<s>
A	O
transmission	B-Architecture
line	I-Architecture
loudspeaker	I-Architecture
is	O
a	O
loudspeaker	B-Device
enclosure	I-Device
design	O
which	O
uses	O
the	O
topology	B-Architecture
of	O
an	O
acoustic	B-Architecture
transmission	I-Architecture
line	I-Architecture
within	O
the	O
cabinet	O
,	O
compared	O
to	O
the	O
simpler	O
enclosures	O
used	O
by	O
sealed	O
(	O
closed	O
)	O
or	O
ported	O
(	O
bass	O
reflex	O
)	O
designs	O
.	O
</s>
<s>
Instead	O
of	O
reverberating	O
in	O
a	O
fairly	O
simple	O
damped	O
enclosure	O
,	O
sound	O
from	O
the	O
back	O
of	O
the	O
bass	B-Device
speaker	I-Device
is	O
directed	O
into	O
a	O
long	O
(	O
generally	O
folded	O
)	O
damped	O
pathway	O
within	O
the	O
speaker	B-Device
enclosure	I-Device
,	O
which	O
allows	O
far	O
greater	O
control	O
and	O
use	O
of	O
speaker	O
energy	O
and	O
the	O
resulting	O
sound	O
.	O
</s>
<s>
Used	O
correctly	O
,	O
such	O
a	O
design	O
ensures	O
that	O
undesired	O
resonances	B-Application
and	O
energies	O
,	O
which	O
would	O
otherwise	O
cause	O
undesirable	O
auditory	O
effects	O
,	O
are	O
instead	O
selectively	O
absorbed	O
or	O
reduced	O
(	O
"	O
damped	O
"	O
)	O
due	O
to	O
the	O
effects	O
of	O
the	O
duct	O
,	O
or	O
alternatively	O
only	O
emerge	O
from	O
the	O
open	O
end	O
in	O
phase	O
with	O
the	O
sound	O
radiated	O
from	O
the	O
front	O
of	O
the	O
driver	O
,	O
enhancing	O
the	O
output	O
level	O
(	O
"	O
sensitivity	O
"	O
)	O
at	O
low	O
frequencies	O
.	O
</s>
<s>
The	O
transmission	O
line	O
acts	O
as	O
an	O
acoustic	O
waveguide	O
,	O
and	O
the	O
padding	O
both	O
reduces	O
reflection	O
and	O
resonance	B-Application
,	O
and	O
also	O
slows	O
the	O
speed	O
of	O
sound	O
within	O
the	O
cabinet	O
to	O
allow	O
for	O
better	O
tuning	O
.	O
</s>
<s>
Transmission	B-Architecture
line	I-Architecture
loudspeakers	I-Architecture
designs	O
are	O
more	O
complex	O
to	O
implement	O
,	O
making	O
mass	O
production	O
difficult	O
,	O
but	O
their	O
advantages	O
have	O
led	O
to	O
commercial	O
success	O
for	O
a	O
number	O
of	O
manufacturers	O
such	O
as	O
IMF	O
,	O
TDL	O
,	O
and	O
PMC	O
.	O
</s>
<s>
As	O
a	O
rule	O
,	O
transmission	O
line	O
speakers	O
tend	O
to	O
have	O
exceptionally	O
high	O
fidelity	O
low	O
frequency	O
response	O
far	O
below	O
that	O
of	O
a	O
typical	O
speaker	O
or	O
subwoofer	B-Application
,	O
reaching	O
into	O
the	O
infrasonic	O
range	O
(	O
British	O
company	O
TDL	O
's	O
studio	O
monitor	O
range	O
from	O
the	O
1990s	O
quoted	O
their	O
frequency	O
responses	O
as	O
starting	O
from	O
as	O
low	O
as	O
17Hz	O
depending	O
upon	O
model	O
with	O
a	O
sensitivity	O
of	O
87dB	O
for	O
1W	O
@	O
1meter	O
)	O
,	O
without	O
the	O
need	O
for	O
a	O
separate	O
enclosure	O
or	O
driver	O
.	O
