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what is "transmission line"?????

Discussion in 'Amps and Cabs [BG]' started by Suburban, Apr 2, 2002.


  1. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    Friends,
    I know the basic theory around sealed cabinets and vented (which I assume is the same as ported, you are welcome to correct me if I'm wrong). I have also read EA's page on transmission line, but I can't say it makes all that much sense....:confused:
    Then I searched this forum, with rather poor outcome. Hence these basic, but difficult, questions:

    What are the principles of a transmission line cabinet?
    How do you design/tune/etc such a cab?
     
  2. A transmission line cab is designed with a computer, and basically takes the rear waves of the speaker down a tunnel of a determined length inside the cab, and ejects them so they either complement or cancel certain frequencies, as the manufacturer deems fit. As this tunnel is made of same or similar wood to the cab, it makes the cab heavier than a regular vented one. This is a hi fi concept, intended to make a small cab sound much bigger, but in the bass guitar application I cant see it really catching on.
     
  3. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Transmission lines and waveguides have been used for years in propagating high frequency radio waves (i.e. microwaves) but have recenlty been adapted to acoustics as well.

    Besides EA, innovators in this area include Bose and V-Dosc (a European PA manufacturer). Here's a description for the Bose website about their Waveguide technology, which is similar. It should help give you a idea of what is going on.

    Acoustic Waveguide® Technology --
    In-Depth.

    The inspiration behind this innovation was a device called an "acoustic waveguide" -- which confines the movement of a sound wave to travel over a desired path. A simple example is a pipe organ, which uses a small amount of air to fill a cathedral with full, rich sound. Another simple example is a flute -- by blowing a stream of air across the mouthpiece, a musician can produce enough sound to fill a large room. However, both these instruments have a serious limitation where loudspeakers are concerned -- production of different notes requires a waveguide of different length. This is created either through fingering, as in the flute, or by selecting another length of pipe, as in the organ.

    Dr. Amar Bose and a senior Bose research engineer, Dr. William R. Short, realized that by mounting a loudspeaker in a tube, the motion of the loudspeaker would be efficiently coupled to the motion of the air. In other words, it would act as a waveguide. Drs. Bose and Short discovered that the waveguide could match the mechanical properties of the loudspeaker for efficient operation maintained over a wide range of notes. This would allow a small-size driver to produce clear sound, without audible distortion, even at high volume levels in the low frequencies.

    Furthermore, additional analysis and measurements
    showed that the tube could be folded into intricate patterns with no ill effect on sound quality. This meant a
    waveguide measuring several feet in length could be
    woven into a small tabletop enclosure, delivering sound with the clarity, depth, and lifelike quality of a component stereo system. For example, the Bose Acoustic Wave® music system, less than a foot high (30 cm), contains a waveguide that's nearly seven feet (2 m) long. The Wave® radio, which is 14 inches (35.6 cm) wide, contains a three-foot (1 m) long waveguide.

    Acoustic waveguide technology is also used in the Bose Acoustic Wave® Cannon[TM] System II. Instead of
    being folded, however, the waveguide is straight, and a
    single high-power woofer is nestled inside the twelve-inch diameter, twelve-foot long barrel-shaped enclosure (30 cm x 3.7 m). Developed for use in large venues such as movie theaters, performance centers, stadiums and nightclubs, this powerful system can reliably produce the high volumes required in these settings. In addition to its power-handling capabilities, Bose waveguide technology gives the Cannon system the installation advantages of easier handling (because the enclosure is constructed of light PVC plastic instead of heavy wood) and a shape that nestles against ceilings or walls to take advantage of
    acoustic bass-loading techniques.

    Link: http://www.bose.com/technologies/acoustics/html/waveguideNfFS.html
     
  4. bben

    bben

    Feb 28, 2002
    Santa Fe, NM
    No offense, Brian, but Bose took something (tuned pipe, driver at about 2/5 of the way between ends) that had been around in the public domain since the 1950s, and patented it as their "waveguide". Aggressive capitalism, from a mid-fi company that is very good at marketing.

    Transmission lines are a different idea, as Marty said. The woofer front is in the room. You take the sound from the back of the woofer, and run it through a long channel stuffed with fiber. This has been popular with do-it-yourself hi-fi speaker builders, especially the British, for decades. The rule-of-thumb has been that the cross-sectional area of the channel is twice the surface area of the woofer. The fiber slows the sound down, and you make the channel length 1/4 wavelength of the deepest sound you want. If you are a traditionalist, you use wool as the fiber and put netting and mothballs in there, too.

    This makes for very big boxes. Amateur speaker builders fiddle endlessly with different ways of folding the channel, tapering it, and reducing the cross-sectional area. I don't know what Euphonic Audio is doing for their transmission line designs, but they are much smaller than the traditional approach. By all accounts they sound good.

    More info in Vance Dickason's "Loudspeaker Design Cookbook" or in old issues of Speaker Builder magazine.
     
  5. If my new EA CXL 112 is any indication, I think it will !
     
  6. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    OK, let's see...
    There is a like a funnel, that collects the air from the rear of the woofer and leads it out. It's tapered, so it will compress the air some, and "wooled" which will to some extent contrawork the compression.
    Did I get it right?

    Then there is the question of how to dimension it. Meaning: is there any way to simulate the response of different configurations of transmission lines?
     
  7. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PA
    its EA's extra mojo that makes its CXL112's sound soooo dang good, but also turns them into back breakers.... oi... Jimmy at Bass Boutique said that the line adds maybe an extra 20 lbs. to the cab. youch!
     
  8. My 112 isn't that heavy, it's about 50 pounds.
     
  9. i agree there not that heavy, they're just deceiving.

    they look so small, people say "all that sound out of that thing?"

    they go to pick it up and POW!!!!

    "bloody el, i didn't think it weighed so much.

    hey, there still the ultimate compact 'portable solution.

    sure you could use small head, like the clarus, or someting by WW, but EA stuff is still more appealing to me than any other cab or head.

    We all think that something small should weigh nothing, their the tardis for bass players :)

    and i'm already loving mine and it's not in the country yet

    stu
     
  10. bben

    bben

    Feb 28, 2002
    Santa Fe, NM
    Suburban, I think you've just about got it. A tuned box relies on Helmholtz resonator theory, e.g. the sound you get when you blow across the opening of a bottle. A transmission line has a tapered chamber behind the woofer instead of a resonating box. The chamber absorbs some of the sound and lets some of it out to reinforce the bass. It isn't really "compressing" anything, though.
     
  11. bben

    bben

    Feb 28, 2002
    Santa Fe, NM
    Hey, Randy, I would be really curious to see what your EA 112 actually weighs. Have you got a bathroom scale?

    I have found by actually weighing a bunch of cabs over the years that some manufacturers tend to understate actual weights.
     
  12. Actually I don't have a scale, I'd rather not know my weight! It's funny about listed weights, even the Bass Player review of the EA cab had two different weights posted!