IMF Transmission Line Stuffing?

Discussion in 'Speakers' started by Nickyrboy, Sep 23, 2014.

  1. Nickyrboy

    Nickyrboy New Member

    Messages:
    7
    Location:
    Bristol, UK
    Hi All

    I am hoping someone can help with a query I have over IMF transmission line stuffing/insulation. I am planning on a building a DIY set of IMF SACM's. Well they won't be true SACM's as I will be using a kef B139, B110, t27 and coles 4001G with the falcon acoustics uprated crossovers. I have managed to get the dimensions of each section of the cabinet and although I am certainly no carpenter I think (fingers crossed) that I can construct the cabinets but it might take me some time! My real query is over stuffing of the transmission tunnel itself. All pictures and info. I can find seem to indicate the trnsmission tunnel isn't actually stuffed at all merely lined with the acoustic foam (egg crate) type. Can anyone confirm if this is the case? Also does every square inch need covering with the acoustic foam internally as some cross section pictures seem to show only some of the panels with acoustic foam. Hopefully someone is an IMF expert and can help me with this query.
    I was also planning to paint all internal panels with a bitumen roof paint prior to installing the foam to help deaden them. Is this a good or bad idea?
    Thanks in advance,
    Nick
     

     

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  2. bare

    bare Super Member

    Messages:
    1,987
    1/2" fibre carpet underlay makes for a passable dampening material Hollow fibre Dacron in a similar thickness mat (tricky to source) works well too.
    No tar.. it's not a roof consequently doesn't need to be waterproof.
     
  3. keilau

    keilau AK Member

    Messages:
    454
    Location:
    Windy City
    Long fiber wool

    Before you start your project, you may want to read through this thread on "transmission line speaker".

    http://www.audiokarma.org/forums/showthread.php?t=584782&highlight=transmission+line

    Bud Fried used the foam in place of long fiber wool. It stakes and fills most of cabinet. View attachment Fried_Studio_V_brochures.pdf JimPA recommended this LOW density open cell polyuerethane foam at Jo-Ann Fabrics

    http://www.joann.com/foam-1inx18inx3...4&prefv1=false

    There are just too much misinformation on the internet on this type of bass loading. The driver components and crossover you mention make me believe that you are trying to build the one designed by B.J. Webb. It is also similar to the commercial TDL Monitor. It was originally published by Audio Amateur (US) and HiFi News (UK) in 1975. You can still found the article on the Falcon Acoustics web site.
    [​IMG]

    There are two type of material that can be used to stuff a TL speaker properly. One is the low density, open cell foam that Bud Fried used in his design. "Low density" mean high porosity, but Bud did not left any technical specific on how to pick the right king. A safer bid is to use the Long Fiber Wool that Dr. Bailey, who invented the TL design, recommended. Long Fiber mean most of the wool fiber will be 5 inches or longer. It needs the larger diameter and its stiffness to be a effective stuffing material.

    Never use fiber glass, Dracon or other polyster fiber. They had been shown not working well or at all for transmission line.

    Dampening the cabinet wall is an entirely different issue. The Brit used to use thick bitumen felt glued to the cabinet wall. But I don't see it at the Falcon Acoustics site anymore. The idea was to absorb the acoustic energy before it gets to the cabinet. Parts Express sells some foam liner with polymer layer embedded. It may work better than a thin coat of paint.
     
    Last edited: Sep 23, 2014
  4. bare

    bare Super Member

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    1,987
    Erm ..speaking of Internet "misinformation" :no:
     
  5. JimPA

    JimPA Distinguished Member

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    3,277
    Location:
    Grover's Corner PA.
    How many T-lines have you built?
     
  6. JimPA

    JimPA Distinguished Member

    Messages:
    3,277
    Location:
    Grover's Corner PA.
    [​IMG]
    Over stuffing a T-line enclosure will cause it to behave like an aperiodic enclosure.
    You want to use the flat foam not the egg crate type.
    You should PM some of the IMF owners on the forum who can better describe the stuffing design layout of their IMF models.