</s>
<s>
A	O
transmission	O
line	O
is	O
used	O
in	O
loudspeaker	O
design	O
to	O
reduce	O
time	O
,	O
phase	O
,	O
and	O
resonance	B-Application
related	O
distortions	O
,	O
and	O
in	O
many	O
designs	O
to	O
gain	O
exceptional	O
bass	O
extension	O
to	O
the	O
lower	O
end	O
of	O
human	O
hearing	O
,	O
and	O
in	O
some	O
cases	O
the	O
near-infrasonic	O
(	O
below	O
20Hz	O
)	O
.	O
</s>
<s>
TDL	O
's	O
1980s	O
reference	O
speaker	O
range	O
(	O
now	O
discontinued	O
)	O
contained	O
models	O
with	O
frequency	O
ranges	O
of	O
20Hz	O
upwards	O
,	O
down	O
to	O
17Hz	O
upwards	O
,	O
without	O
needing	O
a	O
separate	O
subwoofer	B-Application
.	O
</s>
<s>
In	O
practice	O
,	O
the	O
duct	O
is	O
folded	O
inside	O
a	O
conventional	O
shaped	O
cabinet	O
,	O
so	O
that	O
the	O
open	O
end	O
of	O
the	O
duct	O
appears	O
as	O
a	O
vent	O
on	O
the	O
speaker	B-Device
cabinet	I-Device
.	O
</s>
<s>
Essentially	O
,	O
the	O
goal	O
of	O
the	O
transmission	O
line	O
is	O
to	O
minimize	O
acoustical	O
or	O
mechanical	O
impedance	O
at	O
frequencies	O
corresponding	O
to	O
the	O
fundamental	O
free-air	O
resonance	B-Application
of	O
the	O
bass	O
driver	O
.	O
</s>
<s>
Transmission	B-Architecture
line	I-Architecture
loudspeakers	I-Architecture
employ	O
this	O
tube-like	O
resonant	B-Application
cavity	O
,	O
with	O
the	O
length	O
set	O
between	O
1/6	O
and	O
1/2	O
the	O
wavelength	O
of	O
the	O
fundamental	O
resonant	B-Application
frequency	O
of	O
the	O
loudspeaker	O
driver	O
being	O
used	O
.	O
</s>
<s>
Well	O
designed	O
transmission	O
line	O
enclosures	O
have	O
smooth	O
impedance	B-Architecture
curves	I-Architecture
,	O
possibly	O
from	O
a	O
lack	O
of	O
frequency-specific	O
resonances	B-Application
,	O
but	O
can	O
also	O
have	O
low	O
efficiency	O
if	O
poorly	O
designed	O
.	O
</s>
<s>
One	O
of	O
the	O
significant	O
and	O
common	O
problems	O
with	O
a	O
transmission	B-Architecture
line	I-Architecture
loudspeaker	I-Architecture
system	O
is	O
the	O
unwanted	O
phase-cancellation	O
effects	O
of	O
higher	O
line	O
harmonics	O
bleeding	O
from	O
the	O
transmission	O
line	O
and	O
adversely	O
affecting	O
the	O
overall	O
sound	O
field	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
PMC	O
PMC6	O
mid-sized	O
transmission	O
line	O
monitoring	O
loudspeaker	O
,	O
there	O
is	O
a	O
dip	O
around	O
300Hz	O
that	O
is	O
caused	O
by	O
the	O
fifth	O
harmonic	O
of	O
the	O
transmission	O
line	O
’s	O
resonant	B-Application
frequency	O
.	O
</s>
<s>
This	O
type	O
of	O
problem	O
is	O
quite	O
common	O
,	O
and	O
it	O
was	O
readily	O
apparent	O
in	O
other	O
transmission	B-Architecture
line	I-Architecture
loudspeakers	I-Architecture
.	O
</s>
<s>
Another	O
problem	O
is	O
that	O
the	O
sound	O
radiation	O
