It seems to me that a TL is always 4th order. You can certainly damp the pipe output and go from 2nd order with complex poles (Q over .5) to two real poles, but the pole are always there. Secondly, the overal system is more complex, even if the line is well damped. The radiated SPL is the difference between front contribution and rear (okay, the sum of the positive woofer output and the negative rear output). As with the vented box those two contributions are asymptotically aproaching the same level with opposite phase. That is why, even though they are inherently 2nd order individually, there combination is 4th order. Greater line damping may knock down the pipe output to the point where the system degenerates to 2nd order, but at some lower frequency it reverts to 4th order unless the pipe is totally sealed.
David S.
David S.
There seems from what I have read on the net, about how the tl works in a popular conception, that people think that the frequencies where the front radiated and pipe radiated sound are in phase are not resonant phenomenon, but they are.
At these frequencies the impedance at one end is at a minimum and that at the other is a maximum, so that a pressure wave bounces between these mismatches, the energy it looses by radiating being replenished by the driver, if the driver does not continually replenish the energy then the resonance decays exponentially in a way to do with the characteristic geometry of the pipe and whether the driver can be seen as a true stopper for the end.
Up to say 300Hz. a tl might have four such resonances and what you want to do is to damp the top three to an extent that they are a close approach to aperiodic but leave the last one because damping this causes the amount of energy it has to reinforce the front radiation is lessened as you damp it, the problem is a practical one of finding a way of doing this.
Stuffing the line only works partially because there is no sudden difference in damping between the bottom two resonances such that one is aperiodically damped and the other is not.
You could perhaps in theory get closer by providing the pipe with Helmholtz resonators at the top three frequencies but not the last one, but you get a better result by simple providing a resonator for the lowest one, this providing a very effective low pass filter.
Any way you look at the tl you can show that it is not optimum as an enclosure because it is too large and complex to be worthwhile doing.
The only merit I can see is its use in well sealed dwellings that have on average 6db. of room gain bellow around 40Hz. that is average for cold climate situations, a pseudo first order roll off then gives a result that is fairly flat down to a frequency that can be lowered by stuffing the pipe, but here in Australia the typical dwelling has not much room gain to speak of and we need considerably more acoustic power at the low end to sound equally as loud.
You can get the former effect by using a LT and a first order filter and you just need a simple sealed box, and this is a better way of doing it, but admittedly it does not have the mystique that some seem to attribute to the tl.
rcw
At these frequencies the impedance at one end is at a minimum and that at the other is a maximum, so that a pressure wave bounces between these mismatches, the energy it looses by radiating being replenished by the driver, if the driver does not continually replenish the energy then the resonance decays exponentially in a way to do with the characteristic geometry of the pipe and whether the driver can be seen as a true stopper for the end.
Up to say 300Hz. a tl might have four such resonances and what you want to do is to damp the top three to an extent that they are a close approach to aperiodic but leave the last one because damping this causes the amount of energy it has to reinforce the front radiation is lessened as you damp it, the problem is a practical one of finding a way of doing this.
Stuffing the line only works partially because there is no sudden difference in damping between the bottom two resonances such that one is aperiodically damped and the other is not.
You could perhaps in theory get closer by providing the pipe with Helmholtz resonators at the top three frequencies but not the last one, but you get a better result by simple providing a resonator for the lowest one, this providing a very effective low pass filter.
Any way you look at the tl you can show that it is not optimum as an enclosure because it is too large and complex to be worthwhile doing.
The only merit I can see is its use in well sealed dwellings that have on average 6db. of room gain bellow around 40Hz. that is average for cold climate situations, a pseudo first order roll off then gives a result that is fairly flat down to a frequency that can be lowered by stuffing the pipe, but here in Australia the typical dwelling has not much room gain to speak of and we need considerably more acoustic power at the low end to sound equally as loud.
You can get the former effect by using a LT and a first order filter and you just need a simple sealed box, and this is a better way of doing it, but admittedly it does not have the mystique that some seem to attribute to the tl.
rcw
Driver offset, restricted terminus, & taper are other tools you can use to improve the low pass function of the line. Appropriate driver offset can pretty much eliminate the 1st undesirable harmonic leaving only the higher ones to be killed with damping.
Helmholz absorbers have been around for over a decade, particularily championed by the German diy mags.
dave
I agree with u P10. Although the taper spreads the harmonics, it presents a problem. If i wanted to add minimal stuffing at antinodes only, to preserve the fundamental reinforcement as i would in a straight TL, im not sure it would work as well. The helmholtz chamber thing, as P10 says has been done. Visatons Vib130TL kit, which inspired my MJK based TL, has 2 such chambers which are stuffed to damp their notching of (im guesstimating) the F3 and F5.
