Transmission line bass cabs

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


  1. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    I have contacts which would independently test for me in the UK.

    In fact, of the DIY designs on the market, how many people (apart from Bill FM) actually provide curves of their speaker response? I see that Fearful only provide a tuning frequency. I could claim my tuning frequency is 22 Hz -would that make my speaker sell - probably not but it would generate interest.

    This is a good question. Do I have to provide this type of information or let the public decide? I suppose I want to prove to myself and others that I can design good speakers but whats the point if it provides misleading information.

    Bearfaced loudspeakers provide lots of information which is much more meaningful than most technical specs but we have no information as to how they are tested and no plots to look at to judge or compare. The response may be very flat or lumpy and from the information we see, it is difficult to know.

    So, what do you think? Do I do a fearful or a Barefaced? One thing I am not going to do is provide useless information that I cannot justify!

    By the way, thanks to all those offering words of encouragement. I am not a quitter but I also am not ready to enter this market commercially (many reasons). When I do, it will be with something special.
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Eminence shows charts for the predicted LF response of their suggested designs, but AFAIK no one provides measured response, either DIY or commercial.
     
  3. BU - those are good points, and I suppose questions you need to ask yourself - do you want to fudge or be a knowable honest master designer. (I don’t know how accurate BFMs grafts/plots/charts and etc are) Anyway, I’m enjoying reading about your progress (and learning with you) so I too want to encourage you, but I also hope you go the extra mile like John Meyer’s and others like him looking for the best.
     
  4. bobcruz

    bobcruz

    Mar 10, 2004
    CA
    I think Acme provides some FR charts, at least for the new 112s.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Below 250 Hz is a modeled response, so it's not a true measured response chart. Better than nothing, though.
     
  6. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    By how many dB can the measured response deviate from modeled response below 250 Hz, if you have accurate values for the T-S parameters?

    In my view, a small-scale gear maker doesn't need measurements from a commercial lab, in order to stand head and shoulders above most of the industry. Here's what I would hope for.

    1. They understand the design theory, are in control of the parameters (e.g., by buying drivers from a reputable maker) and can perform basic measurements to confirm that the design behaves according to model.

    2. They are open and honest about how they arrived at their ratings.

    I'd be satisfied with Acme's rating method.
     
  7. A lot above 80Hz - but ballpark or fudged test results may be good enough for most average homebrew DIYer’s.
     
  8. Arjank

    Arjank

    Oct 9, 2007
    Above Amsterdam
    Allthough I did not read this thread completely I will give my 2 cents on the TL concept(s).
    TL's have been designed in various ways and have various advantages and disadvantages. The biggest disadvantage (of the ol' TL-design) is what some call the "TL-dip", this dip is mostly centered around the mid/bass frequencies (depends on the TL lenght.
    The best way to compensate this is to use 2 woofers instead of one and position them at 1/5th and at 1/3th of the line. This will give a smoother response and you also need less damping material (you stil need lining to dampen the mid frequencies) so it increases efficiency.
    If you want to design a TL for a basscabinet I would go for the dual woofer 1/5, 1/3 setup.
     
  9. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Maybe I was hoping to publish too much in terms of technical specs. I certainly dont want to publish fudged results as they can be very misleading. I know my cabinet works and it is now being extensively gigged to see how it performs for different music. I would encourage others to build one and try it too.

    This is what I have used it with and the types of music I have tested it gigging:

    Markbass Little mark Tube amp - well balanced and punchy -This is my gigging amp at present.

    Ashdown MAG600 amplifier - softer and less punchy - will sell this amp, it sucks and is heavy (but a second hand bargain).

    I have three more cabs in the pipeline for a "range" which should suit most people or could be used in combination as they are all 8 Ohm:

    Cab 1 - my transmission line cab as discussed. 22 kgs max, 400mm wide, 420mm deep and 700mm high. 500W RMS. designs to be used as a standalone cab or in conjunction with others. Has a tweeter option (with a control to be added). A 6 inch mid version would allow preferred material sizes to be used for a slightly more compact TL rig, est 19 kgs. Includes a crossover.

    Cab 2 - A sub to go with the transmission line to add below the cab. This would be more compact and probably more of a reflex design using the same 3012LF driver. Adds additional low end without changing the tonality of the system. 400mm wide, 420mm deep 460mm high. Around 14kgs. Front ported similar to cab 1. 450W RMS. Could also be used for a powerful vintage sound rig with very little top end. Low pass crossover / no filter option.