from	O
the	O
exit	O
of	O
the	O
line	O
is	O
spread	O
over	O
a	O
quite	O
broad	O
frequency	O
range	O
caused	O
by	O
the	O
hump	O
of	O
the	O
quarter-wave	O
transmission	O
line	O
resonance	B-Application
,	O
whereas	O
the	O
high-Q	O
port	O
resonance	B-Application
of	O
a	O
vented-box	O
loudspeaker	O
rolls	O
off	O
much	O
more	O
quickly	O
and	O
extends	O
over	O
a	O
much	O
narrower	O
frequency	O
band	O
.	O
</s>
<s>
These	O
sorts	O
of	O
issues	O
with	O
transmission	B-Architecture
line	I-Architecture
loudspeakers	I-Architecture
can	O
lead	O
to	O
tonal	O
accuracy	O
problems	O
that	O
cannot	O
be	O
resolved	O
.	O
</s>
<s>
One	O
of	O
the	O
exclusive	O
benefits	O
of	O
a	O
TL	O
design	O
is	O
its	O
ability	O
to	O
produce	O
very	O
low	O
frequencies	O
even	O
at	O
low	O
monitoring	O
levels	O
–	O
TL	O
speakers	O
can	O
routinely	O
produce	O
full	O
range	O
sound	O
usually	O
requiring	O
a	O
subwoofer	B-Application
,	O
and	O
do	O
so	O
to	O
very	O
high	O
levels	O
of	O
low-frequency	O
accuracy	O
.	O
</s>
<s>
One	O
PMC	O
employee	O
was	O
quoted	O
as	O
saying	O
that	O
optimising	O
a	O
transmission	B-Architecture
line	I-Architecture
loudspeaker	I-Architecture
is	O
"	O
like	O
juggling	O
water	O
"	O
.	O
</s>
<s>
The	O
concept	O
was	O
innovated	O
within	O
acoustic	O
enclosure	O
design	O
,	O
and	O
originally	O
termed	O
an	O
"	O
acoustical	O
labyrinth	B-Application
"	O
,	O
by	O
acoustic	O
engineer	O
and	O
later	O
Director	O
of	O
Research	O
,	O
Benjamin	O
Olney	O
,	O
who	O
developed	O
the	O
concept	O
at	O
the	O
Stromberg-Carlson	O
Telephone	O
Co	O
.	O
in	O
the	O
early	O
1930s	O
while	O
studying	O
the	O
effect	O
of	O
enclosure	O
shape	O
and	O
size	O
on	O
speaker	O
output	O
,	O
including	O
the	O
effect	O
of	O
"	O
extreme	O
length	O
in	O
a	O
box	O
baffle	O
"	O
.	O
</s>
<s>
A	O
loudspeaker	B-Device
enclosure	I-Device
based	O
on	O
the	O
concept	O
was	O
proposed	O
in	O
October	O
1965	O
by	O
Dr	O
A.R.	O
</s>
<s>
The	O
article	O
postulated	O
that	O
energy	O
from	O
the	O
rear	O
of	O
a	O
driver	O
unit	O
could	O
be	O
essentially	O
absorbed	O
,	O
without	O
damping	O
the	O
cone	O
's	O
motion	O
or	O
superimposing	O
internal	O
reflections	O
and	O
resonance	B-Application
,	O
so	O
Bailey	O
and	O
Radford	O
reasoned	O
that	O
the	O
rear	O
wave	O
could	O
be	O
channelled	O
down	O
a	O
long	O
pipe	O
.	O
</s>
<s>
If	O
the	O
acoustic	O
energy	O
was	O
absorbed	O
,	O
it	O
would	O
not	O
be	O
available	O
to	O
excite	O
resonances	B-Application
.	O
</s>
<s>
Bailey	O
's	O
article	O
in	O
Wireless	O
World	O
,	O
“	O
A	O
Non-resonant	O
Loudspeaker	B-Device
Enclosure	I-Device
Design	O
”	O
,	O
detailing	O
a	O
working	O
Transmission	O
Line	O
.	O
</s>
<s>
Radford	O
Electronics	O
Ltd	O
took	O
