Transmission line cab

[helmn]

Has anyone tried building a transmission line cab?
I have started, and being that the maths involved is really hard, I got a bit confused but think i'm in the right place.
Having no idea how it's going to sound I was curious of anyone elses' experience.
I can post photos if anyone is interested

[Mr. Foxen]

No good for moving around, the length of the line means lots of wood, and lots of wood means heavy. Sound is down to the physics of the specific cab and speaker, rather than a product of being transmission line. Bear in mind many bass cabs labelled as such aren't actually transmission line but just standard tuned ported cabs, with long ports.

[Bill Fitzmaurice]

A useful TL would have to be at least 2 meters long, while low Qes/Qts pro-sound drivers don't work well in TLs. All in all they're just not a good alignment for electric bass, as they don't work any better than much smaller and easier to construct bass reflex.

[helmn]

Both fair points thanks.
It is more an experiment than something I will be gigging!
Just curious to see what it sounds like, my line works out at about 2.5 metres

[LawrenceH]

I'd be very interested to see what you've come up with, are you using software to assist in this? A strict transmission line is a bit impractical as a bass cab, but a hybrid 'mass-loaded transmission line' would be fun to explore if you feel so inclined. Others will disagree.
My own feeling is that although the empirical descriptions are different for each, a bass reflex, transmission line and horn all behave according to the same physics and a good mathematical model describes all of them successfully - a geometry that's somewhere in between will share some of the features of each approach, so its up to you to decide whether the advantages outweigh the disadvantages for your application/preferences.
Subjectively, TLs can sound very nice for some reason!

[Beer of the Bass]

I've seen it mentioned that some of the existing cabs labelled as transmission lines (like Euphonic Audio) are too short to be true transmission lines and behave more like ported cabs. What does the port stuffing do in these cabs? Subjectively, the cabs "work" but it would be interesting to know what's going on with this design.

[Mr. Foxen]

Stuffing the port in a reflex cab messes with the airflow, which you want as smooth as possible really. I'd guess it drops some midrange reflecting out the port, but really that should be dealt with elsewhere. Stuffing will mess with the port tuning, I'd think that is too inconsistent to be beneficial, its all about mass and airflow.

[Bill Fitzmaurice]

[quote name='Beer of the Bass' timestamp='1370956913' post='2107906']
I've seen it mentioned that some of the existing cabs labelled as transmission lines (like Euphonic Audio) are too short to be true transmission lines and behave more like ported cabs. What does the port stuffing do in these cabs? Subjectively, the cabs "work" but it would be interesting to know what's going on with this design.
[/quote]A true 1/4 wavelength long TL is a resonant pipe. The primary resonance at the 1/4 wavelength frequency is beneficial, being added to the driver front wave radiation. There are also higher frequency resonances at the harmonics of the 1/4 wavelength frequency, and those create undesireable response peaks and dips. The line should be stuffed with sufficient damping to remove the harmonic resonances while leaving the fundamental resonance unaffected.
As for the EA, its operation would be identified by an impedance sweep, but I've never seen one.

[helmn]

Having heard transmission lines in hifi there is something about the sound that I like
Whether that will come through in a bass cab I will find out but like much audio there's trial and error and personal taste
I'm not using software just rough equations.
I will say for anyone who wants to gig one it's getting pretty heavy!
I am probably going to line rather than stuff the TL

[Bill Fitzmaurice]

[quote name='helmn' timestamp='1370974349' post='2108193']
I am probably going to line rather than stuff the TL
[/quote]That's not a choice you can make arbitrarily. You must measure the response and/or the impedance and use as much damping as is required to suppress the harmonics without over damping. If you don't have the required gear to do so there's no way to get it right.

[helmn]

I know it's not going to be perfect by a long shot, hence why I say trial and error. This isn't too serious and in no way a measured experiment!
I don't think this will really help anyone else who has thought about this but I have put it together and will give a quick review of it:
Pros;
'Chest thump' at low volumes
No harshness
Good 'low B' response from a 12" speaker
Not as heavy as I thought it would be
Response is fairly even

Cons;
Not enough top end for some people
Colouration overwhelming at higher volumes

Overall;
I like it, capable of giving a good tonal range and I like feeling the bass without having to turn it up
Not sure if I would gig it but i'm happy. If someone was going to do it properly then I don't think they would be making the last cab they will ever use but it has it's niche!

[helmn]

And as an extra bonus my fretless sounds really double bassy through it!

[Balcro]

Hello helmn,

As a matter of interest, which loudspeaker drive unit did you use for the project?

Balcro.

Edited June 12, 2013 by Balcro

[helmn]

hi
It's a Gallien Krueger replacement 12" 200w 4ohms running off an Ashdown MAG 250 head
They often use a horn type position in hifi speakers, so it is set in the cabinet with a heavy bevel to the opening

[LawrenceH]

Do you think the coloration is the inherent response of your design, or is it cabinet mechanical resonance at higher power levels (ie could stiffer material/bracing fix it)?

[helmn]

It is a dry fit a present so I think the higher volumes are doing odd things to the air flow. But in general TLs aren't used that often in hifi because of the inherent colouration and tend to be efficient at lower volumes so I guess maybe it's going to go funny in a stage situation regardless. I think it would make a great studio/home cab but isn't going to rival traditional designs for stage use really. It has a great character and the driver sound isn't lost in there but as any port will give colouration to the sound, it is just emphasised with the TL. I apologise for what many people will consider waffle but this really is a listening test rather than a measured response!

[malcolm.mcintyre]

I believe they need a lot of amplifier damping, like a big powerful Crown.

[Bill Fitzmaurice]

[quote name='malcolm.mcintyre' timestamp='1371155047' post='2110714']
I believe they need a lot of amplifier damping, like a big powerful Crown.
[/quote]No moreso than any other cab. They were developed by Voigt/Bailey/Bradbury in the days of valves.