<s>
A	O
rotary	B-Device
woofer	I-Device
is	O
a	O
subwoofer-style	O
loudspeaker	O
which	O
reproduces	O
very	O
low	O
frequency	O
content	O
by	O
using	O
a	O
conventional	O
speaker	O
voice	B-Device
coil	I-Device
's	O
motion	O
to	O
change	O
the	O
pitch	O
of	O
an	O
impeller	O
rotating	O
at	O
a	O
constant	O
speed	O
.	O
</s>
<s>
The	O
pitch	O
of	O
the	O
fan	O
blades	O
is	O
controlled	O
by	O
the	O
audio	O
signal	O
presented	O
to	O
the	O
voice	B-Device
coil	I-Device
,	O
and	O
is	O
able	O
to	O
swing	O
both	O
positive	O
and	O
negative	O
,	O
with	O
respect	O
to	O
a	O
zero	O
pitch	O
spinning	O
blade	O
position	O
.	O
</s>
<s>
Since	O
the	O
audio	O
amplifier	O
only	O
changes	O
the	O
pitch	O
of	O
the	O
blades	O
,	O
it	O
takes	O
much	O
less	O
power	O
,	O
per	O
dB	O
of	O
generated	O
acoustic	O
sound	O
level	O
,	O
to	O
drive	O
a	O
rotary	B-Device
woofer	I-Device
than	O
to	O
power	O
a	O
conventional	O
subwoofer	B-Application
,	O
which	O
uses	O
a	O
moving	O
electromagnet	O
(	O
voice	B-Device
coil	I-Device
)	O
placed	O
within	O
the	O
field	O
of	O
a	O
stationary	O
permanent	O
magnet	O
to	O
drive	O
a	O
cone	O
which	O
then	O
displaces	O
air	O
.	O
</s>
<s>
Rotary	B-Device
woofers	I-Device
excel	O
at	O
producing	O
sounds	O
below	O
20Hz	O
,	O
below	O
the	O
normal	O
hearing	O
range	O
;	O
when	O
installed	O
in	O
the	O
wall	O
of	O
a	O
sealed	O
room	O
,	O
they	O
can	O
produce	O
audio	O
frequencies	O
down	O
to	O
0Hz	O
,	O
a	O
static	O
pressure	O
differential	O
,	O
by	O
simply	O
compressing	O
the	O
air	O
in	O
the	O
sealed	O
room	O
.	O
</s>
<s>
Typical	O
subwoofers	B-Application
using	O
moving	O
cones	O
do	O
not	O
transmit	O
energy	O
very	O
well	O
to	O
the	O
air	O
below	O
20Hz	O
,	O
and	O
thus	O
their	O
sound	O
pressure	O
level	O
(	O
SPL	O
)	O
falls	O
off	O
significantly	O
below	O
this	O
frequency	O
.	O
</s>
<s>
The	O
rotary	B-Device
woofer	I-Device
displaces	O
far	O
more	O
air	O
than	O
is	O
possible	O
using	O
moving	O
cones	O
,	O
which	O
makes	O
very-low-frequency	O
reproduction	O
possible	O
.	O
</s>
<s>
Instead	O
of	O
using	O
a	O
moving	O
electromagnet	O
(	O
voice	B-Device
coil	I-Device
)	O
placed	O
within	O
the	O
field	O
of	O
a	O
stationary	O
permanent	O
magnet	O
to	O
drive	O
a	O
cone	O
,	O
like	O
a	O
conventional	O
subwoofer	B-Application
,	O
on	O
a	O
rotary	B-Device
woofer	I-Device
,	O
the	O
voice	B-Device
coil	I-Device
's	O
motion	O
is	O
used	O
to	O
change	O
the	O
angle	O
of	O
a	O
fixed	O
rotation	O
speed	O
set	O
of	O
fan	O
blades	O
in	O
order	O
to	O
generate	O
sound	O
pressure	O
waves	O
.	O
</s>
<s>
Since	O
the	O
audio	O
amplifier	O
only	O
changes	O
the	O
pitch	O
of	O
the	O
blades	O
,	O
it	O
takes	O
much	O
less	O
power	O
to	O
drive	O
a	O
rotary	B-Device
woofer	I-Device
,	O
although	O
a	O
secondary	O
power	O
source	O
is	O
required	O
to	O
drive	O
the	O
fan	O
motor	O
.	O
</s>
<s>
As	O
an	O
analogy	O
,	O
the	O
hub	O
of	O
the	O
rotary	B-Device
woofer	I-Device
's	O
fan	O
is	O
somewhat	O
like	O
a	O
helicopter	O
's	O
swashplate	O
which	O
allows	O
a	O
stationary	O
source	O
of	O
reciprocating	O
motion	O
—	O
the	O
voice	B-Device
coil	I-Device
of	O
the	O
subwoofer	B-Application
—	O
to	O
change	O
the	O
angle	O
of	O
the	O
spinning	O
set	O
of	O
blades	O
.	O
</s>
<s>
A	O
rotary	B-Device
woofer	I-Device
is	O
designed	O
to	O
produce	O
only	O
frequencies	O
lower	O
than	O
20Hz	O
;	O
distortion	O
increases	O
as	O
the	O
input	O
frequency	O
exceeds	O
the	O
fan	O
's	O
rotation	O
rate	O
(	O
20Hz	O
would	O
be	O
about	O
1200	O
rpm	O
for	O
the	O
fan	O
)	O
,	O
which	O
is	O
itself	O
limited	O
by	O
the	O
need	O
to	O
avoid	O
making	O
higher-frequency	O
noise	O
(	O
due	O
to	O
the	O
sound	O
of	O
the	O
spinning	O
blades	O
)	O
.	O
</s>
<s>
The	O
woofer	O
is	O
installed	O
and	O
carefully	O
braced	O
so	O
that	O
the	O
blades	O
lie	O
in	O
a	O
circular	O
opening	O
,	O
so	O
that	O
air	O
can	O
be	O
moved	O
between	O
an	O
external	O
chamber	O
,	O
such	O
as	O
the	O
attic	O
of	O
a	O
house	O
,	O
and	O
the	O
main	O
listening	O
space	O
;	O
if	O
the	O
rotary	B-Device
woofer	I-Device
were	O
not	O
installed	O
in	O
such	O
a	O
"	O
baffle	O
"	O
and	O
placed	O
directly	O
in	O
the	O
main	O
space	O
instead	O
,	O
the	O
generated	O
sound	O
pressure	O
from	O
the	O
rear	O
of	O
the	O
unit	O
being	O
180	O
degrees	O
out	O
of	O
phase	O
would	O
almost	O
completely	O
cancel	O
that	O
from	O
the	O
front	O
the	O
end	O
,	O
the	O
result	O
of	O
which	O
would	O
not	O
be	O
fit	O
for	O
its	O
intended	O
application	O
.	O
</s>
<s>
Six	O
rotary	B-Device
woofers	I-Device
are	O
installed	O
as	O
part	O
of	O
an	O
immersive	O
Niagara	O
Falls	O
attraction	O
known	O
as	O
Niagara	O
's	O
Fury	O
,	O
located	O
in	O
the	O
Table	O
Rock	O
House	O
,	O
to	O
provide	O
low	O
frequency	O
extension	O
down	O
to	O
less	O
than	O
1	O
Hertz	O
,	O
to	O
emulate	O
those	O
waves	O
created	O
by	O
the	O
falls	O
.	O
</s>
