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
    That's one of the first things you do after building a prototype, otherwise you can't really know what you've got.
     
  2. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    So when looking at the impedance chart, if it was a true Transmission Line the impedance curve would look more like a sealed cabinet with a single impedance peak? Compared to a ported enclosure which has 2 impedance peaks.

    I dont really completely grasp the benefit of a TL or what technically makes a system a true TL. But just looking at various TL designs all they seem to be is a med/small volume ported enclosure with a very long low tuned port. And by adding a little taper to the port, it just tunes the port lower. Similar to using a smaller port tube to get a lower frequency with less length. And simple stuffing the hell out of the port at the larger end would help get rid of alot of chuff/smearing since the smaller end would behave like a high velocity small port.

    and hopefully without derailing the thread what is the real difference between a LLT (long low tuned) compared to a EBS (extended bass shelf).
    Is a LLT basically tuned low below the Fs of a driver, but just using a larger or smaller enclosure volume.

    All in all, it seems a transmission line is really just a ported cabinet tuned extremely low. And the impedance curve appears similar to a sealed box. Since low tuning will cause the first impedance peak to rise. And the 2nd peak can seem to disappear. But is just a much smaller peak hiding below 20hz in the 5 to 15hz range, depending on the tuned frequency.

    Since it is known that a ported enclosure will start to behave like a sealed enclosure if it is tuned to low.
     
  3. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    A true transmission line is a speaker in a pipe. The best way to understand it is as an open baffle speaker where the baffle has been folded backwards to make the pipe. There is no 'enclosure' in the traditional size - the woofer acts more as though in free air and the back wave is delayed and thus phase shifted by the pipe length, with output highest at the resonant modes of the pipe. The pipe acts like an organ pipe, not like a helmholtz resonator as in a ported cab.
     
  4. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    oh ok now that makes sense. And thought provoking. Seems like their is a trade off to anything. If you could create an ideal acoustic delay line for every frequency. their would still be cancellation where the 2 sound waves meet.

    I wonder if instead of having the TL line exit like a forward facing port. You would want it to either exit the rear of the enclosure to try and create a 360 degree waveform in phase. Or have the TL exit on a 90 degree baffle to the speaker. To try and get the 2 sound pressures to recombine and work as a single driver in phase.
     
  5. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    As you spotted you can't delay each frequency by the exact right amount to get omnidirectional output, so the TL relies on delaying a narrow bandwidth by the right amount to get in-phase reinforcement in that frequency range and then using stuffing to damp the resonance at other frequencies.
     
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The main advantage to a TL is that it can give bass reflex style frequency response and sensitivity using higher Q drivers that aren't well suited to bass reflex alignments. That was of some benefit when the TL was in its heyday, 40 years ago, when most drivers were high Q. Today, not so much. I only employ it in one design, loaded with high Q drivers.
     
  7. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Thats interesting you think that. Take your organ pipe and give it slightly more volume at the closed end - the frequency drops. to compensate slightly shorten the pipe. If you keep doing this, you end up with a reflex port.

    The behaviour of either undamped system would be similar and there is a very gradual transition between the two (such as tapering which also lower the tuning frequency (or shortens the line slightly).

    However, damp a reflex port and you kill the output. With the huge cross sectional area of the TL "port" you can add significant damping and not completely kill the LF output but you will reduce the upper components. The down side is a larger box but as part of the line acts as a box and part of it a port, the volume is not a small as you may calculate.

    My model takes the 39 litre chamber and adds 15 litres to this to assume box volume (includes the first section of the line). Then if you take the rest of the line to be assumed to be a reflex port you get approximately the response of a reflex mode. Calculate the frequency based on the distance from the back of the cone to the teminus and you get a different result. The two systems combine to create a flatter/wider output with little loss of efficiency and an average impedance which is slightly lower (smoother).
     
  8. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    With very little damping, mine is working just fine with a lower Q driver (around 0.32 Qts). All the notes are smooth and audible and there is a massive authority to the low end.

    In terms of impedance testing being "the first thing you should do with a prototype", I disagree. Listen to it first and if you like what you hear, its a good indication that something is working. You can have a fantastic impedance plot but it could still sound like a bag of spanners. Most amps can cope with the wildly fluctuating impedance of sealed and vented boxes and a TL (or similar hybrid) would tend to maintain a more constant impedance across the range which can't be a bad thing.

    Measurements are useful to make comparisons on paper but it's the sound at the end of the day that matters and everyone has a preference for a slightly different sound. Mine is for a clear smooth bass where all the notes sound equal in level and low in colouration. Its easy to make something that booms at one or two notes very loudly and make bold claims about its efficiency. In the real world for small venues, I think quality rather than quantity should win.

