a
distance
of
22
p,
from
the
back
plate.
This
distance
is
reduced
to
20
p,
when
the
200 V
polarization
voltage
is
applied.
The
polarized cartridge capacity,
which
is
of
the
order
of
60
pF,
is
measured
individually
for
each
cartridge
(see
the
calibration
chart).
The
insulation
is
verified
as
higher
than
105
M.Q.
Silica Gel Cap
UA
0135.
This
device,
supplied
with
all
B & K
microphone
cartridges
will
allow
the
microphones
to
be
stored
in
a
hot,
high
humidity
atmosphere
without
running
the
risk
of
condensation
occuring
inside
the
microphone
cartridge
when
taken
into
a
cooler
atmosphere
for
use.
It
will
also
serve
as
an
effective
dust
cover.
Microphone Cartridge
166o38
Fig. 1.5.
The
Silica Gel Cap
VA
0135
mounted
on
a
1"
Microphone
Cartridge.
The
cap
is
made
of
a
high
temperature
resistant
material
and
filled
with
an
indicator
type
moisture
absorbent
(Fig. 1.5).
In
this
dry
condition
this
ab-
sorbent
is
blue
but
when
its
colour
becomes
pink
then
the
absorbent
is
saturated
with
water
and
should
be
restored
to
its
dry
condition
by
heating.
This
"drying
out"
process
can
be
carried
out
repeatedly,
simply
by
heating
the
absorbent
in
an
oven
for
1/2-
1
hour
at
a
temperature
from
100°C (212°F)
to
150°C (300°F).
When
the
microphone
is
stored,
the
absorbent
is
only
exposed
to
the
air
within
the
microphone
cartridge.
To
continue
this
protection
against
un-
necessary
saturation
when
the
microphone
is
in
use,
the
cap
should
be
placed
in
the
upper
part
of
the
cartridge
hole
in
the
microphone
box,
which
will
11
4131
• 4132-
DESCRIPTION
Fig. 1.6.
The
Silica Gel Cap UA 0135 placed
in
the
cartridge
hole
in
the
microplwne
box.
seal
against
the
rim
of
the
cap
(Fig. 1.6).
From
the
dry
to
saturated
con-
dition,
the
cap
will
absorb
approximately
o.17
mg
of
water.
1"
microphone
cartridges
have
an
internal
volume
of
1 cm3,
and
the
total
air
volume
enclosed
under
the
cap
when
mounted
on
a
cartridge
is
approxi-
mately
2 cm3.
The
water
content
of
saturated
air
is:
9.4 X 10-6
gram/cm3
at
10°C
17.3 X 10-6
gram/cm3
at
20°C
30.4 X 10-6
gram/cm3
at
30°C
51.1 X 10-6
gram/cm3
at
40°C.
Cleansing
of
the
Microphone
Diaphragm.
CAUTION. On
no
account
should
the
diaphragm
whicli
is
only
5
ft
(o.0002")
thick
come
into
contact
with
fingers
or
other
objects.
The
protecting
grid
should
only
be
removed
when
absolutely
necessary.
This
grid is
an
effective
protection
against
mechanical
damage
but
cannot
stop
liquids
or
dust
from
falling
on
the
diaphragm.
Should
the
diaphragm
become
contaminated
it
may
be
dried
off
with
cotton-wool
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.
Description
of
the
Cathode
Followers
Type
2612
and
2613*).
The
Cathode
Followers
Type
2612
and
2613
are
designed
for
connection
to
the
B & K
Condenser
Microphone
Cartridges
or
Accelerometers,
transforming
the
high
source
impedance
of
the
transducer
to
a 750 n
output
impedance.
*)The
cathode-follower
Type
2630
is
described
in
"Accessories''.
12
4131
•
4132
-
DESCRIPTION
They
are
electrically
identical
units.
The
mechanical
construction,
however,
is
different
in
that
the
Cathode
Follower
Type
2612 is
fitted
to a
flexible
goose-neck,
while
Type
2613 is
supplied
with
a 2 m
long
multi-core
cable
as
shown
in
Fig. 1.9.
The
outside
diameter
of
the
cathode
followers
is
identical
to
that
of
the
microphone
cartridge-s, i.e. 23.77
mm
(o.936"),
over
a
length
of
48.4
mm.
To
ensure
high
operating
stability
under
conditions
of
large
temperature
changes
and
external
vibration,
the
components
used
have
been
carefully
selected
and
are
mounted
on
glass
laminated
boards.
