<s>
The	O
chief	O
electrical	O
characteristic	O
of	O
a	O
dynamic	O
loudspeaker	O
's	O
driver	B-Device
is	O
its	O
electrical	O
impedance	O
as	O
a	O
function	O
of	O
frequency	O
.	O
</s>
<s>
It	O
can	O
be	O
visualized	O
by	O
plotting	O
it	O
as	O
a	O
graph	B-Application
,	O
called	O
the	O
impedance	O
curve	O
.	O
</s>
<s>
The	O
most	O
common	O
driver	B-Device
type	O
is	O
an	O
electro-mechanical	O
transducer	B-Algorithm
using	O
a	O
voice	B-Device
coil	I-Device
rigidly	O
connected	O
to	O
a	O
diaphragm	B-Device
(	O
generally	O
a	O
cone	B-Device
)	O
.	O
</s>
<s>
The	O
voice	B-Device
coil	I-Device
in	O
moving	O
coil	O
drivers	O
is	O
suspended	O
in	O
a	O
magnetic	O
field	O
provided	O
by	O
the	O
loudspeaker	O
magnet	O
structure	O
.	O
</s>
<s>
As	O
electric	O
current	O
flows	O
through	O
the	O
voice	B-Device
coil	I-Device
(	O
from	O
an	O
electronic	O
amplifier	O
)	O
,	O
the	O
magnetic	O
field	O
created	O
by	O
the	O
coil	O
reacts	O
against	O
the	O
magnet	O
's	O
fixed	O
field	O
and	O
moves	O
the	O
voice	B-Device
coil	I-Device
(	O
and	O
so	O
the	O
cone	B-Device
)	O
.	O
</s>
<s>
Alternating	O
current	O
will	O
move	O
the	O
cone	B-Device
back	O
and	O
forth	O
.	O
</s>
<s>
The	O
moving	O
system	O
of	O
the	O
loudspeaker	O
—	O
consisting	O
of	O
the	O
cone	B-Device
,	O
cone	B-Device
suspension	O
,	O
spider	O
,	O
and	O
voice	B-Device
coil	I-Device
—	O
can	O
be	O
modeled	O
as	O
an	O
effective	O
mass	O
(	O
spring	O
–	O
mass	O
system	O
)	O
,	O
a	O
mass	O
suspended	O
by	O
a	O
spring	O
.	O
</s>
<s>
This	O
system	O
has	O
a	O
characteristic	O
mass	O
and	O
stiffness	B-Algorithm
,	O
and	O
a	O
resonant	B-Application
frequency	O
at	O
which	O
the	O
system	O
will	O
vibrate	O
freely	O
.	O
</s>
<s>
This	O
frequency	O
is	O
known	O
as	O
the	O
"	O
free-space	O
resonance	B-Application
"	O
of	O
the	O
loudspeaker	O
and	O
is	O
designated	O
by	O
Fs	O
.	O
</s>
<s>
At	O
this	O
frequency	O
,	O
the	O
voice	B-Device
coil	I-Device
is	O
vibrating	O
in	O
the	O
speaker	O
's	O
magnetic	O
field	O
with	O
maximum	O
peak-to-peak	O
amplitude	B-Application
and	O
velocity	O
.	O
</s>
<s>
The	O
impedance	O
is	O
at	O
its	O
maximum	O
at	O
Fs	O
,	O
shown	O
as	O
Zmax	O
in	O
the	O
graph	B-Application
.	O
</s>
<s>
For	O
frequencies	O
just	O
below	O
resonance	B-Application
,	O
the	O
impedance	O
rises	O
rapidly	O
as	O
the	O
frequency	O
increases	O
towards	O
Fs	O
and	O
is	O
inductive	O
in	O
nature	O
.	O
</s>
<s>
At	O
resonance	B-Application
,	O
the	O
impedance	O
is	O
purely	O
resistive	O
.	O
</s>
<s>
The	O
frequency	O
Fs	O
and	O
the	O
frequencies	O
above	O
and	O
below	O
it	O
where	O
the	O
impedance	O
is	O
Zmax	O
are	O
important	O
in	O
determining	O
the	O
loudspeaker	O
's	O
T/S	B-Device
parameters	I-Device
.	O
</s>
<s>
These	O
can	O
be	O
used	O
,	O
for	O
example	O
,	O
to	O
design	O
a	O
suitable	O
enclosure	O
for	O
the	O
driver	B-Device
,	O
especially	O
for	O
low	O
frequency	O
drivers	O
.	O
</s>
<s>
In	O
fact	O
Fs	O
is	O
itself	O
one	O
of	O
the	O
Thiele/Small	B-Device
parameters	I-Device
.	O
</s>
<s>
This	O
is	O
the	O
minimum	O
value	O
in	O
the	O
impedance	O
vs.	O
frequency	O