    Gigged the "TL hybrid" cab last night and it performed admirably at around half volume on the MB little mark tube on flat response and a little VLE to tame top top end. Easily filled a club with a single 12 with 200 people in it and had plenty in reserve.
    :bassist:
     
  9. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Bass below 100 Hz would be omni directional. In the far field (long way from speaker) it would not matter if the vent was on the front, side or rear - you still get the same output. By putting it on the rear though, you can extend the distance slightly between speaker and output making it tunned a few Hz lower. However, it would not then work well up against a wall as it would be blocking the port.

    Another problem in the mid bass is that any residual port emissions are at different distances from a player close to the speaker) - keeping both very tightly packed prevents any cancellation between the two low frequency sources (possibly relevant above 100 Hz) and to my ears at least, it sounds more solid at all frequencies. Also, keeping the port at floor level maximises output (and the floor extends the end effect making it appear slightly longer).

    I favour a front port so it performs consistently away or against walls. Due to the massive area, there is no chuffing even flat out and you also feel more of the bass around the ankles on the low notes!
     
  10. im learning about this kinda stuff in college so im going to be a complete nerd and show it to my lecturer.
     
  11. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    When you get the hang of it, its no black art but a little lateral thinking can provide an effective "different" design. However, 99% of people have not grasped many of the basics.

    I hope to see you posting on here with confidence once you graduate and you could be the next "Guru" :)
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    -100. An impedance plot will reveal things that you simply cannot hear, including leaks in the cab and errors in the crossover. If you're going to produce a professional quality product you have to use professional engineering and testing techniques.
     
  13. +1000 to both listening and professional testing.
     
  14. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    +1 to listening AND testing combined. The danger with relying on one only is you can end up with sillyness of :- "Do you think this speaker sounds good?"

    "hmmm ... I don't know until I see the impedance chart."
     
  15. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    So with the 39 liters chamber (1.33 cubic feet) and the first section of the line 15 liters (.52 cubic) so a combined total of 54 litres or 1.90 cubic feet.
    1.9 is really close Volume for at least 3 reflex type alignments that work with the 3012LF, the only difference really between
    BB4, QB3 and C4 would be the tune freq around the drivers resonant frequency.

    Question= When you calculate your system as a reflex system what is the tuning frequency ?

    Moving on

    To make a long story short, when i look at the response chart that you posted. Which really seemed to turn up the roar and caused people to really want to pick on you. Was because the bass output below 50hz was pretty dam good. Way better than what any builder could get with a standard reflex cabinet.

    Your point Bass Unique, on the listening test was a good point. We are still listening to a 3012LF same dam speaker in many "Boutique" builds. so its going to sound fine, makes many happy.

    Otherwise technically if you model the 3012LF in 1.4 to 1.9 cubic feet. And tune it to some ridiculous low value like 20hz to try and model what seems to be a low tuned port. Its impossible to get the low frequency output shown on your response chart with such a small volume of 54 liters (1.9cu).

    In fact if you model the 3012LF in 4 cubic feet (113L) or as a EBS3 extended bass shelf. Aside from a few peaks and dips your response graph looks almost exactly like the EBS modeled response.

    [​IMG]

    So i think the main advantage of your system would be. A frequency response similar to a EBS alignment ( very large box), but with the power handling and cone excursion of a BB4, QB3 alignment (small box)


    In 113 liters (4 cubic feet) the cone excusion of a 3012LF would climb threw the roof compared to 39/54 liters (1.4 to 1.9 cubic feet).

    [​IMG]

    so many would be willing to discredit you, I would much rather you spend just a little time, trying to get a impedance chart and do another Frequency response chart, in your backyard with a standard chart. otherwise dont lose sleep over use internets people, i enjoy seeing others work. Since im sure you have spent many hours of your time on this.
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You'd clarify that using AkAbak or HornResp for the modeling. Both are capable of doing so with his alignment, whereas WinISD and other similar box programs are not.
     
  17. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    Well yes of course, But winisd is uses TS parameters, which includes mechanical and electrical values of speaker being modeled. And is capable of showing basic cone excursion of a driver in a certain cabinet volume.

    Bass Unique from what i understand has compared his system as a hybrid of a reflex and transmission line.

    I was simply trying to show that his system is similar to a reflex cabinet, but is not a reflex cabinet, since yes, you cannot model it in a reflex software like winisd. Since it would be impossible to get so much bass output from a small volume. unless it was a large volume, like a extended bass shelf.

    But again in Bass Unique case...he has created a hybrid capable of getting a Extended bass shelf response, with a much smaller cabinet volume.

    I just used winisd real quick to show cone excursion of a 3012LF
    in a volume similar to bass unique's design. To show how a smaller cab can help raise power handling and lower excursion.

    And make a graph which may technical show the advantages of Bass Unique's system. Since ive just compared its frequency response to a 3012LF in a EBS3 alignment. But yes of course it is absolutely in no way a model of what it really is.

    But i was hoping we could at least agree that its not impossible for a 3012LF to create as much bass output as Bass Unique has shown.

