Transmission line bass cabs

Discussion in 'Amps and Cabs [BG]' started by Bass Unique, Nov 3, 2011.


  1. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If there is a notch the design was botched. A properly stuffed TLine's terminus output has a very narrow bandwidth, just like a reflex port. The stuffing supresses the harmonics of the pipe frequency, leaving only the Fp output.
    TLines are not higher in sensitivity than reflex, as they're both direct radiators. TLines can have a bit smoother response than reflex, but it's obtained at the expense of a larger, heavier cab and more expensive out the door cost due to the added complexity and material cost. That makes them far more useful in hi-fi situations, where portability is not a concern, than pro-sound.
    Their main advantage is a more tube friendly impedance curve, as when properly done it's the same as that of a sealed cab, while the response is the same as a vented cab. That makes them attractive for users of tube power amps for hi-fi.
     
  2. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    In my experience a stuffing density that will strongly suppress the 1 wavelength antinode will be dense enough to absorb some energy down at 1/4 wavelength, so it's a juggling of tradeoffs. Line geometry also plays a role.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    For sure, though it's not too difficult if you watch the impedance bumps that accompany the pipe harmonics and only add enough stuffing to flatten them out. Like every alignment there are unavoidable compromises.
     
  4. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Yes... and again relying on my limited experience, by the time we have the optimium stuffing to smoothe that impedance curve, we're getting less reinforcement from the terminus than we would with a reflex box. One possible solution there is to start out with a relatively high-Q woofer to begin with, in anticipation of adding a lot of stuffing to the line, but now we're trading off efficiency.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Actually TLines work better with Q values of .5 or higher, very much like sealed cabs. If I was going to do one I'd be looking at the B810 driver in a vertical 4x10, using the cabinet height to its advantage with respect to the necessary line length. If you look at my TLAH I made it a TL as it was over three feet high anyway with an 80Hz passband, so why not take advantage of that length and use it? You could do the same with a vertical 4x10 and get at least a six foot line with only one bend.
     
  6. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I've spent a lot of time on transmission lines and despite their value in low SPL environments such as hi-fi speakers I've yet to find any benefit over ported cabs with modern drivers other than that of novelty! I I do have a TL hi-fi speaker in the works but I'd take with a bucket of salt the dubious claims I've just seen on Wikipedia that I hope the OP will remove... ;)
     
  7. Mike Arnopol

    Mike Arnopol Inactive Supporting Member Commercial User

    Jan 4, 2005
    Chicago
    Owner of MAS Soundworks
    I built my first transmission line (I. M. Fried) when I was sixteen. In absolute terms I love transmission lines. My favorite type of bass response. (my home hifi speakers are transmission lines with electrostatic highs and mids). I've experimented with transmission lines for live sound use.
    That being said, as stated by people much more knowledgeable than me Tl's are not too practical for our usage. Even tuned at 40 hz the line will be pretty long.
    Efficiency is low. The termination of the TL contributes very little sound. The pro sound drivers mentioned aren't designed for TL's. All of the drivers I've seen used for TL's are very low efficiency and high qts.
    TL's need to be crossed over pretty low. So you really need a mid-bass driver along with it.
    The pro sound transmission lines mentioned are not full transmission lines. There is no way I know of to get enough TL length in a manageable box size. Then you add the additional weight of the copious amount of wood needed for the TL and you get a pretty heavy box.
    Too bad, because I love the quality of bass with TL's.
     
  8. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Lots of interesting comments -thanks eveyone.

    Re: the notch at the higher frequency - the stuffing is the key here - ignore the impedance curve for a min. Imagine it was a straight pipe and not flared. You have a high SPL at the woofer cone and low velocity of air movement. At the teminus you have a high air velocity and low SPL (its an advantage to maintain a reasonably large area compared to a reflex port here to maintain efficiency - and at the throat too). If you put stuffing in by the terminus, you kill the low frequency cut off you want to boost. Put stuffing in lightly to not block the channels. However, tease it out and make it even but regular over the first 1/3 to 1/2 of the line only. This will suppress the higher frequency which causes the notch without killing the low end. Don't stuff the last 1/4 at all or it will reduce your bottom end (good if thats what you want though!). lots of room for errors or tweaks here...