up	O
this	O
innovative	O
design	O
and	O
briefly	O
manufactured	O
the	O
first	O
commercial	O
Transmission	B-Architecture
Line	I-Architecture
loudspeaker	I-Architecture
.	O
</s>
<s>
Although	O
acknowledged	O
as	O
the	O
father	O
of	O
the	O
Transmission	O
Line	O
,	O
Bailey	O
's	O
work	O
drew	O
on	O
the	O
work	O
on	O
labyrinth	B-Application
design	O
,	O
dating	O
back	O
as	O
early	O
as	O
the	O
1930s	O
.	O
</s>
<s>
Bailey	O
hit	O
upon	O
the	O
idea	O
of	O
absorbing	O
all	O
the	O
energy	O
generated	O
by	O
the	O
bass	O
unit	O
inside	O
the	O
cabinet	O
,	O
providing	O
an	O
inert	O
platform	O
for	O
the	O
drive	O
unit	O
to	O
work	O
from	O
;	O
unchecked	O
,	O
this	O
energy	O
produces	O
spurious	O
resonances	B-Application
in	O
the	O
cabinet	O
and	O
its	O
structure	O
,	O
adding	O
distortion	O
to	O
the	O
original	O
signal	O
.	O
</s>
<s>
Because	O
the	O
line	O
is	O
operating	O
over	O
several	O
octaves	O
with	O
the	O
drive	O
unit	O
,	O
cone	O
excursion	O
is	O
reduced	O
,	O
providing	O
higher	O
SPLs	O
and	O
lower	O
distortion	O
levels	O
,	O
as	O
compared	O
with	O
bass	O
reflex	O
and	O
infinite	O
baffle	O
loudspeaker	B-Device
enclosure	I-Device
designs	O
.	O
</s>
<s>
The	O
complex	O
loading	O
of	O
the	O
bass	O
drive	O
unit	O
demands	O
specific	O
Thiele-Small	B-Device
driver	I-Device
parameters	I-Device
to	O
realise	O
the	O
full	O
benefits	O
of	O
a	O
TL	O
design	O
.	O
</s>
<s>
This	O
lowers	O
the	O
resonant	B-Application
frequency	O
of	O
the	O
drive	O
unit	O
,	O
negating	O
the	O
need	O
for	O
a	O
highly	O
compliant	O
device	O
.	O
</s>
<s>
Filling	O
half	O
the	O
cross	O
section	O
throughout	O
the	O
line	O
's	O
entire	O
length	O
treats	O
the	O
TL	O
as	O
an	O
infinite	O
baffle	O
,	O
basically	O
damping	O
high	O
frequencies	O
and	O
wall-to-wall	O
resonances	B-Application
.	O
</s>
<s>
One	O
such	O
is	O
to	O
design	O
a	O
closed-box	O
loudspeaker	B-Device
enclosure	I-Device
,	O
then	O
building	O
a	O
transmission	O
line	O
of	O
the	O
same	O
volume	O
tuned	O
to	O
the	O
closed-box	O
loudspeaker	O
's	O
resonance	B-Application
frequency	O
.	O
</s>
<s>
Benjamin	O
Olney	O
–	O
originated	O
the	O
idea	O
of	O
a	O
duct	O
in	O
speaker	B-Device
enclosure	I-Device
design	O
,	O
which	O
he	O
termed	O
an	O
"	O
acoustic	O
labyrinth	B-Application
"	O
,	O
while	O
working	O
for	O
Stromberg-Carlson	O
as	O
an	O
acoustic	O
engineer	O
and	O
studying	O
the	O
effect	O
of	O
enclosure	O
size	O
on	O
output	O
sound	O
.	O
</s>
<s>
USA	O
–	O
New	O
York	O
Acoustics	O
,	O
loosely	O
associated	O
with	O
New	O
York	O
Audio	O
Labs	O
,	O
kits	O
and	O
plans	O
for	O
8	O
"	O
and	O
10	O
"	O
driver	O
TL	O
speaker	B-Device
cabinets	I-Device
,	O
active	O
in	O
the	O
mid	O
1980s	O
.	O
</s>
