I admit that I don't know much about TL's. I'm doing reading and trying to understand them. Several places that I have found list these rules as 2 basic principles that make up a TL. Is that wrong?
Well, it's not that it's wrong, but way overgeneralized. The driver will be placed in an enclosure that still has a large air mass behind it. This will change its resonant frequency (Fs) like it does in a sealed enclosure. To get an idea of what the new Fs is, you need to calculate the sub in a sealed enclosure of equivalent airspace. Winisd Alpha will tell you this value. For some subs, an airspace as small as 2 cubes may give you the Fs of the driver in free-air while others may take massive enclosures.
As for determining cross sectional area of the line, you need to take in quite a few factors before you just pull anything out of your ***.
First, you need to know whether or not you're going to taper the line and by how much. This line taper ratio is very critical and will affect the final sound of the system. Then, you have to know how much stuffing you're going to use (or no stuffing at all). Then, you need to know the resistance factor for your sub which can be found based on the Qts of the driver being used. You will also need the peak value of the shape function, BL, Sd, and Re to calculate the optimal line area. If you take all these values and multiply them and then divide by BL^2, you will have a unitless value that will correspond to the ratio of taper you have chosen for the line. If you choose not to taper, then you multiply this number by the Sd of the driver and you get your final line area. Sometimes it's bigger, sometimes it's smaller.
Stuffing is important to really smooth out the impedance of the line and it's use will actually change the length that you need to get the proper tuning. It also helps break up higher order harmonics of the line tuning and can give you a more pure sound. Stuffing really helps in frequencies much higher than what it typically used for sub systems. In these ranges, you will have some serious issues with peaks and valleys in the frequency response due to standing waves and the line being out of phase with the sub. Stuffing isn't too critical when you're just dealing with sub bass frequencies but it will definitely help with the impedance smoothing.
Another important part of transmission lines is where to put the driver. Sometimes you can get away with putting it right at the closed end of the line and other times, this will hurt your sound. Off-setting the driver helps break up and suppress some of the upper order harmonics that would usually be heard. However, moving the driver further down the line will actually lessen your low frequency extension. It's all a bit of a compromise game.
T-Lines are very interesting and can sound very nice. They can get pretty large when working with larger drivers though and that makes them a little out of place in car audio. It can be done and it can work well but they won't be as at home as they would be in a nice large room. As far as output is concerned though, I think ported enclosures definitely have a one up but t-lines don't have as steep of a roll off after you go below tuning. Power handling is also reduced in a t-line but sensitivity is greatly increased.
Argh.. too much typing... go read for yourselves at
http://www.quarter-wave.com and be prepared to follow some math.
