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Quarter Wave / T-Line tutorial-UPDATED


Forevrbumpn

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So what kind of group delay is there with 1/4 wave t lines?

Pretty much the same as a ported box of similar size/tuning. The larger you make it, the more delay you get.

You can model T-lines in HornResp, that way you can see how much group delay there is and compare it to whatever other design you want.

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So what kind of group delay is there with 1/4 wave t lines?

I think one of the big advantages of transmission lines, back loaded horns, front loaded horns and tapped horns is that the phase shift is half as much as a ported box.

Here's why:

SCDB.png

In a ported box, the output from the back of the cone is 180 degrees out of phase. That's why ported boxes don't sound "tight." If you have a ported box tuned to 30hz, the output at 60hz will be significantly out-of-phase with the output at 30hz. This is important because bass notes have harmonics. IE, if you have a bass note at 30hz, there will be harmonics at 60hz, 120hz, etc. So the most important way to achieve "tight bass" is to keep those harmonics in phase. This is difficult with a ported box; using my example above, the fundamental at 30hz will be 180 degrees out of phase with the harmonics at 60hz. The 120hz harmonic will *also* be out of phase if the subwoofer crossover is set below 120hz. (This is because the 120hz harmonic won't be coming from the sub; it will coming from the front stage.)

I hope all of that makes sense. If you've ever wondered why your car stereo doesn't sound "tight", that explains it. I put my money where my mouth is; my subwoofer is a tapped horn and I'm crossed very high (200hz) to keep the phase response intact.

29082009480.jpg

In a horn or transmission line, the rear wave is delayed half as much. It's delayed ninety degrees. The reason is because the output from the back of the cone is 180 degrees out of phase, but it travels a quarter wavelength to reach the mouth. Put those two together, and you have ninety degrees of phase shift, half as much as a ported box.

Now, the obvious way to have tight bass would be to use a sealed box. But here's the cool thing; it's actually possible to make horns with flatter phase than sealed boxes. The reason is because sealed boxes have a sloooooow rolloff, and when they're rolling off, the phase is shifting. So if you juggle the parameters right, you can actually flatten out the phase of a horn to the point where it's flatter than a sealed box. Admittedly, it is fairly trivial to fix the phase of a sealed box using EQ.

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  • 1 month later...

I've always fancied trying a T line but could do with a little help with the maths as I struggle badly with it these days due to an illness that wiped a lot of my. The sub I'm wanting to use is a JBL GT3 12

Sd (cm2) 527

Mounting depth 6 1/4"

Fs 28.2

Qms 5.53

Qes 0.60

Qts 0.54

Xmax (In) 0.43

If i've worked it out right the line needs to be 10ft long & using an online converter cone area is 81.68sq" (sub measures 10" from centre of suspension on one side to the other) Now i'm stuck on how to work out the port area & how wide & high the line needs to be. I'm happy for it to be tuned so to speak higher to 34-35 Hz

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  • 3 weeks later...

your mouth would be equal to the cone surface area. so, if your sub is 6.25" deeep, make the line depth 7". if cone area is 81.6, you could make your mouth 7x 11.5"

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  • 4 weeks later...

Hi guys, I'm building a tline for a 12 inch sub, 30hz. It's a 2.86 meter line as I calculate it, and so I had two questions.

I understand the port dimensions need to be equal to or slightly larger than the cone, however I can't seem to find anything about its ratio. I'm wondering if I can have (begin extreme for arguments sake) a 1 inch wide port that's like 40 inches high, so a 40:1? Does the port ideally need to be square? Can it be 1:2 or 1:10, ect ect.

The other thing I'm wondering is how important the tuning is, does it need to be a 30hz if that's it's resonant frequency? Will I get away with 32 or 35 or 40hz, and simply have a higher peak?

Thanks a million for your help in advance and I'm glad there is such a great forum like this around.

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Hi guys, I'm building a tline for a 12 inch sub, 30hz. It's a 2.86 meter line as I calculate it, and so I had two questions.

I understand the port dimensions need to be equal to or slightly larger than the cone, however I can't seem to find anything about its ratio. I'm wondering if I can have (begin extreme for arguments sake) a 1 inch wide port that's like 40 inches high, so a 40:1? Does the port ideally need to be square? Can it be 1:2 or 1:10, ect ect.

