<s>
An	O
electret	B-Algorithm
microphone	I-Algorithm
is	O
a	O
type	O
of	O
electrostatic	O
capacitor-based	O
microphone	B-Application
,	O
which	O
eliminates	O
the	O
need	O
for	O
a	O
polarizing	O
power	O
supply	O
by	O
using	O
a	O
permanently	O
charged	O
material	O
.	O
</s>
<s>
Modern	O
electret	B-Algorithm
microphones	I-Algorithm
use	O
PTFE	O
plastic	O
,	O
either	O
in	O
film	O
or	O
solute	O
form	O
,	O
to	O
form	O
the	O
electret	O
.	O
</s>
<s>
Electret	O
materials	O
have	O
been	O
known	O
since	O
the	O
1920s	O
and	O
were	O
proposed	O
as	O
condenser	O
microphone	B-Application
elements	O
several	O
times	O
,	O
but	O
they	O
were	O
considered	O
impractical	O
until	O
the	O
foil	O
electret	O
type	O
was	O
invented	O
at	O
Bell	O
Laboratories	O
in	O
1961	O
by	O
Gerhard	O
Sessler	O
and	O
James	O
West	O
,	O
using	O
a	O
thin	O
metallized	O
Teflon	O
foil	O
.	O
</s>
<s>
This	O
became	O
the	O
most	O
common	O
type	O
,	O
used	O
in	O
many	O
applications	O
from	O
high-quality	O
recording	O
and	O
lavalier	B-Algorithm
use	O
to	O
built-in	O
microphones	B-Application
in	O
small	O
sound	O
recording	O
devices	O
and	O
telephones	O
.	O
</s>
<s>
There	O
are	O
three	O
major	O
types	O
of	O
electret	B-Algorithm
microphones	I-Algorithm
,	O
differing	O
in	O
the	O
way	O
the	O
electret	O
material	O
is	O
used	O
:	O
</s>
<s>
Back	O
electret	O
An	O
electret	O
film	O
is	O
applied	O
to	O
the	O
back	O
plate	O
of	O
the	O
microphone	B-Application
capsule	O
and	O
the	O
diaphragm	O
is	O
made	O
of	O
an	O
uncharged	O
material	O
,	O
which	O
may	O
be	O
mechanically	O
more	O
suitable	O
for	O
the	O
transducer	O
design	O
being	O
realized	O
.	O
</s>
<s>
Unlike	O
other	O
condenser	O
microphones	B-Application
,	O
electret	O
types	O
require	O
no	O
polarizing	O
voltage	O
,	O
but	O
they	O
normally	O
contain	O
an	O
integrated	O
preamplifier	O
,	O
which	O
does	O
require	O
a	O
small	O
amount	O
of	O
power	O
(	O
often	O
incorrectly	O
called	O
polarizing	O
power	O
or	O
bias	O
)	O
.	O
</s>
<s>
Other	O
types	O
include	O
a	O
1.5V	O
battery	O
in	O
the	O
microphone	B-Application
housing	O
,	O
which	O
is	O
often	O
left	O
permanently	O
connected	O
as	O
the	O
current	O
drain	O
is	O
usually	O
very	O
small	O
.	O
</s>
