What's the difference between a TL, a ML-TL and a TQWT?

[Bricolo]

I see how TLs work, but how do they differ from ML-TLs and TQWTs?

The ML-TL looks like a classical BR box, what's more in it?

And in terms of sound?

[Timn8ter]

Transmission Line speakers are generally assumed to be using quarter-wavelength standing waves inside the enclosure to produce the bass emanating from the port so they are also referred to as Quarter Wave tubes. ML-TL resembles a bass reflex but is utilizing these standing waves to load the port. A classic TL may appear to be a long pipe with the driver at one end and the other end open. Usually some poly-fil to help damp the driver. A ML-TL may be a box with the driver mounted on the front baffle near the top of the box and the port at the bottom of the box very much like a BR but calculated to use a quarter-wave. This enclosure may use damping material. The TQWT acts as a cross between a horn and a TL. By tapering the pipe the resonances will spread over a wide range of frequencies theortically producing smoother response. The top of the pipe is usually filled with damping material. The results you get will vary depending on the driver used and how well the pipe has been designed. The rest is personal preference. Some think TQWT are honky like some horns but I've heard some very good ones.

[MJK]

Nice response, I think that you have summed it up very well. But I think that you missed one point.

The TQWT acts as a cross between a horn and a TL. By tapering the pipe the resonances will spread over a wide range of frequencies theortically producing smoother response.

Actually, when the line expands like a TQWT, I agree a cross between a horn and a TL, the lowest standing waves increase in frequency and start to bunch up and overlap giving the TQWT more SPL efficiency at the bottom end but TQWT bass will not be as low in frequency as the same length TL. This can also lead to a rippled response.

If you think about it, a 30 Hz TQWT will be much longer then a 30 Hz straight TL. The TQWT's lowest standing quarter wave is at a frequency that is higher then the length would indicate if you used the equation for a straight TL.

f = c/4L

By about the 5th or 6th quarter wavelength standing wave, the equation above becomes more accurate for the TQWT. So the first quarter wavelength mode is higher in frequency then would be expected for a straight TL and each of the following modes are still higher in frequency but not by as much until they converge with the TL frequencies at about the 6th mode.

Boy that was a real babble. Maybe an example will help. I just made this up for demonstration purposes. Suppose we had a TL and TQWT both with a length of 112 inches. The frequencies might look like the following table.

Mode__Straight TL __TQWT (SL>S0)
1/4_____30 Hz______80 Hz
3/4_____90 Hz_____130 Hz
5/4____150 Hz_____175 Hz
7/4____210 Hz_____220 Hz
9/4____270 Hz_____275 Hz
11/4___330 Hz_____330 Hz

The closer spacing of the first three modes in the TQWT causes the peaks to overlap a little and provide a summed bass SPL response that is sligthly higher then would have been achieved with the straight TL. Again the bass will not go as low for this sample TQWT.

The opposite trend will happen if you use a tapered TL (SL<S0).

Adding a port, or mass loading, to a quarter wavelength enclosure allows the length to be shortened for the same fundamental standing wave. This is like adding a lump of clay to the end of a ruler clamped to the counter in your kitchen. When the lump of clay is added, and the length is unchanged, the ruler will vibrate at a lower frequency. The type of vibration (displaced shape) remains the same but the frequency is reduced.

I think I just confused myself. I hope that helps,

[MJK]

Just one more input. If you look at Attachment C to my TL alignment table document, you will see a study showing the response of the tapered, straight, and expanding TL other wise known as the TQWT. I think that the three plots show better what I was stuggling to describe. Sometimes pictures speak much clearer then words.

[Bricolo]

Thank you both for answears.

Martin: so, adding a port lowers the Fp (or keeps it the same with a smaller box). Does it have any disadvantage?

[MJK]

Adding a port to a TL, producing a ML TL design, allows you to shorten the line length which results in a more compact enclosure for the same tuning frequency. This is a real advantage IMO, I am tired of making big enclosures.

[Bricolo]

yes, I realize that it's a real advantage

but I was asking if there was any disadvantage in doing this?

[Illusus]

Would adding a little bracing in an ML TL disrupt the standing wave used to load the port? I find my ML TLs are rock solid (I used 3/4"mdf and 5/8" Baltic Birch laminate) and are fine without bracing but a friend is interested in a pair, he does not want them built as thick as mine. I was thinking to use a few dowels to reduce the resonances large panels may create.

P.S. Thanks for all your help so far mr.King. The congestion I was trying to solve earlier(if you remember) has been diminishing with further driver break in and the use of felt on the inside of the basket legs. I am really impressed with the ML TL design.

[Timn8ter]

Dowels have worked fine for me. Any distruption has to be less than the adverse effect of a vibrating enclosure panel.

[planet10]

Augspurger showed that the "restricted terminus" -- his terminology for an ML -- also improves the low-pass function of the line, meaning less ripple with the same damping or the same ripple with less damping and more terminus output.

dave