    Enclosed above is a picture of a Fried T-line showing some of the foam stuffing of WmartinB's sub woofers. This should give you a general idea of how the foam should be placed in the line.

    [​IMG]
     

     

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  7. keilau

    keilau AK Member

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    454
    Location:
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    But the misinformation is not difficult to spot with some simple measurements. Dr. Bailey originally design the "non-resonant" transmission line" to get very low frequency response without getting the boominess of a vented (bass reflex) speaker design. The TL line is characterized by a slow 6 dB/octave slope of the bass response, while a bass reflex has a steeper 12 dB/octave slope. You can go to the original Wireless World to understand why we want the slower slope. Below is a figure from Bailey's paper.

    Pages from Bailey-1965-Non-resonant-Loudspeaker-Enclosure-Design.jpg

    Most of us do not have the facility to measure the bass region response curve, but can easily measure the impedance curve using an audio signal generator (feeding to a power amplifier).

    In the picture below, curve #1 is a KEF B-139 mounted on a TL stuffed with long fiber wool. Curve #2 is a bass reflex box tuned to the driver. It can achieve a low response, but the bass is ringing. Curve #3 is a closed box design where the response drops off early. Curve #4 is typical of a TL cabinet, but stuffed with Dracon or similar.

    Ted-Jordan-Aperiodic-Design 5.jpg

    When someone claims to have built a TL speaker, a quick look at the response and impedance curve will quickly tell the truth. A twin peaks in the impedance is a dead give away.

    Never use fiber glass, Dracon or other polyster fiber. They had been shown not working well or at all for transmission line.
     
  8. Spalls

    Spalls Making up for lost time Subscriber

    Messages:
    1,167
    Location:
    Kirkland, WA
    For what it is worth, my TLS-80s have plain open-cell foam, not egg crate. The strips that are used in the lower end of the enclosure are twisted.

    I hope that our project meets your expectations :thmbsp:
     

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  9. Nickyrboy

    Nickyrboy New Member

    Messages:
    7
    Location:
    Bristol, UK
    All Thank you very much for your responses they have given me a lot of food for thought and information I didn't have before. The link from Keilau to the "lively" discussion about transmission line stuffing was a very good one. I should have explained myself a bit more in my initial post I am no stranger to transmission line or 1/4 wave speakers being extremely lucky and having a few pairs at the moment. I am certainly no engineer or true audiophile just a guy who likes listening to and enjoying music.
    I have some original IMF pro monitors that use fibreglass stuffing, some IMF studio 50's that use open cell foam flat type (twisted in the end of the line), some TDL studio 1's that use open cell foam egg crate style and some DIY Webb R50 clones that use long fibre wool. The reason I say this is purely to show that I am lucky enough to have an example of each type of recommended transmission line stuffing already. They do all exhibit slightly different properties but to be fair each speaker has a different cabinet volume set of drive units and crossover so it isn't really fair to compare them based purely on their internal stuffing. If I had to though I would say the long fibre wool does give the "fastest" and "flatest" bass reponse but doesn't go as low as my big IMF pro's however I strongly think this is down to cabinet volume.
    I am looking to build a cabinet clone of the IMF SACM speaker albeit with diffeent tweeter and supertweeter. This is the IMf that houses each drive unit in a totally seperate enclosure stacked on top of one another. All pictures I have seen on the internet of old cutaway diagrams show that the SACM, main KEF b139 cabinet, had the open cell egg crate type insulation bonded onto a few internal panels not all of them can anyone who has or has seen a pair confirm this possibly? The reason I ask is it just doesn't seem logical to me that this would be sufficient for a large cabinet like this.
    You may be wondering why I would go to the effort of building these with some big transmission line speakers kicking about?! Purely because all my current speakers were secondhand bargains and have damage to the cabinets of varying degrees I just wanted to build a modern clean cabinet, magnetic grilles etc, and thought why not try and build the best deisgn I could. Am I wrong and there is a better transmission line cabinet I could build that would suit the driver combination I have.
    Thanks again for all the input so far guys.
     

