IMF ALS30

[walpurgis]

The length of the line is the main consideration. Like reflex cabinets, you can 'cheat' with the bass response by having a port resonance slightly below the driver bass resonance in system. Meaning that the port is excited by output from the driver during its roll-off. Say for instance, the bass driver resonates at 40 Hz and rolls of below that, tuning the port/line a little way into this roll-off, say 35Hz, allows the two to sum and extend the bass output level to remain nearer flat to lower frequencies.

There are loads of books and websites that would explain better than me, take a look.

[Lawrence001]

The length of the line is the main consideration. Like reflex cabinets, you can 'cheat' with the bass response by having a port resonance slightly below the driver bass resonance in system. Meaning that the port is excited by output from the driver during its roll-off. Say for instance, the bass driver resonates at 40 Hz and rolls of below that, tuning the port/line a little way into this roll-off, say 35Hz, allows the two to sum and extend the bass output level to remain nearer flat to lower frequencies.

There are loads of books and websites that would explain better than me, take a look.

Is it really cheating, I thought that was the primary aim of the reflex design. Just semantics of course. The thing I'm really interested in with TLs is whether the line is designed to get rid of unwanted back waves from the woofer, or enhance the bass output, I've read conflicting views. I guess it can do both, plenty of long haired wool to dissipate higher frequency energy but leave the intended LF energy to output from the ports. The other thing I'm interested in is the phase effect of the quarter wave line. Ie if the back wave (180deg out of phase) travels down the line at 330m/s it will emerge 90 or 270deg out of phase at the frequency the 1/4 wave line was defined (presumably the resonant frequency of the bass driver, ignoring the impact of any stuffing) which is surely bad?

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[Macca]

In theory a transmission line should absorb all the rear output of the drivers, in practice this never happens and the low frequencies are used to augment bass response. BuT AFAIK they are in phase with the driver output, at least on the TLS50.

[walpurgis]

Is it really cheating, I thought that was the primary aim of the reflex design. Just semantics of course. The thing I'm really interested in with TLs is whether the line is designed to get rid of unwanted back waves from the woofer, or enhance the bass output, I've read conflicting views. I guess it can do both, plenty of long haired wool to dissipate higher frequency energy but leave the intended LF energy to output from the ports. The other thing I'm interested in is the phase effect of the quarter wave line. Ie if the back wave (180deg out of phase) travels down the line at 330m/s it will emerge 90 or 270deg out of phase at the frequency the 1/4 wave line was defined (presumably the resonant frequency of the bass driver, ignoring the impact of any stuffing) which is surely bad?

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Firstly, a tapered transmission line can be closed at the end away from the drivers and be treated as (hopefully) a total absorbtion system. However using the output from an open line is a neat trick to reinforce and extend bass, so why not use it.

Secondly, only part of the port bass output will be in phase, but the wavelengths at low frequency are so big that the output that is moving out of phase either side of the chosen resonant frequency will not be out by much, so won't affect the bass delivery negatively by causing cancellation.

Compared to a quarter wave line, the proportion of port output that is in phase will be halved in an eighth wave length design or doubled in a half wavelength type. A half wavelength line will totally invert, so the whole wave exiting the port will be in phase with the output from the front of the main driver at the designed frequency.

Most transmission line speakers are quarter wave types like IMF, but the B&W DM2 is an eighth wavelength design.









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[Lawrence001]

You've hit my ultimate point there, why are quarter wave lines the norm when a half wave will give the bass output in phase. Obviously cabinet size is an issue, but why didn't eg. IMF make their biggest speakers half wave?

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[walpurgis]

why didn't eg. IMF make their biggest speakers half wave?

I've not built a half wave box, but I'd imagine the bass output may be rather prodigious and tricky to keep under control with no useful benefit in performance over a quater wave, the extra complexity and bulk would be unwelcome too. I'd bet money that IMF would have experimented with this though.

A good quarter wave speaker can sound amazingly good through the bass. I've heard loads and built a few.

Even small ones deliver excellent sound, here's some info on a development of the seventies Daline design: https://www.blackdahlia.com/html/tip_80.html

The original used a 5" KEF B110 and went down to 30Hz believe it or not.

[Macca]

The original used a 5" KEF B110 and went down to 30Hz believe it or not.

Ah but what are your distortion levels at 30Hz with a tiny driver like that? There's no free lunch.

[walpurgis]

Ah but what are your distortion levels at 30Hz with a tiny driver like that? There's no free lunch.

Can't say offhand, but it was a well regarded design. I may even have the original series of articles that covered the build somewhere.

[Lawrence001]

I just missed one on ebay that went for peanuts a few years ago I was very upset.

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[walpurgis]

Horns are good fun too. There are several types, but the main ones are, constant width backloaded folded horn, quarter wave horn and flare coupled reflex (which is a hybrid). I've also built horns in the past.