    Re: the Q of the driver - there are things you can do with the compression chamber (volume behind the driver) to tailor the box to suit almost any driver (within reason above 0.3 but below 0.6 Qts I would guess). The TL is more tollerant of drivers than reflex or sealed boxes when its done correctly - it does change the output bandwidth slightly but ideally, you want a smooth non-peaky bass response closer to a sealed box than a reflex but with higher efficiency and full bottom - advantages of both types.

    Re the size - my line is 1.6m long - for pro sound this is not very long when you consider that my box is smaller than other comparable boxes. The taper makes the line shorter for a given frequency and also helps kill the notches and standing waves. The volume of my box as a reflex would be 90 litres (approx 3.5 cu foot) before you remove volume for driver, baffles and ports. This would give a lot of low end to my 3012LF driver anyway but have little cone control....

    Re the crossover frequency - mine is low at 250 Hz and this is the reason to use a large mid to handle the essential lower mid region for a bass - this is also set in a closed type transmission line (although I have to admit, this is not long enough - it turns into a damped sealed box at its low end).

    Re the impedance curve - if you do stuff the line sufficiently to flatten the impedance curve (unlikely to be very flat on pro-sound where efficiency matters compared to hi fi) then it DOES NOT look like a sealed box. The sealed box curve usually shows a single narrow peak at the drive box resonance. The reflex has two, one at the port Helmholtz resonance and also the driver box resonance (unless you coincide the two... thats another story). The TL typically still has two like a reflex but the damping will reduce these to two small humps only. (The other reason for peaks is poor box design without good bends - a major source of compromise in even the most prestigious brands!)

    Right last one - the cost and complexity - it does not have to be difficult - I have one 180 degree turn and one 90 degree turn (no stuffing in the last 400 mm to ensure that there is still high output at the first line resonance). Although this is more complex than a reflex box normally is, it is still easy to do if you have an accurate bench saw. The cost would be higher but this is the reason to make the difficult bit (the design) available to the public so the bidding DIY'er can build his(or her) own at low cost knowing it is a proven design. Weight is still low at 22 kgs (although I think I could trim it here and there slightly and shave off another 2 kgs no problem). The wood is relatively thin at 12mm (except the 18mm baffle) but this is ok as there is so much bracing.

    As you can tell - I am a big fan of the design and when you look in to it and build one for yourself, you will see there are audible advantages. I have just returned from a rehearsal and I can already tell that the sound I am producing is less coloured and I have a bigger range of tonality available from my bass. It also has a monstrous bottom end if you want it too - the studio guy plugged in his bass and said he has never heard a speaker sound so flat in response (ie not boomy) with his bass.

    I need to gig with this to provide a report for high level use but it won't be finished for a few weeks. I will however, do a measurement comparison between by Mark bass 410 HR and the TL box. From listening to it, it certainly has more low bass grunt than the MB 410 (not bad considering cone area) although it wont take that much power - depends what you want out of your cab. My MB tube amp wont give it more than 300 watts ever...

    Don't forgeth the MB 410 is around £900 new and most much less than £600 second hand on ebay. I built this cab (including the finish etc not yet fitted) for less than £400 including drivers. If this was mass produced, the materials would also get cheaper.
     
  9. father of fires

    father of fires Commercial User

    Nov 29, 2006
    BALTIMORE CITY
    Chief of Medicine at Damnation Audio
    The engineering prowess in the last several posts is inspiring.
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If it swims like a duck, quacks like a duck, and flies like a duck...it's a duck. A TL has a single impedance peak, at Fp, just like a sealed cab has one at Fb. If your TL has an impedance that looks like a VB it is a VB. That doesn't mean it can't have some TL characteristics, perhaps it's operating as a mass-loaded TL; if your cab has a rear chamber that's probably the case, as a TL doesn't have a rear chamber either. This doesn't invalidate your design, but it does mean that you should call it something other than a TL, as it's actually a hybrid design.
     
