So, why is it exactly that 1/4 wave transmission lines only seem to work in large boxes?

[dfuller]

Is this just a limitation of the loading, or is it more about most commercial designs being inadequately engineered? I know most smaller ones tend to have fairly poor extension, but I see no reason it shouldn't be possible with enough folding and/or tapering to get an appreciably low box tuning out of one. So what am I missing?

 

[sq225917]

You're not missing anything, it's just physics, you can only go so low with a smaller box as it means shorter or thinner line, shorter equals higher frequency and go too thin and it stops being a transmission line and just becomes a long resistive leaky port.

 

[voodooless]

It can be done, but it’s not that easy. I’ve made a long transmission line for a Vifa wideband speakers years ago that reached down to about 50 Hz in a small bookshelf. It took quite a bit of work to get that correctly simulated, and shoving all of those folds in there.

Obviously, this thing can’t go very loud, and the bass output efficiency isn’t that good. It sounded quite good though, amazing bass for such a small speaker.

 

[DanielT]

Then there is the question of what you gain by having a long, i.e. large TL box compared to other design principles? Among other things, we have this ting with the sensitivity that voodooless mentions. It has been pointed out by others:

Here is an example:

The short answer is that transmission lines should be avoided as far as possible. The cons outweigh the pros. For bass purposes, it is practically a matter of quarter-wave pipes (since it is not possible to create a reflection-free termination) and here both closed boxes and bass reflex dito win by far. Quarter wave pipes are simply hopeless in speaker context and should only be used in musical instruments - in organs they are fine.

Only when you have a large area and very low moving mass (band or electrostatic element), and the membrane thus becomes very sensitive to reflections on the back, can there be reason to use a transmission line.

SEAS THOR (a TL design) which is apparently considered "legendarily successful" may serve as a comparison object. Below is what happens when you start from the same cavity volume and element drivers as in THOR and use bass reflex, closed box and quarter wave pipe.

SEAS THOR:


Then the fact that they need to be damped so much to achieve good FR, which can suffer for their efficiency:

It is about optimizing the design in terms of frequency response, voltage sensitivity/efficiency and distortion within the framework of the driver cost, cavity volume, etc. Here the SEAS THOR leaves a lot to be desired, the bass reflex solution completely surpasses it on all parameters.

If I e.g. construct a speaker with a volume of 300 liters and a high-class 15" bass driver that gets a reasonably constant frequency response down to 20 Hz, but with a sensitivity of 80 dB, 2.83 V 1 m half-space, I have a sub-optimized construction.
(mostly translated via google translate so I may have missed some nuances)

faktiskt.io • Visa tråd - Transmission Line högtalare

 

[kemmler3D]

Is this just a limitation of the loading, or is it more about most commercial designs being inadequately engineered? I know most smaller ones tend to have fairly poor extension, but I see no reason it shouldn't be possible with enough folding and/or tapering to get an appreciably low box tuning out of one. So what am I missing?

Aside from the acoustic issues, I think it's a problem mostly with construction. You can make a really long TL in a small box with lots of folds, as long as you aren't worried about viscous losses. It's hard to build something like this out of wood, but people have 3D printed them, because with 3D printing the effort to build a crazy shape is the same as a simple one. Not sure how successful they are acoustically, though.

https://www.reddit.com/r/3Dprinting/comments/wj7dvn/i_made_a_transmission_line_bluetooth_speaker

3D Printing Transmission Line Speakers

Anyone who has played with speakers on the workbench knows the huge difference enclosure design makes to the frequency response of an audio system. Speakerheads spend hours tinkering with designs a…

hackaday.com

Google Image Result for https://cdn.review-images.pimoroni.com/upload-2875a11c54c19dc2d59ea6ed4ee26073-1608137927.JPG?width=400&optimizer=image

 

[DanielT]

Or long enough with the right diameter bendable plastic drainage pipe.

Curled in a box with sand around the pipe for stability. Maybe a terrible idea, but it wouldn't cost a lot to test it. But, good luck fixing damping material appropriately in that drainage pipe.

Such a drainage pipe TL should probably be seen mostly as an acoustic experiment.

 

[Geert]

PMC is probably the most famous manufacturer of TL speakers, and they also use it successfully in small near field monitors. As mentioned before, they dampen the whole transmission line.

https://pmc-speakers.com/technology/atl/

We used to have a pair in the studio, next to ATC's. PMC's for their low end, and the ATC's for their mid range qualities.

