What is a "transmission line" speaker?


Maybe I'm a dummy, but I don't know what differentiates a transmission line speaker from any other type of cone drive speaker. Please explain.
jphilips
I am not sure, but I think the speaker in the middle of this webpage is a transmission line?

http://www.exoticaudio.org/PAGE12.html
Sean,
Thanks for your elaborate explanation.
Do I understand correctly that a TL speaker can be tuned to a specific listening environment?
Example?
The best speakers I've ever heard were a transmission line design. I'd tell you the company/model but I never see them up for sale and I don't want more competition when that day finally arrives. I hate being "ratty" but I've been waiting for 3 years.
Outstanding post Sean!!

I've been living happily with my Kinetic Audio Labyrinths (http://www.kineticaudio.com/audiophile.com ... those are
my set of speakers in the picture!!). They are similar in
concept to Bud Fried's old IMF designs.

These are true 1/4 wavelength TLs. A 12" woofer with a 17hz
fs at the end of a 9' folded line with a 10" round terminus
on the rear of each speaker. f3 of the woofer when in the
line is 16hz. No subwoofers needed in my room!

The pros and cons of these speakers are as per Sean's post:

1) Big and heavy. 48"(h) x 18"(d) x 15"(w) at 250lbs each.
Pass the "knuckle rap" test with no problems.
2) Relatively inefficient. Woofer has 92db sensitivity
free-air but is lowered to approx 86-87db in the line. I
vertically biamp to maximize current flow.
3) Much easier to place than vented-box speakers since the
terminus does not (or should not in a proper TL design)
radiate any sound. Although not optimal for imaging, I've
got my "Labs" right up against the front wall due to lack
of space (although plenty of space to the sides). No bass
problems (bloating, etc) whatsoever.
4) Bass transients are so good it's scary with no overhang
at all. Reproduction of kick drums at high SPLs is matched
by only a very small number of other loudspeakers.
5) Woofer distortion is extremely low, although the low 70hz
crossover certainly plays a significant part in that. I
think this characteristic more than anything is what
sometimes gets TL bass labeled as "dry" or "sterile."

For the curious, my system (2-channel only):
Kinetic Audio Labyrinth loudspeakers
Wadia 860 CDP
Panasonic SV-3700 DAT (use 860 as playback DAC)
Fanfare FT-1A FM tuner
Sony DVD-V (2-channel output to pre for movies).
NEC S-VHS (output to pre for TV shows).
ARC LS-5mkII preamp
2 ARC D-400mkII amps (vertically biamped)
All cables and ICs DIY: Canare/Neutrik balanced ICs,
Canare/Canare/Neutrik RCA->XLR hybrid IC (for DVD->pre and
SVHS->pre), Canare/Neutrik AES/EBU digital cable
(DAT->CDP), Canare 4S11 speaker cable with ST Connections
dual gold bananas.
Thanks to you all, especially Sean. Another audio mystery solved for me. I am very interested in Meadowlarks for my bedroom system.