<s>
A	O
contact	B-Algorithm
microphone	I-Algorithm
,	O
also	O
known	O
as	O
a	O
piezo	O
microphone	B-Application
,	O
is	O
a	O
form	O
of	O
microphone	B-Application
that	O
senses	O
audio	O
vibrations	O
through	O
contact	O
with	O
solid	O
objects	O
.	O
</s>
<s>
Unlike	O
normal	O
air	O
microphones	B-Application
,	O
contact	B-Algorithm
microphones	I-Algorithm
are	O
almost	O
completely	O
insensitive	O
to	O
air	O
vibrations	O
but	O
transduce	B-Algorithm
only	O
structure-borne	O
sound	O
.	O
</s>
<s>
Contact	B-Algorithm
microphones	I-Algorithm
can	O
be	O
used	O
to	O
amplify	O
sound	O
from	O
acoustic	O
musical	O
instruments	O
,	O
to	O
sense	O
drum	O
hits	O
,	O
for	O
triggering	O
electronic	O
samples	O
,	O
and	O
to	O
record	O
sound	O
in	O
challenging	O
environments	O
,	O
such	O
as	O
underwater	O
under	O
high	O
pressure	O
.	O
</s>
<s>
Contact	B-Algorithm
microphones	I-Algorithm
based	O
on	O
piezo	O
materials	O
are	O
passive	O
and	O
high-impedance	O
,	O
and	O
they	O
sound	O
tinny	O
without	O
a	O
matching	O
preamp	O
.	O
</s>
<s>
The	O
most	O
commonly	O
available	O
contact	B-Algorithm
microphone	I-Algorithm
element	O
is	O
made	O
of	O
a	O
thin	O
piezoelectric	O
ceramic	O
round	O
glued	O
to	O
a	O
thin	O
brass	O
or	O
alloy	O
metal	O
disc	O
.	O
</s>
