There are MANY variations on what most people call a "transmission line" nowadays. To try and make things simple, the sound coming out of the back of a woofer is typically loaded into the speaker box. Directly behind the woofer is a chamber that feeds into what one might think looks something like a maze or "labyrinth". At the end of the labyrinth, one would have an opening in the box. By varying the overall length, width, taper and density ( achieved by various amounts of "acoustic stuffing" ) of the labyrinth along with the size of the opening, you can effectively tune the output of the box and the velocity of sound exiting it.
As a side note, ALL vent's of ANY design have a point of maximum flow and will operate at various levels of efficiency above and below that point. As such, one really needs to calculate the amount of airflow required to achieve the desired listening levels and then tune their vents with that in mind. There are newer ports on the market with flared ends which help to broaden the efficiency range of their operation, making the vent less susceptible to over and under flow conditions.
The end result of all of the work that goes into a TL is a vented speaker that does not suffer from most of the side effects of "normal" i.e. ported or passive radiator designs. However, TL's do introduce side effects of their own into the system. Compared to most vented speakers, transmission line bass is typically much cleaner with better attack, definition and less ringing. It sounds very similar to that of a good sealed box woofer with a low Q. On top of that, the amplifier can load into the driver much easier and control it with less effort compared to most vented speakers due to the very noticeably smooth impedance curve. Most vented designs have one very huge impedance peak or two lesser ( but still quite high ) impedance peaks whereas TL's remain VERY smooth and even in impedance, even at resonance. The high impedance peaks of most vented designs are what make the woofers uncontrollable, resulting in the typical "bloat" that most vented designs suffer from. After all, how much power can most amps deliver into a load that is somewhere between 40 - 90 ohms* ??? Obviously, not much and that is why most vented designs tend to ring and lack definition. Most sealed and TL designs keep their impedance peak at the point of resonance somewhere at or below 20 ohms. That is, if they are designed properly. TL's typically are slightly better ( read as "lower" ) in terms of impedance at resonance than most sealed designs. I have seen TL's that never varied below 6 ohms or above 10 ohms across their entire operating range. Obviously, this makes for a VERY nice and controllable load for most amps.
Due to the fact that the driver does not suffer from severe oscillation at resonance, this gives the TL a slightly slower roll-off rate as compared to a sealed box and what is MUCH better than that of a comparable ported or passive radiator design. Most people tend to hear and think of this as very good "extension" of low frequencies. The bass fundamentals are still reproduced pretty solidly, but maybe not at the greatest levels. Distortion is also typically pretty low IF the labyrinth or "line" is properly tuned and designed.
Another "benefit" of a typical TL design is that, due to having to build the "tunnel" within the speaker cabinet, the cabinet has a lot of bracing that most other cabinets would never receive. The effect is that of having a very sturdy cabinet with minimal resonances. The drawback is that you also have all of that wood inside the box, so they are typically pretty heavy and more costly to produce compared to most other speakers of similar size.
The one major drawback to a TL design is that they are typically LESS efficient than a sealed design. Venting a woofer typically increases the output by 2-3 dB's over the same woofer in a sealed box and that is why most manufacturers do it. Taking that one step backwards, TL's might end up being 4-6 dB's LESS efficient than if the same woofer was used in a box with a port or passive radiator. As such, TL's typically require quite a bit of power to play loud. However, due to their very clean and extended bass ( if using a good sized driver and a well tuned cabinet ), one can listen at slightly lower levels than what one is accustomed to and still experience what one would call a very "full" and powerful sound.
The reason that you don't see a lot of TL's floating around is that they are MUCH harder to design, require a lot of fine tuning on an individual basis, are hard to mass produce and cost more to build and ship due to the greater amount of materials and weight. As such, they have found greater support from DIYer's that are willing to put forth the effort and cost that it takes to one of these designs "right".
Hope this gave you a good basic run-down on TL's. They are worth checking into if your prefer quality over quantity. If you like sealed designs, you'll probably like TL's too. They might be TOO tight, clean or "dry" if you are a big fan of "normal" vented designs. As with anything though, you can find TL's that will sound more like a vented design than that a sealed design. If that is what floats your boat, so be it : ) Sean
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