transmission line help

Shallenn,
The basic math is trivial: Make the length of the line equal to 1/4 (some people use 1/8) of the wavelength of the driver's free air resonance. The end of the tube nearest the driver will be somewhat larger than the surface area of the cone, the port at the far end no smaller than the area of the cone. Stuff the line with loose wool, polyester, etc.
Done.
Okay, now for the fine tuning.
The stuffing will slow down the velocity of the sound waves travelling down the line. As a consequence, the line can be slightly shorter than the raw numbers would indicate. Most people use a factor of .9 or thereabouts. Since the line will be longer then most folks would want in their listening room, you'll probably want to fold it. Treat each fold as though you were directing a light beam, i.e. put diagonal reflectors in the corners to ease the pressure wave around the turn. Remember not to constrict the cross section of the line in the turn.
There are programs to do all this for you, but the math is such that you can do it in just a minute or two with a pocket calculator; it would take longer to download some fancy program than to do the math yourself.
Transmission lines are far more forgiving than reflex or sealed enclosures. If you end up with a line somewhat longer or shorter than optimum, or if the stuffing is more or less dense, you'll still get excellent results.
Transmission lines are not efficient, but they are capable of stunning sound quality, and are the only enclosure design that will actually lower the resonant point of the driver itself, giving deeper bass than the same driver would give in another type of enclosure.

Grey
 
Some one can tell me I am nuts here...

But I have always thought of horns vs TL as doing the same things, but one is from the back and the other is from the front.

Now of course, what pros are doing with sound reinforcement gear is changing the picture... take a look at Meyers Lab web site. I wonder how much of that "LARGE" scale work applies to our small rooms?
 
Dan,
I'd like to note that speaker enclosure design is as loaded a minefield as, say, tubes vs. transistors. People get *emotional* about their personal favorites. So far, we haven't really had any conflicts of that nature here--but I assume that it's only a matter of time. Let's hope that this isn't the thread that starts the head-butting.
The one thing that horns have that no one can take away is efficiency. The theoretical efficiency of a horn approaches 50%, which is an entire order of magnitude better than even a reflex cabinet. But...there ain't no such thing as a free lunch. The efficiency comes with several penalties. The primary one is distortion. Horns are good for about two octaves before the various distortion mechanisms begin digging in their heels. As a practical matter, commercial designs often go for three octaves or more. The distortion manifests itself as a 'honking' quality to the sound; in severe cases, you might almost liken it to the sound of someone speaking through a length of pipe, almost echoic in nature.
Another problem is that it's difficult to time-align a horn. Historically, the problem first reared its head in movie theaters when a scene appeared in which someone was tap-dancing. The speakers of the day made no attempt to time-align, and it sounded as though there were two dancers when in fact there was only one. The solution was simple: move the drivers carrying the upper frequencies back until they lined up with the bass unit. The same strategy works today, but since the speakers aren't hiding behind a screen, you're faced with some pretty odd looking enclosures that are going to be prone to edge diffraction due to all the gymnastics you had to go through to get the drivers time-aligned.
Then there's the size...but in this case the transmission line suffers from the same fault, so that levels the playing field.
I like the efficiency of horns--who wouldn't--but the price you pay has always seemed to me to be too high, particularly in the sound quality category.
Transmission lines are roughly in the same category as reflex and sealed enclosures as far as efficiency goes, i.e. low to mid-single digit percentages. However, I've never seen a distortion mechanism postulated for TLs that's inherent in the speaker design. In fact, they tend in some ways, due to the fact that they "lose" the rear wave from the driver, to approach the "cabinetless" ideal that people seek. There's no pressurized cabinet behind the driver, which makes the reflex people happy, and there's no port coupled to a resonant chamber (in the conventional sense) to give boomy bass, so the sealed folks are happy. Now, note that the rear wave does reinforce the radiation from the front of the driver, so in that way they are somewhat like reflex speakers, but they don't do so by having a resonant cavity. In fact, if you've got a resonance in your TL, you did something wrong.
Yes, you can clearly see my bias here. I see horns as ideal for PA systems and such, and have played bass through a number of variants over the years. However, I've yet to hear one that I thought qualified as high end in sound quality. (Caveat: I haven't heard some of the newer horns that are all the rage in high end circles. Could they change my mind? Possibly, but not probably.) Transmission lines are, in my opinion, more suited for high fidelity use. The disclaimer I need to make here is that my current woofers are planar; my subs are sealed. I do not currently have a pair of TLs in the house, but my memories of the ones I've owned and heard in shops are fond.
Sawzall,
Horns and transmission lines are different beasts, and work according to different principles. A horn is best thought of as a transformer that couples the diaphragm to the room, thus performing impedance matching: A little bit of air moving at really high velocity gets swapped for a lot of air moving at lower velocity. A transmission line is better thought of as a resistor (or perhaps inductor) that attenuates the sound coming off the driver. No impedance matching is involved. Note that either horns or transmission lines may come off of the front or back of the driver; makes no difference. In the case of horns, you can horn load both the front and back of the driver if you can cope with the out of phase aspect (some people horn load the front, loading the rear into a reflex, then mount a horn on the port for the reflex). With a transmission line, you'll only be doing one side.

Grey