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BRUEL & KJAER 4131 - Page 6

BRUEL & KJAER 4131
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4135
• 4136- 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,
the
mechanical
tension
of
the
diaphragm
,
and
the
spacing
between
plate
and
diaphragm.
The
low
frequency
limit
of
the
linear
range
is
set
by
the
"cut-off"
(or
time
Cartridge
Cathode Follower
Mechanical I
Electrical
Elasticity
C+LlC
~-~
Mechanical Damping
Eo
R~
· /
-~
Polarization
Circuit
C =
Polarized
cartridge
capacity
llC = Variation of
capacity
producing
the
signal
Cs =
Stray
capacity
of
connection
to
cathode
follower
R;.C;
=Input
impedance
of
cathode
follower
1621
61
Fig. 0.2.
Simplified
diagram
of
the
condenser
microphone.
(The
electrical
leakage
in
the
cartridge is neglected).
4

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