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  10. MJKing

    MJKing Active Member

    Messages:
    219
    Location:
    Clifton Park, NY USA
    I completely agree with bare.

    I have designed, built, and measured a few myself and have collaborated on quite a few more.

    Fiber Glass, Dracon, or other Polyster Fiber can work very well in a TL to damp unwanted standing waves. My TL software has been used to design hundreds of TLs and when measurements are made of the finished speakers the correlation with predictions is very good.
     
  11. canuckaudiog

    canuckaudiog On a quest for high fidelity

    Messages:
    7,793
    Location:
    Edmonton, AB

    Hey Martin,

    I would love to hear your comments on the classic IMF transmission line speakers and the t-line they use as well as their overall design. They seem to work well and I would like to know more if you don't mind indulging me :)

    As for the OP - I believe you can find plans for DIY IMF SACMs.. Look around for them. As far as I know they were official plans and described in detail all the panels, stuffing, crossover parts, drivers etc. Personally if it were me I would go with a more modern transmission line design because many strides have been made in that area of speakers, notably they are far more efficient and more predictable to build (with a big thanks to MJKing). IMFs are great but are early in their engineering for t-lines.

    If you do want a nice pair of IMFs, just wait around for a good second hand pair to show up. It'll be way cheaper and far more guaranteed to give you what you want.

    Just my two cents :) I personally own the IMF RSPM Mk IV and have had many other models - so I know why you like them ;)

    Jeffrey


    Sent from my iPhone using Tapatalk
     

     

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  12. MJKing

    MJKing Active Member

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    Location:
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    Sorry, I don't know anything about the IMF TLs or the drivers used in them. I have never looked closely at any of those designs.
     
  13. JimPA

    JimPA Distinguished Member

    Messages:
    3,277
    Location:
    Grover's Corner PA.
    These are the enclosure and stuffing plans for the Fried O4 1/4W T-line sub woofer and C3L mid bass enclosure.
    This enclosure uses a 10" driver.

    All of the Fried T-line enclosure volumes are 1.25 Sd x line length.
    The O4 line length is 10.4 feet being approximately 27.5hz 1/4 wave.
    The taper ratio of the O4 model is 2.5 Sd reducing to .75 Sd.

    The Fried C3L model use a 6.5" driver.
    The line length is 3.9 feet being 75hz 1/4 wave. The taper ratio is 2.5 Sd reducing to .5 Sd.
    The C3L cut off frequency is lowered by reducing the taper ratio to .5 Sd
    The C3L enclosure is flat to 60hz.

    I.M. Fried had written a great deal about using the resistive foam in his T-line speakers over the years.

    http://www.friedproducts.com/Bud Fried/Further Reflections on Loudspeakers.pdf

    http://www.friedproducts.com/Bud Fried/WhatAboutLoudspeakers.pdf

    I have tried fiberglass in the Fried C3L model just to see if it would work and it acts like a no pass filter in the T-line.


    [​IMG]






    [​IMG]
     

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    Last edited: Sep 24, 2014
  14. keilau

    keilau AK Member

    Messages:
    454
    Location:
    Windy City
    Why you should not use polyster fiber for TL

    Would you show one example of the response curve and impedance curve of a transmission line you built? If you use Dracon etc., I suspect that you will end up with a bass reflex box. No matter what you call it, a twin peak impedance will show that is the bass reflex design.

    The long fiber wool or open cell foam stuffed transmission line absorbs the higher frequency back wave and act as a phase inverter at the driver resonant frequency. Dracon will not do either function.

