A Transmission Line Speaker With The Design Work To Back It Up

We love the world of audiophiles here at Hackaday, mostly for the rich vein of outrageous claims over dubious audio products that it generates. We’ve made hay with audiophile silliness in the past, but what we really like above that is a high quality audio project done properly. It’s one thing to poke fun at directional oxygen free gold plated USB cables, but it’s another thing entirely to see a high quality audio project that’s backed up by sound design and theory to deliver the best possible listening. [Davide Ercolano]’s transmission line speakers are a good example, because he’s laid out in detail his design choices and methods in their creation.

Starting with the Thiele-Small parameters of his chosen driver, he simulated the  enclosure using the Hornresp software. As a 3D-printed design he was able to give it paraboloid curves to the convoluted waveguide, making it a much closer approximation to an ideal waveguide than a more traditional rectangular design. In the base is a compartment for an amplifier module, with additional Bluetooth capability.

We’d be curious to know how well 3D printed plastic performs in this application when compared for example to something with more mass. However we like these speakers a lot; this is how a high quality audio project should be approached. We’ve delved into speakers more than once in the past, but if you’re looking for something really unusual then how about an electrostatic?

27 thoughts on “A Transmission Line Speaker With The Design Work To Back It Up

    1. Rather than a box with baffles and a port, the internals of this speaker are specifically designed to be a waveguide. For practical reasons the waveguide is folded up, but it’s still there.

      You can then do various things with that waveguide – add dampening along the path, or choose a resonant frequency that adds or subtracts from your output audio.

  1. You guys need to do your homework. This has nothing to do with electrons moving through wires, but waves moving through the air. No, not all speakers are transmission lines. Or maybe they are, but with miserable impedance matching.

  2. Hi Old Guy! Davide here!
    Thanks for your comment, I’m looking forward to suggestions to improve the text in the project page. My focus till now it’s been to share my design approach but you’re right, I didn’t explained what actually is a transmission line enclosure.
    I will quote the same text from wikipedia, it is quite explenative.
    Hope you will follow the project, I’m going to share the “desktop version” (without bluetooth).
    Stay 3D ;)

  3. I built these years ago – they are superb. I still have them.
    P. Atkinson – State of the Art Loudspeaker – Hi-Fi News – April 1976
    I think there was a pretty good description of the design as well as how to build.

  4. Since this structure depends on the path being a quarter wave relative to wavelength it will necessarily be narrow in bandwidth; +-5% relative to center frequency.

  5. Eh. Kids.

    Don’t understand a term? Drag over it to highlight, right-click, select “Search [fave search engine] for ‘[term]’ “, et voilà!

    If I haven’t looked up at least two new terms every day, it’s a dull day (or I was offline and outside playing).

  6. I suppose if you wanted to make something with more mass you could use a cnc router to make sections out of wood and then glue the speaker cabinet together. If you have a cnc router that is.

  7. Actually,….. transmission line speakers were the standard from the invention of audio devices. I have an Edison standard model B gramaphone that has an original if somewhat beat up magnolia horn and the throat of the feedhorn almost perfectly fits an earbud. If you play the earbud without the horn you get expected results but you put the horn on it which efficiently couples the power of the earbud, I would guess about a 20 dB increase in volume and able to listen at a comfortable level throughout a medium sized room. No bass response below (guessing) 200Hz but surprisingly clear audio.
    Modern design finds the forward audio can be coupled more efficiently, like on the Edison, with a horn, and that using proper coupling and phase delay you can get a significant increase in bass amplitude by delaying the backside speaker audio to put it more in phase with the forward audio.
    Funny thing is that most serious speaker designs in the pro sound business and high end audio used feedhorn and transmission line coupling even from the early days. Keep in mind that the Altec Voice of the Theater was first manufactured in 1945. There have been a bunch of examples from huge JBL arrays to home stereo Klipsch horns, etc.
    Biggest difference between then and now is in 1945 they had to do the complex shape and distance, phase angle, etc using pencil, paper and slide rules.

    1. I was amazed what I heard out of a Sousaphone with a over the ear headphone up to the mouthpiece when was in a band room at a high school once. There was bass! Before Altec, they were Western Electric. Watch any movie from the 30’s. The breakup of the Bell empire started long before the 80’s. Their 200 watt Beachmaster PA guided GI’s onto the Pacific islands in WW2! Imagine being dug in and hearing us coming in!

  8. I built a set of large transmission line speakers about 20 years ago and I still love them. They stand about 1.2m tall and contain 3.6m of waveguide folded up, with concrete backed ceramic tile reflectors positioned on each corner of the folds. They were pretty easy to DIY with basic tools and carpentry skills, even easier if you had a saw table.

    The main drivers are 8″ and only 60w, but they produce an amasingly clean sound. Driven from a 100W/ch amp they have no problem filling the room even at low volume levels. There’s a 1″ tweeter installed too but they dont participate in the transmission line.

    If i ever built speakers again, I’d definitely stick with transmission line but maybe switch from 8″ woofer to 10″ and add a midrange, as the frequency response transition from 8″ to tweeter isn’t ideal. It would make the folding more complex though.

  9. The transmission line is really quite complex if done correctly particularly if the free air resonance of the woofer is considered. You could significantly increase bass response by making a properly (acoustic) impedance matched infinitely long transmission line or terminating the transmission line with a weighted dampener like the Epicure in the 80’s even though you’re dumping 1/2 of the speaker power.

  10. One interesting thing about 3d printed speakers is that it’s the one place where infill >25% is ligit. You want the resonance to be in the spaces, not inside the walls. They don’t necessarily need 100% infill, but 40% or 50% can go far.

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