    Because when you do model a 3012LF in Winisd, it is capable of getting that much output.
    And if you look close at Bass Unique's graph the output, makes a "shelf" that is about -3dB down from 90 hz down. Very very close to EBS3. Aside from the peaks and valleys, this is very much possible the trade off of using a Transmission Line hybrid.

    attachment.jpg

    And again the only way to get this in a reflex type is in 4 cubic feet,
    and again Bass Unique has designed a system that can do it with 1.9 cubic feet.

    So again in no way am i trying to model this with winisd, you cant, period. But we can show that a 3012LF can produce that much bass output. And we can show that doing so in a smaller box would increase power handling.

    Im just want to know if its true or not, And again this is the age old quest for "More Bass" in a smaller box.

    which we all know is impossible. Without trade offs.
     
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    I haven't seen adequate documentation so far to come to that conclusion. I'm not saying that it's impossible, but before he can make such a claim he has to produce corroborating data.
     
  19. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    yes yes yes, indeed.

    Thats what i want to see. And we have already giving Bass Unique a hard time about how he gets is data.
    So hopefully we can all be understanding that he doesn't have access to super duper amazing test equipment.

    But hopefully can sorta outline, data he can show. so we can firgure out whats happening and try to model it with the correct software.

    So Bill if Bass Unique was to drag that cabinet outside and retest the frequency response to a standard graph. and record the impedance curve of the driver operating in his enclosure. would that be enough to prove it?

    and im sure Bass Unique knows how to test speaker systems.

    But could you the mighty BFM give us a quick crash course how you would measure the frequency response outside. (aside from what you already posted) and how to measure the impedance curve without fancy computer software?

    and yes i know, the internet has plenty of info, just more interested in your approach.
     
  20. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    The last few posts have been pretty interesting as I have had the same problem modelling the response duirng my design phase. However I will confirm the following:

    1) The output in the low bass should be similar to an EBS design of a larger volume (in fact larger than the entire cabinet volume if it were a reflex). This is because there is more than one modelled resonance present in the same octave band. The "reflex" port tuning is somewhere in the 33 to 37 Hz range extending the low end beyond that of a typical line of the length I have. The TL tuning is around 53 to 58 Hz (undamped). Actual tuning is difficult to model as the extra volume in the line may or may not be considered part of the volume or the resonator so I had to do some experiments. However, since the system "resonance" is fairly broad, the actual tuning probably does not matter +/-10%.

    2) The cone excursion is much better controlled than a standard reflex in a large box (above and below the system resonances). This should improve power handling (and if power handling is higher, you can put more in to get more out etc...).

    3) I have used Horn resp (not that sucessful in modelling TL I must add!) and Win ISD to obtain my models. Neither explain the output or behaviour on their own. The closest alignment to the cone excursion I have found (as I have two resonances (reflex and Line modes) is a 6th order bandpass in Win ISD - although the output is inphase not out of phase as the model would show) when the two frequencies are close together as they are both generated from the back of the cone.

    Hopefully the impedance plot should show something similar to the bandpass with the SPL output similar to the EBS. Based on my calculations, the group delay would be no worse than a reflex below 40 Hz (and closer to a sealed box over 50 Hz). However, dont forget we are not talking mega efficiency. Smooth response and a useable level is the aim here whilst maintaining a box smaller and lighter than a small horn or traditional TL. Its all about the sound and portability, not about output claims.

    I agree that testing is required on a commercial design (which I must add this is not) and we are probably at that point. However, what I dont agree with is running scientific tests first without listening as soon as you complete a prototype. BFM has made it clear that testing matters more to him than the sound (although he only provides data on 1/3 octaves and I have never seen an impedance plot published!). Ultimately, however a speaker performs on tests is less relevant, it's the sound that matters as that is what we produce to the public.

    If it was all about testing then there would not be many valve amplifiers on the market when solid state exists but most people still prefer the sound of valves. They have much higher distortion levels (but if you listen, the distortion is smoother and less noticable as they are mostly even orders).

    So, yes testing is required and, in order to build a speaker than can take the power and deliver the goods, it is essential. However, unless we build things on a whim and listen to the results, we are building a clinical lab standard speaker which can still sound dull and unexciting. We would never break the mould unless we try things which are new. Live a little!

    If you check the models, the 9.2mm Xmax is not exceeded until below 30 Hz. Xlim is not likely to be exceeded at all, even with the EBS - (if it was, the fearful speakers would not work so well). This design, although it can take 450W thermally, will never see more than 300W and I wanted to design it to work as an 8 ohm speaker on my amp. A commercial version of this speaker would be designed to take at least 450W (probably a little over 500W through a crossover, similar to a fearful). Not surprising really as its the same speaker.

    The laws of physics cannot be broken. However, as most people dont understand the acoustic principles involved (although they believe the compromised models), they are typically only 50% there. Look how technology is progressing and we are discovering new ways to achieve goals every week that was not thought possible before. Come on everyone, think outside the "box". :)
     
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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