<s>
A	O
compression	B-Device
driver	I-Device
is	O
a	O
small	O
specialized	O
diaphragm	O
loudspeaker	O
which	O
generates	O
the	O
sound	O
in	O
a	O
horn	B-Device
loudspeaker	I-Device
.	O
</s>
<s>
Horn-loaded	O
compression	B-Device
drivers	I-Device
can	O
achieve	O
very	O
high	O
efficiencies	O
,	O
around	O
10	O
times	O
the	O
efficiency	O
of	O
direct-radiating	O
cone	O
loudspeakers	O
.	O
</s>
<s>
They	O
are	O
used	O
as	O
midrange	B-Device
and	O
tweeter	B-Device
drivers	O
in	O
high	O
power	O
sound	O
reinforcement	O
loudspeakers	O
,	O
and	O
in	O
reflex	O
or	O
folded-horn	O
loudspeakers	O
in	O
megaphones	O
and	O
public	O
address	O
systems	O
.	O
</s>
<s>
were	O
the	O
first	O
to	O
discuss	O
the	O
benefits	O
of	O
using	O
a	O
large	O
radiating	O
diaphragm	O
with	O
a	O
horn	O
of	O
smaller	O
throat	O
area	O
as	O
a	O
means	O
of	O
increasing	O
the	O
efficiency	O
of	O
horn	B-Device
loudspeaker	I-Device
drivers	O
.	O
</s>
<s>
They	O
found	O
that	O
the	O
bandwidth	O
of	O
the	O
transducer	O
could	O
be	O
extended	O
to	O
higher	O
frequencies	O
using	O
their	O
phase	B-Architecture
plug	I-Architecture
.	O
</s>
<s>
The	O
paper	O
described	O
the	O
first	O
generation	O
compression	B-Device
driver	I-Device
with	O
a	O
field	O
coil	O
magnet	O
and	O
phase	B-Architecture
plug	I-Architecture
,	O
It	O
used	O
aluminum	O
diaphragm	O
with	O
an	O
edge	O
wound	O
aluminum	O
ribbon	O
voice	O
coil	O
.	O
</s>
<s>
The	O
first	O
commercial	O
compression	B-Device
driver	I-Device
was	O
introduced	O
1933	O
when	O
Bell	O
Labs	O
added	O
a	O
Western	O
Electric	O
No	O
.	O
</s>
<s>
555	O
compression	B-Device
driver	I-Device
as	O
a	O
mid-range	B-Device
driver	I-Device
to	O
their	O
two-way	O
"	O
divided	O
range	O
"	O
loudspeaker	O
which	O
was	O
developed	O
in	O
1931	O
.	O
</s>
<s>
Today	O
the	O
majority	O
of	O
compression	B-Device
drivers	I-Device
,	O
either	O
by	O
inheritance	O
or	O
design	O
,	O
are	O
based	O
on	O
the	O
guidelines	O
outlined	O
by	O
Smith	O
.	O
</s>
<s>
using	O
a	O
more	O
accurate	O
analytical	O
acoustical	O
model	O
of	O
the	O
compression	B-Device
driver	I-Device
geometry	O
.	O
</s>
<s>
From	O
this	O
work	O
improved	O
phase	B-Architecture
plug	I-Architecture
design	O
guidelines	O
have	O
been	O
deduced	O
to	O
eliminate	O
all	O
traces	O
of	O
acoustical	O
resonance	O
in	O
the	O
compression	O
cavity	O
.	O
</s>
<s>
In	O
some	O
sound	O
reinforcement	O
and	O
studio	B-Device
monitors	I-Device
the	O
high	O
frequency	O
drivers	O
are	O
protected	O
by	O
current	O
sensing	O
self-resetting	O
circuit	O
breakers	O
.	O
</s>
<s>
The	O
net	O
effect	O
is	O
that	O
as	O
the	O
power	O
increases	O
the	O
filament	O
consumes	O
an	O
increasing	O
share	O
of	O
the	O
total	O
power	O
thus	O
limiting	O
the	O
power	O
available	O
to	O
the	O
compression	B-Device
driver	I-Device
.	O
</s>