Actually I, and I'm confident others like Planet10 and Scottmoose, to name a couple, try to take the mystique (and myths) out of TLs. TLs are, after all, one of several types of enclosures whose primary purposes, IMO, are to provide and enhance bass output. The fact that you seem to think they're flawed and not worth pursuing is okay for you, but others shouldn't be discouraged from trying them just because you view them with disdain (unfairly IMO).
Paul
Substitute "typical BR" and the "you" becomes "me". Not found very many that don't benefit from an open cell foam plug (aka a sopisticated sock) in the port.
dave
All that I am doing is pointing out that the tl is a complex and elaborate way of doing something that can be done more simply.
People like the original poster to this thread, who lives in Australia, should be told this, and that in practical terms, especially over here, you can get better results in a simpler way by using a reflex box. This has the advantage over here of considerably more acoustic output at low frequencies and typically that's what is needed.
rcw
People like the original poster to this thread, who lives in Australia, should be told this, and that in practical terms, especially over here, you can get better results in a simpler way by using a reflex box. This has the advantage over here of considerably more acoustic output at low frequencies and typically that's what is needed.
rcw
id agree that reflex could offer higher ultimate bass efficiency and damping of cone movement. Id attribute that to the higher Vb/Vas ratio in many TLs. I liken the effect to that of reflex loading a acoustic suspension woofer, but lesser in its degree. If youre playing LOUD then reflex is better, IMO. That said, a damped TL like the Vib130 is attractive, with the 6db roll off even if it starts at around 80hz.
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Everything is a compromise. To attain this a BR is typically tightly tuned. To do that you choose a set of T/S parameters from the T/S curves, As soon as you change the volume or even within the dynamics of the music, the tuning goes off. The only exception is with drivers that have horizontal T/S curves. Adding an R to the vent -- like the damping in a TL does, helps make the box more tolerent to the changing driver parameters,
dave
No mystique here, as I know to be the case with both Paul & Dave also. I make no apologies for regarding the many types of QW loading as perfectly acceptable and useful forms of cabinet configuration, and I will continue to use them as and when they may be appropriate.
I do find it interesting that all QW boxes should be casually written off as 'too large and complex' though. I'll remember that when I next design an MLTL, with zero internal panels, or a single fold, high taper ratio pipe, which typically requires a single internal panel.
I do find it interesting that all QW boxes should be casually written off as 'too large and complex' though. I'll remember that when I next design an MLTL, with zero internal panels, or a single fold, high taper ratio pipe, which typically requires a single internal panel.
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As pointed out by Thiele in his original paper, and later by Keele the major thing that varies in practical drivers is compliance.
Keele stated that production examples of very well made tightly toleranced drivers, such as those made in those days by such as Altec and JBL varied by around 20% but showed that this makes practically no difference to measured performance, in fact Thiele observed that compliance variation does not really matter at all.
This can be shown by the fact that you can compliance scale just about any driver to just about any alignment, and in fact the tuning is far from critical in most instances.
I have not seen any research into the effect of compliance variation in transmission line enclosures, but I doubt if they are more tolerant to compliance variation than are bass reflex ones.
rcw
Keele stated that production examples of very well made tightly toleranced drivers, such as those made in those days by such as Altec and JBL varied by around 20% but showed that this makes practically no difference to measured performance, in fact Thiele observed that compliance variation does not really matter at all.
This can be shown by the fact that you can compliance scale just about any driver to just about any alignment, and in fact the tuning is far from critical in most instances.
I have not seen any research into the effect of compliance variation in transmission line enclosures, but I doubt if they are more tolerant to compliance variation than are bass reflex ones.
rcw
Scott, I agree with you, my TQWT is foldless 20:1 taper, 1m tall 20 x 30cm footprint. Thats small, similar to small stand mounted monitors. Internal volume 27 litres. A similar max flat comes out at 13 litres with a port too small for my liking. The TL has double Vb and vent area. I could grossly mistune a reflex, double the Vb and vent area, maintaining vent length constant. What then do I have? A reflex right?
I don't know about that, but ask the guy who is trying to reconcile the differences in box alignment between the factory specs and what he measured on his computer. 2 valid sets of data, 2 very different boxes.
dave
Actually, most production lot variations in drivers are self-compensating, like the ratio fs/Qts (or its reciprocal) and [Vas x (fs squared)], and TLs have no trouble whatsoever in ignoring those variations IME; the drivers are "identical" in that they give virtually the same predicted performance in a TL.
Paul
Paul
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