    According to Ken Kantor, not all polyfill work for closed box either:
    More information at Ken Kantor's blog:

    http://auralization.blogspot.com/
     
  15. JimPA

    JimPA Distinguished Member

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    Location:
    Grover's Corner PA.
    Nickryboy I have seen in a few cases where speakers bought used have had the type stuffing used completely changed.

    Fried and Wright both learned about 1/4W T-line designs from Dr. A.R..Bailey and Arthur H.. Radford.
    That is the story that I was told being a customer of I.M.Fried.
     
  16. Nickyrboy

    Nickyrboy New Member

    Messages:
    7
    Location:
    Bristol, UK
    Wow even more great infornation thanks guys.
    Canuckaudiog, or others, if you think a more modern design might be the way to go rather than an Imf diy version could recommended me one? Nothing fearsomely large but I was happy to build the sacm it can be of a reasonable size but nothing over 5 feet high! I'm very intrigued to hear more modern designs if they surpass the IMF's as they have impressed me quite a bit. The only modern t-lines I have heard are pmc's which although are great speakers don't come close to the larger cabinet transmission lines of yesteryear.
     

     

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  17. JimPA

    JimPA Distinguished Member

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  18. keilau

    keilau AK Member

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    Location:
    Windy City
    Bailey, Radford and Bud Fried are the audio genius of their generation. They invented the TRANSMISSION LINE speaker for very specific reasons, a tight, extended and non-boomy bass.

    The transmission line bass is hard to describe until you listen to a DYI Bailey, or TDL or IMF design. Because of its non-boomy nature, it actually sound thin on initial audition. But it's all there. It is non-fatiquing over long period

    On the other hand, a bass reflex design has the crowd pleasing ringing bass. It may sound pretty good if you never heard a Fried transmission line.. It is not surprising that some want to call their bass reflex a "transmission line" with no intention to duplicate the slow decay nature (6 dB/octave) of a true TL.
     
  19. MJKing

    MJKing Active Member

    Messages:
    219
    Location:
    Clifton Park, NY USA
    A straight TL I built about 15 years ago for a pair of Focal 8" mid woofers. The TL was tuned to about 45 Hz and the entire length was stuffed with ~0.5 lb/ft^3 of Dacron Hollofil II fibers (pillow stuffing). I eventually ended up with only the first 2/3 of the length stuffed. These are the empty and stuffed measurements of electrical impedance and the SPL for the woofer and open end. Comparing the two plots for each measurement you can see how a TL works and how the stuffing impacts the response.

    [​IMG]

    [​IMG]

    Empty and Stuffed Impedance

    [​IMG]

    [​IMG]

    Woofer Response Empty and Stuffed

    [​IMG]

    [​IMG]

    Terminus Response Empty and Stuffed

    [​IMG]

    [​IMG]

    Summed Response Empty and Stuffed

    [​IMG]

    [​IMG]

    The measurements show that Dacron fiber is very effective (the first peak in the impedance curve is supressed) and that the TL behaves as it should. The higher order standing waves are damped out and only the fundamental contributes to the bass output. There is no phase inversion at the tuning frequency only a 90 degree shift associated with a quarter sine wave along the length. And the best part is that the MathCad models for this geometry and stuffing configuration correlate very closely to the above measured results. If you can calculate an answer that matches test data then you must have captured the significant properties/behaviors of the design correctly.
     
    Last edited: Sep 25, 2014
  20. MJKing

    MJKing Active Member

    Messages:
    219
    Location:
    Clifton Park, NY USA
    The test data from the previous post pulled into MathCad and plotted over the modeled predictions.

    Impedance - Empty TL

    [​IMG]

    Impedance - Stuffed TL

    [​IMG]

    Woofer and Terminus SPL - Empty TL

    [​IMG]

    Woofer and Terminus SPL - Stuffed TL

    [​IMG]

    I believe the correlation is extremely good and conclude that the math/physics based MathCad model accurately captures the empty and stuffed TL behavior.
     

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