<s>
A	O
noise-canceling	B-Algorithm
microphone	I-Algorithm
is	O
a	O
microphone	B-Application
that	O
is	O
designed	O
to	O
filter	O
ambient	O
noise	O
.	O
</s>
<s>
The	O
development	O
is	O
a	O
special	O
case	O
of	O
the	O
differential	O
microphone	B-Application
topology	O
most	O
commonly	O
used	O
to	O
achieve	O
directionality	O
.	O
</s>
<s>
All	O
such	O
microphones	B-Application
have	O
at	O
least	O
two	O
ports	O
through	O
which	O
sound	O
enters	O
;	O
a	O
front	O
port	O
normally	O
oriented	O
toward	O
the	O
desired	O
sound	O
and	O
another	O
port	O
that	O
's	O
more	O
distant	O
.	O
</s>
<s>
The	O
microphone	B-Application
's	O
diaphragm	O
is	O
placed	O
between	O
the	O
two	O
ports	O
;	O
sound	O
arriving	O
from	O
an	O
ambient	O
sound	O
field	O
reaches	O
both	O
ports	O
more	O
or	O
less	O
equally	O
.	O
</s>
<s>
The	O
microphone	B-Application
's	O
proximity	O
effect	O
is	O
adjusted	O
so	O
that	O
flat	O
frequency	O
response	O
is	O
achieved	O
for	O
sound	O
sources	O
very	O
close	O
to	O
the	O
front	O
of	O
the	O
mic	O
typically	O
1	O
to	O
3cm	O
.	O
</s>
<s>
Sounds	O
arriving	O
from	O
other	O
angles	O
are	O
subject	O
to	O
steep	O
midrange	O
and	O
bass	O
rolloff	B-Algorithm
.	O
</s>
<s>
Commercially	O
and	O
militarily	O
useful	O
noise-canceling	B-Algorithm
microphones	I-Algorithm
have	O
been	O
made	O
since	O
at	O
least	O
1935	O
(	O
Amelia	O
Earhart	O
used	O
one	O
on	O
her	O
1935	O
flight	O
from	O
Hawaii	O
to	O
California	O
)	O
and	O
have	O
been	O
made	O
since	O
the	O
1940s	O
by	O
Roanwell	O
,	O
Electro-Voice	O
and	O
others	O
.	O
</s>
<s>
Another	O
technique	O
uses	O
two	O
or	O
more	O
microphones	B-Application
and	O
active	O
or	O
passive	O
circuitry	O
to	O
reduce	O
the	O
noise	O
.	O
</s>
<s>
The	O
primary	O
microphone	B-Application
is	O
closer	O
to	O
the	O
desired	O
source	O
(	O
like	O
a	O
person	O
's	O
mouth	O
)	O
.	O
</s>
<s>
In	O
a	O
noisy	O
environment	O
,	O
both	O
microphones	B-Application
receive	O
noise	O
at	O
a	O
similar	O
level	O
,	O
but	O
the	O
primary	O
mic	O
receives	O
the	O
desired	O
sounds	O
more	O
strongly	O
.	O
</s>
<s>
Thus	O
if	O
one	O
signal	O
is	O
subtracted	O
from	O
the	O
other	O
(	O
in	O
the	O
simplest	O
sense	O
,	O
by	O
connecting	O
the	O
microphones	B-Application
out	O
of	O
phase	O
)	O
much	O
of	O
the	O
noise	O
is	O
canceled	O
while	O
the	O
desired	O
sound	O
is	O
retained	O
.	O
</s>
<s>
The	O
internal	O
electronic	O
circuitry	O
of	O
an	O
active	O
noise-canceling	O
mic	O
attempts	O
to	O
subtract	O
noise	O
signal	O
from	O
the	O
primary	O
microphone	B-Application
.	O
</s>