relationship	O
,	O
which	O
is	O
always	O
higher	O
than	O
the	O
DC	O
resistance	O
of	O
the	O
voice	B-Device
coil	I-Device
,	O
i.e.	O
,	O
as	O
measured	O
by	O
an	O
ohmmeter	O
.	O
</s>
<s>
The	O
graph	B-Application
shows	O
the	O
impedance	O
curve	O
of	O
a	O
single	O
loudspeaker	B-Device
driver	I-Device
in	O
free-air	O
(	O
unmounted	O
in	O
any	O
type	O
of	O
enclosure	O
)	O
.	O
</s>
<s>
A	O
home	O
hi-fi	O
loudspeaker	O
system	O
typically	O
consists	O
of	O
two	O
or	O
more	O
drivers	O
,	O
an	O
electrical	B-Algorithm
crossover	I-Algorithm
network	I-Algorithm
to	O
divide	O
the	O
signal	O
by	O
frequency	O
band	O
and	O
route	O
them	O
appropriately	O
to	O
the	O
drivers	O
,	O
and	O
an	O
enclosure	O
that	O
all	O
these	O
components	O
are	O
mounted	O
in	O
.	O
</s>
<s>
But	O
due	O
to	O
the	O
complex	O
and	O
variable	O
nature	O
of	O
the	O
driver/crossover	O
load	O
and	O
its	O
effect	O
on	O
the	O
phase	O
relationship	O
between	O
the	O
voltage	O
and	O
current	O
,	O
the	O
current	O
will	O
not	O
necessarily	O
be	O
at	O
its	O
minimum	O
when	O
the	O
voltage	O
across	O
the	O
output	O
devices	O
is	O
maximum	O
-	O
this	O
results	O
in	O
increased	O
power	O
dissipation	O
in	O
the	O
amplifier	O
output	O
stage	O
which	O
manifests	O
as	O
heating	O
in	O
the	O
output	O
devices	O
.	O
</s>
<s>
The	O
phase	O
angle	O
varies	O
most	O
near	O
resonance	B-Application
in	O
moving	O
coil	O
loudspeakers	O
.	O
</s>
<s>
See	O
Power	B-Application
factor	I-Application
for	O
more	O
detail	O
.	O
</s>
<s>
So	O
much	O
so	O
,	O
that	O
differences	O
in	O
practice	O
between	O
a	O
16-ohm	O
nominal	O
impedance	O
driver	B-Device
and	O
a	O
4-ohm	O
nominal	O
impedance	O
driver	B-Device
have	O
not	O
been	O
important	O
enough	O
to	O
adjust	O
for	O
.	O
</s>
<s>
The	O
amplifier	O
damping	O
factor	O
,	O
which	O
is	O
the	O
ratio	O
of	O
the	O
nominal	O
load	O
impedance	O
(	O
driver	B-Device
voice	B-Device
coil	I-Device
)	O
to	O
amplifier	O
output	O
impedance	O
,	O
is	O
adequate	O
in	O
either	O
case	O
for	O
well-designed	O
solid	O
state	O
amplifiers	O
.	O
</s>
<s>
Tube	O
amplifiers	O
have	O
sufficiently	O
higher	O
output	O
impedances	O
that	O
they	O
normally	O
included	O
multi-tap	O
output	O
transformers	B-Algorithm
to	O
better	O
match	O
to	O
the	O
driver	B-Device
impedance	O
.	O
</s>
<s>
This	O
is	O
significant	O
since	O
the	O
ratio	O
between	O
the	O
loudspeaker	O
impedance	O
and	O
the	O
amplifier	O
's	O
impedance	O
at	O
a	O
particular	O
frequency	O
provides	O
damping	O
(	O
i.e.	O
,	O
energy	O
absorption	O
)	O
for	O
the	O
back	O
EMF	O
generated	O
by	O
a	O
driver	B-Device
.	O
</s>
<s>
In	O
practice	O
,	O
this	O
is	O
important	O
to	O
prevent	O
ringing	O
or	O
overhang	O
which	O
is	O
,	O
essentially	O
,	O
a	O
free	O
vibration	O
of	O
the	O
moving	O
structures	O
in	O
a	O
driver	B-Device
when	O
it	O
is	O
excited	O
(	O
i.e.	O
,	O
driven	O
with	O
a	O
signal	O
)	O
at	O
that	O
frequency	O
.	O
</s>
<s>
A	O
properly	O
adjusted	O
damping	O
factor	O
can	O
control	O
this	O
free	O
vibration	O
of	O
the	O
moving	O
structures	O
and	O
improve	O
the	O
sound	O
of	O
the	O
driver	B-Device
.	O
</s>