    Cab 3 - A small but loud rig for a fast clean sound for small venues on its own or for adding above cab 1 (or cab 2) to make the sound louder and more agressive - uses a 3012 HO driver. 400mm wide, 360mm deep, 400mm high, rear ported. Est 11 kgs. 400W RMS (this is a re-build as I already have this cab in use and it sound fast, punchy and fantastic). no crossover.

    Cab 4 - practice, small Jazz rig using a single 10 (I wanted to do an 8 but think it would be compromised). Possibly a Eminence Deltalite 2010. This would be super light and smaller in width than the others. 280mm wide, 320mm deep, 280mm high. Est, 6 kgs max. 250W RMS. no crossover but a tweeter option (4 kHz and above). MB 250 amp would suit this really well.

    Cabs 3 and 4 would have a combo option for adding a MB or Genz micro head to the back if required.

    Well thats the plan anyway. Any of these rigs (or a combination of them) should provide 90% of people with what is required of a cabinet, unless you are super bass heavy and want to use a 2 kW rig..). A MB 800W amp or Genz 900W amp would make the most of the cabs when used in pairs/4 ohm mode.

    Watch this space. Hope I get more time in the new year to build - I have the building bug again!

    :hyper:
     
  10. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    I agree with the principle for a classic line and, although it increases apparent efficiency, you do get slightly less output at the lowest notes (despite less damping). However, by incorporating a small cavity directly behind the driver, this effect is reduced as it introduces "reflex like" modes as well. Also, bear in mind the cabinet dimensions and the size of the woofers used in pro audio, there is also ready some "smearing" of line length due to the size of the cone and the cavity at the line end.

    If you want high output for a sub, tune the line length (1/4 wavelength frequency) and the reflex tuning of the long (lightly damped) "port" to the "cavity" to a similar frequency. This would however give a boomy one not bass effect). This is the difficult bit because it is difficult to truely define where the cavity finishes and the "port" starts.

    The modelling tools generally only work for classic lines or reflex cabs, not hybrids. Using two tools will yield two results independently. There are multiple systems active here which also "interact" and provide a new system tuning. By slightly shifting the two tuning frequencies apart, you end up with broader band output and still a high efficiency across the range. There is a lot of trial and error involved, not to mention geometry which discourages bass cancellation. For this reason, I need to build and test each cab to prove it works before I can publish a design.

    In my TL(esque) design, the transmission line is tuned to the resonant frequency of the driver in the box (to control the speaker well) at around 53 Hz (including taper). Any virtual reflex mode is tuned slightly lower the enhance the low end further. The "reflex" tuning (at approximately 37 Hz) boosts the 30 to 50 Hz region adding weight to the sound. However, the coupled mass in the line and the damping material provide better cone loading at ultralow frequencies than a traditional reflex. The other way to do it is to make the reflex mode fill in the notch created by the line length (at a higher frequency). This can also be cancelled at the terminus of a classic line using a Helmholtz resonator to prevent back wave cancellation at the cone but I consider that method to be a botch as it is destructive in terms of energy, not using the energy innovatively!

    In short, apart from approximately 20% larger enclosure, I cant find any disadvantages. Also, all the cabinet bracing is actively helping the tuning and not just dead wood like other reflex designs so the weight is not really much higher.
     
  11. Arjank

    Arjank

    Oct 9, 2007
    Above Amsterdam
    Bass Unique, do you undestand the principle of the design I suggested?
    You could also add a helmholz-absorber(chamber) at the closed end of the line, tuned to the "dip" frequency. This will smooth out the dip a bit.
    You can simulate a TL line with two woofers positioned at 1/3rd and at 1/5th of the line with this program, it's very accurate.
    AJ-Systems
    see design nr 3 and 4 in chapter 2.3 of the online manual
     
  12. Blue

    Blue

    Jun 19, 2004
    Southeast Penna

    Totally disagree with the "not ... designed well" statement ... and gather "there are not many designs around like this" is strictly a commercial cost issue.
     
  13. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Yes, I understand the principle. Just to explain it to others, by shifting the woofer from the closed end to part way along the line (1/3 / 1/5 positions are at odd multiples not even but nearly opposite in phase), the dip is less excited (the dip is at even multiples). Using the two woofers smears the location of the sources along a significant proportion of the line.