  11. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    A "true" transmission line flattens the impedance peaks so much that they do appear as a single entity but much lower Q (mostly due to damping). If you have a large sealed box and completely stuff it, it would look similar (is it a duck or a goose?)

    For pro audio - I admit, it must be a hybrid but whats the problem with that? There have been many designs over the years which are technically hybrids and the best of these still exhibit some of the good characteristics of a TL. If this can be done with no loss of efficiency over a reflex (careful with that stuffing) and a few of the advantages (named above in previous posts) then it is making progress over a vented box.

    Yes it is slightly more complex and room for more error and would be more expensive to make. However, consider now that to obtain quality neo drivers is the dominant financial outlay on a design, why stick them in a poor cab? My idea is to allow proven designs to be provided to the masses so that they can DIY with confidence, keeping cost low and build a cut above the rest cab.

    Obviously, it wont float eveyones boat but for those that believe that there are genuine advantages in a "hybrid" and take the time to build a cab properly, there are very few disadvantages!
     
  12. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I'd like to see some actual proof of the advantages of a quasi-TL - everything I've read and all my own research suggests that with low Qts high Vd drivers there are none. I've heard plenty of marketing spiel claiming better transient response and lower group delay but not a single AES paper that backs this up - show me the money! ;)
     
  13. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    ok - dont jump the gun too quickly! I have made one cab so far for this application and all seems to be working well (no obvious disadvantages). I will sort out some proper outdoor test results in a meaningful format to see what comes out. In the mean time on this wet day, here is a comparison in my office using my new cab and a standard reflex box. Obviously there is some room boost going on (you can see the ripples form room modes) and the total volumes of the two boxes are different which would favour the TL box. However, note where the roll off starts (and the roll off rate is lower on the TL) and also that there is no notch at twice the line frequency.

    bass box comparison in room.jpg
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Your testing protocol is obviously in error. Any differences would show up only below 100Hz, not above, where the response of the two formats would be nearly identical.
     
  15. thudfromafar

    thudfromafar

    Dec 12, 2007
    Chicago
    cooool
     
  16. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    oh dear... looks like someone does not want to believe it!

    Well I did say it was only in a room so not that scientific and I also said they are different boxes (which also involves different dimensions etc.) and the midrange output will be different as one has a reflected backwave and the other does not - surprised experts did not think of this....

    I have not tested them in isolation either - the Red trace is through its crossover and the blue trace has not crossover which would affect the higher frequencies slightly but only marginally below 200 Hz, I suspect most of it is the internal reflected wave (or I was stood in a different positon affecting room modes slightly?)
     
  17. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Only if the cab is defective.
     
  18. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    I agree - too many variables to make a good comparison and the test was probably in favour of the new cab.... but it was all I had to show off this week :)

    Cab defective? Thats why I am trying to improve on reflex designs!

    Mind you, I have built some crackers in the past - how about a 3012HO in 22 litre box, port tuned to 37 Hz as a perfect practice amp - weighs 7 kgs. Gigged with it alone last weekend as well - does lack a little bottom but its impressive for its size. I tried it on top of the new cab yesterday and ran them together 4 ohms - its a big sound in many ways.

    I will endeavour to do a scientific test next week of the "TL-hybrid" and hopefully there will be less doubt. I will try to report results in a "Barefaced" type format for comparison.

    have a good weekend all and thanks for the comments.

    Tim
     
  19. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Washington, Utah
    John K Custom Basses
    yo Tim. i think what you're doing is great and have always loved TL speakers. keep at it and don't get discouraged. i'm looking forward to reading more of your posts.
     
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    Primary TB Assistant

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