Use
has
been
made
of
silver
plated
printed
circuitry
and
the
amplifier
tube
chosen
meets
the
Mil-EIC
specifications.
The
complete
component
assembly
is silicone
treated
which
provides
a
high
resistance
to
moisture.
In
the
interests
of
good
electrical
connection
and
low
noise
the
contact
between
cathode
follower
and
microphone
cartridge
is
made
through
gold.
The
influence
of
vibrations
is
described
in
Figs. 1.32
and
1.33.
Before
mounting
each
tube
is
tested
for
sensitivity
to
shock
and
during
the
final
tests
the
cathode
followers
are
exposed
to
a
sound
pressure
of
130 dB
where
the
microphonics
is
measured
at
all
frequencies
from
20
to
20ooo Hz.
The
signal-microphonics
ratio
is
greater
than
50 dB
with
reference
max
.
allowable
:
level
:
40db
-
time
: 1.6sec.
__
1sec
.
senes
series
f62f67
Fig. 1.7.
Test
of
a
tube
EF731. Microphonics are
excited
by
means
of
two
series
of
lateral
shocks
and
recorded
whereby
the
amplitude
and
duration
of
transients are closely checked.
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Fig. 1.8.
Recording
of
the
output
from
a Cathode
Follower
with
a
dummy
microphone
of
60
pF
when
exposed
to
a
sound
field
of
130 dB in
the
fre-
quency
range
from
20 to 20ooo Hz.
The
output
signal is
at
any
frequency
more
than
50
dB
below
that
obtained
with
the
microphone
cartridge in place.
13
41St . 4132-
DESCRIPTION
Fig. 1.9.
Cathode
Followers
2612-2613
with
Part
Numbers.
to a
microphone
with
a
sensitivity
of
5
mV/~-tbar.
See
Fig.
1.8.
The
input
is
internally
shielded
by
means
of
a
"guard
ring"
which
is
connected
to
the
cathode
of
the
tube.
In
this
way
the
input
capacity
is
reduced
to
an
absolute
minimum.
See
Fig
. 1.10.
By
extending
the
internal
shield
it
is
possible
to
use
14
4131
•
4132
-
DESCRIPTION
.....------
........
-Polarizing
Voltage
Microphone
Cartridge
+
f592t
3
Fig. 1.10. Basic diagram
of
a
cathode
follower
showing
the
principle
of
the
((guard
ring"
system.
Meters Meters
of
cable
of
cable
AC
3029 AC3028
1s used as
used
. ,
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3.33 feet
3 ' 6 8
noo'i's
2 3 ' 6 8
nK
Cfs
2 3 ' s 6 s
100K
Cfs
2 3 '
X1000JJJJF
100
80
60
50
40
30
20
10
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1000K
Cfs
159215
Fig. 1.11. Graph
of
cathode
follower
high
frequency
performance
with
different
lengths
of
connecting
cable
attached
to
the
output.
To
illu.'ftrate
the
use
oj
the
graph
the
standard
B & K
microphone
cables are
drawn
in.
The
cables
AC
3028
and
AC 3029 are
the
types
of
cable
used
by
B & K,
but
without
connecting
plugs
and
sockets
in
the
ends.
15
U31
· 4132- DESCRIPTION
the
microphone
cartridge
at
some
distance
from
the
cathode
follower.
See
Accessories:
Extension
Rod
UA
oo:i9.
The
Cathode
Follower
Type
2612 <'an
be
mounted
either
directly
on
one
of
the B & K
Microphone
Amplifiers
or
Frequency
Analyzers,
or
connected
via
a
seven-conductor
extension
cable.
\Vhen
used
with
an
extension
cable
it
can
be
mounted
on
a B & K
microphone
stand.
The
total
capacity
of
the
extension
cables
will,
to
a
certain
extent,
load
the
cathode
follower
and
lower
the
high
frequency
cut-off
point
which
will
produce
distortion
at
high
sound
pressure
levels. At
normal
levels,
this
will
only
be
of
importance
when
very
long
cables
are
used,
but
this
does
not
infer
any
serious
limitation
in
the
practical
use
of
the
microphones.
The
relationship
between
high
frequency
cut-off,
cable
length
and
harmonic
distortion
are
given
showing
the
limiting
values
in
Fig. 1.11.
