I'm trying to build my own speakers using off the shelf drivers, and after hearing several transmission line speakers have decided that is the direction I absolutely want to pursue. I'm fairly competent at electrical and mechanical calculations, but I have to admit the math (and the lack of quality literature) is pushing my abilities.
What I would appreciate is a simple gut check; whereas most TL designs are two way with 5.5-6.5 inch midbass drivers, I was hoping to use an 8" driver in the TL with a mid dome I already have my hands on. My concern would be focusing too much reinforcement on low bass (30-50Hz) at the expense of the midbass. I like bass, but I don't need a system that sounds like it's all subwoofer.
Basically, if I tune the TL to a quarter wavelength of the driver (34Hz) would this focus too much energy too low?
Additionally, someone posted fairly nice measurements of a TL and it showed frequencies of up to 400Hz coming out of the line (fully stuffed) at less than -3dB. At that point you've gone around multiple 360 degree circles of phase, wouldn't you get fairly awful cancellations/peaks if this is the case?
I think you have some misconceptions.
First you need to understand there are a whole variety of deigns under the TL banner.
However I'm pretty hair shirt as to what I regard as a TL in the traditional sense.
So here I'm going to discuss the reverse tapered aperiodically damped line. This is what PCM build, although in third part test they appear to be under damped, as there are still two peaks of impedance.
The next issue is that you need to pay attention to the all the Thiel/Small parameters. The whole object in my view is to build an essentially non resonant system, with bass augmentation, ideally over about 1.5 octaves or so. A line with output at the port at 400 Hz only -3 db would not be a properly designed line. The object is to have system Qt to be in the 0.5 range.
Now the total system Q can never be less than the driver Qts in any speaker design. The bass cut of frequency will never be below the free air resonance of the driver, Fs. I think a driver with a Qts of around 0.3 to 0.35 is the sweet spot for TLs.
Now after the work of George L. Augspurger, we know that we have to also calculate line length and volume. You can't pull the pipe parameters and especially the pipe tuning frequency out of the air, just like you can't with a vented reflex system.
The dimensions of your pipe are going to be determined by the T/S parameters of the driver.
So you need to calculate, pipe length, Lp. pipe frequency Fp, and pipe volume Vp.
Having got to that point, you need to determine taper. I find a 3:1 taper works well. You do need to fold the pipe properly, or you will have high output of mid range frequencies. At the turns you need to keep the volume linear, with corner pieces.
You place the driver/ drivers about 1/3 the distance from the closed end to suppress the odd harmonics. (A closed pipe only generates odd harmonics).
Without damping the line will have two peaks of impedance, just like a reflex box. You add damping uniformly throughout the line, but staying a little short of the open end, until you only see one peak of impedance on measurement. Do not damp past that point.
I would recommend you read the three papers by George Augspurger before designing your line. He also has a disc for modelling the line for purchase.
You can see my TL systems via my signature. This will give you an idea of typical line geometry.
I hope this helps.
