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The Tradenda Transmission Line Enclosure


ElliottDesigns
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This is the transmission line design I have so far for the bass section of my loudspeaker, which I have recently named the Tradenda, tradenda being transmission in Latin.

I'll post a few images to explain the idea.

Schematic

^ This is the schematic of the enclosure, the circles essentially represent where the loudspeaker sits. The dark grey area represents a certain density of mineral wool for absorption.

image

^ This is the estimated frequency response of the GRS 8sw-4 in the enclosure at the 50W signal I will be providing the driver. I know it's very peaky, but luckily that can be solved with DSP, which will reduce the volume down to about 88db, however, luckily this is modeled in 2pi, or half-space (the equivalent of the speaker sitting in an infinite grass field). So it's essentially the worst-case scenario in terms of output, so I'm pretty happy with that.

As for the higher resonances, most of those should be quite severely reduced in the fact that there will be a 180-degree bend in my transmission line, which has proved to damp higher-order resonances in the line quite well. This should also give us back a lot more high-frequency gain.

I will also be experimenting with thin layers of absorption on the walls, but I have a feeling all this will do is reduce the output and not much else, still worth a try though.

As you can see I have used the fact that the driver has such a high Qts to achieve a very low-frequency output, which when EQ'd should hopefully be flat down to 20hz. Not bad for an 8" driver, eh?!

image

^ As you can see, even at 50W (my planned wattage) we are still below the 6mm xmax for this driver.

image

^ We are also below 10m/s for mouth velocity, which is literally inaudible, nowhere near chuffing territory.

image

^ Group delay may seem bad, but 30ms is the point at which group delay becomes audible and even then I plan to delay everything somewhat using my FIR filters to flatten the group delay. As for the giant peak at 20hz, that is pretty common anyway, but with everything getting flattened anyway the difference in group delay between 20hz and the rest will just be about 35ms, and it's often said group delay is much less noticeable at low frequencies.

I think that's everything, other than the fact it might be smaller than what you'd expect for 20hz. The line opening is just 52cm^2, with the maximum line cross-sectional area being 500cm^2 (about 22cm x 22cm, unless the fold makes it wider which is a possibility). Also, the height will be about 1m since it's folded.

Ahah, yes. Just a quick sim of the power response if put between two walls (in a corner). This does however still disregard all other walls, this time the floor and ceiling too.

image

In fact this is probably louder than this speaker will ever run, an EQ'd state of about 95db, with the fact that due to the Harman in-room curve my bass will be 6db higher than the rest anyway, it means everything else is at 91db. 85 is too loud for me already XD. I think I'm happy with this, lets just hope that when I build it, it comes out this good.

Hope you enjoyed looking through the sim of my design!

Elliott

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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ElliottDesigns
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I was wondering if anyone else here has made a transmission line before, and if so, how close was it to your simulation?

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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TVOR-Ceasar
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The calcs I have only require the Fs and driver size. See attached:

image

Sorry it's all in imperial.

-Charlie
https://www.youtube.com/user/TheTrueVoiceOfReason


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TVOR-Ceasar
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Now if you want to drop that down to 20 Hz:

image

BTW, what are you using, HornResp?

-Charlie
https://www.youtube.com/user/TheTrueVoiceOfReason


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ElliottDesigns
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@tvor-ceasar Yep, it's hornresp, and it was a lot of trial and error to get that output down to 20hz without getting too funky of a response whilst keeping excursion and velocity below distortion points. I was just wondering about any other transmission line builds people have done.

Also, you'll find my transmission line design isn't very standard, hence why I had to use a program like hornresp rather than one that uses standardised geometry such as yours. I also used my own number for the damping material since I know what I'll be using. Seems like your program is great for getting quick results though!

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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ElliottDesigns
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To elaborate, it's the combination of the filled chamber behind the driver and the tapered nature of the transmission line that allows for the lower extension and lower velocity at the given wattage. The absorption also does wonders for the frequency response and even higher resonances within the line. That's where most transmission lines go wrong you see, nearly all place the driver at the start and don't consider tapers either.

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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TVOR-Ceasar
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That's the wonderful thing about this particular simple equation, it gives you your main box volume that the driver reacts to and the average overall cross-sectional area and the length of the "port"/line, for what is a horn but a specialized port. As long as you have the average cross section from start to finish, you can taper/flare the line. Generally it would be something along the idea of hinging on the center point and making the entrance smaller and the exit wider. As for the fill, what my calcs say is to put that part of the fill in the line area to help slow / dampen the energy in order to reduce all that fractured energy that can happen in such a long outlet. And if course, you can stuff the main box as you see fit. 

Just like anything else in this hobby, it's a scientific art. And only through practicing our art can we refine the science needed to make the art that much better.

-Charlie
https://www.youtube.com/user/TheTrueVoiceOfReason


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TVOR-Ceasar
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Yeah, I had another page open on my phone that had a horn calculator from a major manufacturer, but it got shut on me and I lost the page. I've been unable to find it again. Oh well. 

-Charlie
https://www.youtube.com/user/TheTrueVoiceOfReason


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ElliottDesigns
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No matter, I think I'm in the situation where I'm happy with my design anyway. So now it's just build and measure! Might be a while till it's built though, had someone crash into my car last Friday and the insurance is taking forever because the other party counterclaimed 🙄. So yeah, materials are stationary for now, but the next update will either be a cad model or a build process update.

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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TVOR-Ceasar
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@elliottdesigns Re: accident. Man, that really stinks. At least you are alright. Hopefully the Insurance Co. settles quickly and fairly.

-Charlie
https://www.youtube.com/user/TheTrueVoiceOfReason


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ElliottDesigns
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@tvor-ceasar Yeah, hopefully. Thanks Charlie

Elliott Dyson - Mechanical Engineering Student and 3D printing & Design Freelancer


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