    Very easy to model in a straight un-bent line. (thanks - will look at your link). Once you taper the line, there are less dips anyway and some low pass filtering occurs at the temination. By setting the lengths of the line so that bends are positioned so that the distance to the first bend (and subsequent bends but less important) are not equal to the dip frequency (ie half way along the line) and fitting damping material at the points where the dip frequency has maximum velocity only (and near minimum velocity for the 1/4 wavelength frequency), you can kill the dips and maintain output.

    Yes you can put the Helmholtz resonator at either the close end or the open end but not in the middle or it wil have a much reduced effect at the dip frequency (but may be beneficial to other higher frequencies if accurately tuned and positioned).


    My design does not show a classic dip (based on my crude measurements). It does not use the Helmholtz resonator and the dip is also not audibly present playing.

    However, bear in mind this is "not a true classic line" as it would be considerably longer (and cabinet larger) for this size woofer. The whole point of this is to provide more information to the masses as to how this type of cabinet can be designed for our applications and how to compress the size without significantly compromising the performance.
     
  14. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    I am surprised at the cost issue as most good designs would still incorporate braces so material costs should not be significantly more. As there is so much bracing, the thickness of the outer wood can also be reduced overall lowering weight further.

    I suspect it is more of a lack of innovative thinking and confidence that spawns the cost argument. The design and testing costs more but once it is complete, its down to materials and assembly only. Most large companies have an R&D department so there would be design/test costs involved. Smaller companies (and DIY kit builders etc) tend to do this for the love of it, so the design part is pretty much a hobby and if you can sell a proven design, without the need for any manufacturing or shipping costs, it removes the cost part altogether.
     
  15. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Modeling doesn't take into account cabinet dimensions and construction, which can easily give 6dB deviations from predicted response. It wouldn't be a broadband deviation, but rather peaks and valleys in the response. Modeling certainly has its uses, but it's no substitute for actual measured response.
     
  16. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Agreed. We use the model to ensure we can obtain approximately the bass response desired and to ensure that we are not going to cause over excursion or similar issues.

    Actual response would vary significantly. A simple example would be a square box or a long thin pipe shape. Both have similar volumes but due to a significantly different aspect ratios, the response would be different due to the standing waves etc inside the box affecting the cones output. there is also some radiation from the box walls which will vary depending on the natural modes of the panels. these are not taken in to account in most loudspeaker models so its best to minimise them with damping and braces.

    The exception (of course!) is in a totally "no-compromise" true transmission line where the rear wave should be completely absorbed down to the cut off frequency. Then, if it is made accurately, the measured response should closely match the model. In reality, getting a closer match is as good as it gets and cabinet radiation cannot be completely eliminated.
     
  17. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Side view inside line.jpg


    Here is the inside view of this line before I put on the final side panel. I subsequently also fitted more polyfil directly behind the driver, mostly in the lower section.

    Was then varnished and carpeted for protection and finish. I used a Hexagonal perforated steel grille which I painted gloss black.

    Found the pics and thought you may like to see inside. I have added approximate dimensions as well for interest.

    The cavity has a larger cross section than the driver and is around 39 litres and the start of the line following the cavity is around 3/4 of the drivers surface area (this would usually be 1 without a chamber). I then tapered the line to just under half the surface area at the teminus.

    Ideally, the line would be slightly longer at the start (and a whole driver in area). The mid chamber could be shrunk dramatically with a 6 inch instead and the line could extend in to the space where the 10 mid was. This would extend the line and move the woofer from the line end by about 1/5 distance. My trials of that technique did not work as well as this design.
     
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Between the large rear chamber and the gradual line taper it may be acting more like an LLT (long low tuned) reflex than a TL. That would explain the apparently stronger than expected low frequency response. An impedance chart would help reveal what's actually going on.
     
  19. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Bill - it probably is. I have not completed an impedance chart yet but this design probably would look mor elike a reflex with two humps. However, the reflex mode tuning would be different to the 1/4 wave tuning which broadens the badwidth of the terminus output.
     
  20. lelandcrooks

    lelandcrooks Commercial User

    Jan 6, 2011
    Kansas
    Owner SpeakerHardware.com
    This thread is eye candy. Geek out baby!
     
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    Primary TB Assistant

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