Type
2613 is
intended
for
direct
mounting
on
a B & K
Microphone
Stand
and
may
also
be
suspended
by
means
of
its
own
cable.
The
power
supplies
necessary
to
operate
the
cathode
followers
are
provided
by
the
B &K
amplifiers
via
the
multi-socket
at
the
front
panel
(Fig. 1.12)
or
by
Microphone
Power
Supplies
Type
2801
and
2803 (see Accessories).
SPECIFICATIONS:
Input
Impedance:
270
M.Q
in
parallel
with
3
pF.
Output
Impedance:
750
.Q.
Transmission
Loss: Voltage
loss:
o.8 dB ·± o.2 dB.
Self-generated
Noise Level:
Approx.
20
t-tV
with
the
input
loaded
by
a
capa-
citor
of
60
pF.
(20
Hz-20
kHz).
Maximum
Input
Voltage for
4%
Distortion: 20 volts RMS
(with
load
im-
pedance
>50
k.Q).
Tube:
EF
731 (5899),
specially
tested
for
low
microphonics.
Accessory
included:
Input
Adapter
JJ
2612
fitting
the
B & K
coaxial
plugs,
for
direct
connection
of
accelerometers
etc.
instead
of
a
microphone
cartridge.
Ground
Polarization
Voltage
200V
D.C.
Cathode
Filament
6.3V
D.C.
200mA
(max. ripple 2 mVolt)
Plate
150V
D.C
2m
A
(max. ripple 60)-IVolt)
Connect
io
n
tor
4408
1
59216
Fig. 1.12.
Plug
connections
for
the
cathode
followers
(viewed
from
outside).
16
GENER
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20
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100
__
____
1oo_o
____
~
__
s
____
1
o_ooo_~~
Fig. 1.13.
Typical
calibration
chart
as
supplied
with
the
microphone
cartridges.
The
automatic
plotting
process
used
in
production
has
an
accuracy
of
o.2
dB
up
to
10
kHz
and
o.5
dB
up
to
20
kHz.
(Contains
a
series
ca
pacitor
of
a
few
thousand
pF
for
blocking
the
200 volts
polarization
voltage).
General
Characteristics
(Cartridges+
Cathode
Followers).
1.
Sensitivity.
The
sensitivity
of
both
cartridges
measured
at
the
output
of
the
associated
cathode
followers
are
within
the
limits
3.
5--6
mV/,uhar.
Each
cartrid~e
is.
however,
individually
calibrated
and
supplied
with
its
specific
calibration
chart
when
delivered,
see Fig. 1.13.
The
calibration
is ca
rried
out
at
250 Hz
and
with
a
cathode
follower
presenting
the
exact
nominal
characteristics:
gain
o.8 dB
and
input
capacity
3
pF
(see
the
technical
data
for
the
cathode
followers).
The
open
circuit
sensitivity
,
which
is
calculated
from
the
overall
sensitivity,
is
also
indicated
on
the
calibration
chart
together
with
the
polarized
cartridge
capacity.
When
this
capacity
is
exactly
60
pF
the
open
circuit
sensitivity
is 1.2 dB
higher
than
the
overall
sensitivity
measured
at
the
output
of
the
reference
cathode
follower.
17
4131
· 4132- DESCRIPTION
2.
Frequency Response.
Each
microphone
cartridge
is
provided
with
an
individual
frequency
response
determined
by
the
electrostatic
actuator
method
(Fig. 1.13)
giving
pressure
conditions.
The
free-field
response
at
a
particular
incidence
is
obtained
by
adding
to
this
pressure
response
the
relative
free-field
correction
given
in
25
db
15
010
10
0
Type
4131
+2612/13
.
100
100
Type 4132+2612/13
.R
10dB
1000
10000 20000
c/s~
1000
Hz
10000
ro
~
~
~
--~
10
100
1
ooo
Hz
1oooo
111m
Fig. 1.14. Typical frequency characteristics
of
the one-inch microphones
with
protecting grid.
180°
pressure response. 160°
0 --
140°
10
120°
4131
r=
4132
100°
r 80°
-'U
60°
db
40°
phose angle 20°
00
10
10 100 --
.,.
.00000
10
100
1000
cjs
10000
1~1n~
Fig. 1.15. Typical pressure-response phase angle characteristics
of
the
micro-
phone cartridges. Below 50 Hz,
the
phase
shift
becomes slightly negative
because
of
the
cathode follower.
18
Table of contents
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