 

[dfuller]

Then there is the question of what you gain by having a long, i.e. large TL box compared to other design principles? Among other things, we have this ting with the sensitivity that voodooless mentions. It has been pointed out by others:

Here is an example:

The short answer is that transmission lines should be avoided as far as possible. The cons outweigh the pros. For bass purposes, it is practically a matter of quarter-wave pipes (since it is not possible to create a reflection-free termination) and here both closed boxes and bass reflex dito win by far. Quarter wave pipes are simply hopeless in speaker context and should only be used in musical instruments - in organs they are fine.

Only when you have a large area and very low moving mass (band or electrostatic element), and the membrane thus becomes very sensitive to reflections on the back, can there be reason to use a transmission line.

SEAS THOR (a TL design) which is apparently considered "legendarily successful" may serve as a comparison object. Below is what happens when you start from the same cavity volume and element drivers as in THOR and use bass reflex, closed box and quarter wave pipe.

View attachment 410415

SEAS THOR:
View attachment 410416View attachment 410414

Then the fact that they need to be damped so much to achieve good FR, which can suffer for their efficiency:

It is about optimizing the design in terms of frequency response, voltage sensitivity/efficiency and distortion within the framework of the driver cost, cavity volume, etc. Here the SEAS THOR leaves a lot to be desired, the bass reflex solution completely surpasses it on all parameters.

If I e.g. construct a speaker with a volume of 300 liters and a high-class 15" bass driver that gets a reasonably constant frequency response down to 20 Hz, but with a sensitivity of 80 dB, 2.83 V 1 m half-space, I have a sub-optimized construction.
(mostly translated via google translate so I may have missed some nuances)

faktiskt.io • Visa tråd - Transmission Line högtalare

See, the thing is - I'd consider THOR to be a small box, all things considered. It's not, not really, but compared to the big TL loaded boxes out there, it's not very large. A 2m-ish TL length makes for a 1/4 WL of ~43hz. The big ones are closer to 3m or so.

PMC is probably the most famous manufacturer of TL speakers, and they also use it successfully in small near field monitors. As mentioned before, they dampen the whole transmission line.

I'm not sure I'd call anything PMC has done with small box TLs successful...

 

[Geert]

I'm not sure I'd call anything PMC has done with small box TLs successful...

In the context of the question in the OP, they did for sure. The PMC was a third the size of the ATC and it blew the ATC out of the water in the low end. The result was that they both had to go. The ATC because it became clear their low end wasn't representative, and the PMC's because the low end was the only positive about them. If your comment refers to the latter, I understand.

 

[DanielT]

See, the thing is - I'd consider THOR to be a small box, all things considered. It's not, not really, but compared to the big TL loaded boxes out there, it's not very large. A 2m-ish TL length makes for a 1/4 WL of ~43hz. The big ones are closer to 3m or so.


I'm not sure I'd call anything PMC has done with small box TLs successful...

Bigger boxes then, which leads to the last quote in my post #4:
If I e.g. construct a speaker with a volume of 300 liters and a high-class 15" bass driver that gets a reasonably constant frequency response down to 20 Hz, but with a sensitivity of 80 dB, 2.83 V 1 m half-space, I have a sub-optimized construction.

However, I suspect that there is a belief that it is a question of a qualitatively different type of bass that you get from a TL compared to sealed or bass reflex. That a TL gives a deeper, faster, airier, firmer or whatever you can think of type of bass. But if you believe that, what do you base it on?

Why would a TL vs a well constructed bass reflex bass box/subwoofer, given they have the same physical size, give a better bass performance, bass sound?

 

[MaxwellsEq]

Bigger boxes then, which leads to the last quote in my post #4:
If I e.g. construct a speaker with a volume of 300 liters and a high-class 15" bass driver that gets a reasonably constant frequency response down to 20 Hz, but with a sensitivity of 80 dB, 2.83 V 1 m half-space, I have a sub-optimized construction.

However, I suspect that there is a belief that it is a question of a qualitatively different type of bass that you get from a TL compared to sealed or bass reflex. That a TL gives a deeper, faster, airier, firmer or whatever you can think of type of bass. But if you believe that, what do you base it on?

Why would a TL vs a well constructed bass reflex bass box/subwoofer, given they have the same physical size, give a better bass performance, bass sound?

Is there anything specific about heat dissipation and extraction?

When main monitors are driven loud for hours on end, the magnets are sometimes too hot to touch (personal experience). Smaller sealed boxes are obviously a limit at one extreme, but TL and reflex may be better?

 

[voodooless]

That a TL gives a deeper, faster, airier, firmer or whatever you can think of type of bass. But if you believe that, what do you base it on?

I think this stems mostly for the type of drivers used back in the day. They weren’t very good for deep reflex tuning, so a TL could actually perform better. Nowadays, creating deep bass with modern drivers is a relative breeze, so the need for transmission lines dwindled.