The other thing I'm wondering is how important the tuning is, does it need to be a 30hz if that's it's resonant frequency? Will I get away with 32 or 35 or 40hz, and simply have a higher peak?

Thanks a million for your help in advance and I'm glad there is such a great forum like this around.

I'd maintain an 8:1 ratio, which is the same you should respect in slot ported enclosures to avoid compression. And no, you do not have to tune at Fs but I'm not a fan of tuning above Fs as the boxes I've done that were didn't impress me. But I don't like peaky bass, I like bandwidth so there's that.

Also, you can reduce the line area and save some space while also lowering the tuning of the line. I don't know how to calculate the decrease in tuning frequency but I've had a lot of success with reducing line area and having a clean sounding line. Just stay at or above 60% of Sd and you'll be happy.

Tapers also lower tuning and they increase the number you'll get on the meter, if that matters to you.

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Oh wow thanks heaps for your response.

I reduced my line to 10cms wide and it saved me plenty of room, I can tune to FS now without an issue, at a 6:1 ratio, thanks heaps for that.

I wont be tapering this line as its my first build, and I feel it will overcomplicate it. I will make the mouth were the sub sits larger than the other channels, and the channel after the first bend narrower than the sub mouth channel, but larger than the rest of the lines width, effectively tapering a little. This improves SQ from my understanding, correct me if Im wrong or this is miss applied.

I had another couple of questions if you can answer.

Im building a 30hz line for a German Maestro 3SUB12. Id be happy to email you a pic of my design direct if you shared your details, I can figure out how to post lol.

First I wanted to ask how many lines you have built, or seen built? Seems you El Jefe have a lot of experience and have a lot of posts as well. Have you considered writing a new post or guide? Rather a myth busting of tlines if you will. Seems the math is extremely simple, but there are a lot of myths out there. Rather than retype a lot of the same it would be good to be able to read through a concise updated resource with the standard questions.

Q1, how important is the scoop or flow of air, especially in regards to the first turn, from the sub air. Do the 45s really help direct air or is pressure? Im playing with changing the first 45 to being a smaller one, thus giving me more length in the box, I need to find another 45cms. Does using perfectly kerfed mdf offer outstanding performance over the 45s or is this of negligible advantage.

Do the number of folds matter, is there a sound difference between 2 and 20 folds?

Does one need to look at the frequency of the driver, as though the tline was a sealed box, as it suggests in other guides, or by this stage are we getting diminished returns.

Does one measure the distance of the line from the middle of the walls, or the shortest distance from woofer to mouth (as though a piece of string was fed end to end and pulled tight)

Im hoping to build this weekend, and have the design perfected by then, so as to avoid redesigning on the fly and having the project spill over to another weekend.

I appreciate any help or further experience you may have and wish to share, kind regards, Myles.

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Tapers don't improve sound quality in as much as I'm aware but frankly, since I haven't done a controlled comparison between tapered, consistent and horn lines I can't say that with any real authority. But I have built a lot of t-lines for drivers ranging from 3" to 18" and for sub bass as well as full range audio and I've never found any appreciable sound quality loss from any particular deviation from convention.

That said, tapers do tend to produce higher SPL than consistent dimension lines but as with anything, there is always a compromise. That compromise has been in the increased potential for "vent" noise, which is why I caution to maintain a 2:1 ratio that starts with full Sd or more. Other than that I haven't noticed any problems with tapers and I've done them for both subwoofers and full range loudspeaker systems.

As for the presence of wave guides (typically called 45s), I'm not altogether sure the importance of their roles, again owing to a lack of controlled comparison. I have, however, seen and heard a line that had no wave guides and while the system was definitely loud, it wasn't the best sounding setup. But I have to add the caveat that I didn't do any critical listening to it either. Of course, the vehicle that line was in is an older SUV with NO deadener and not much in the way of front stage presence so the poor sound quality could easily be blamed on rattles and poor frequency response because I'm definitely keen on the importance of sound quality.

As far as the importance of being precise with wave guides I don't see the utility in spending a lot of time making sure they're perfectly rounded or exactly the right size to make the line "perfect" either. I've built a few lines (not for paying customers) where I was only concerned with speed and saving materials and in those cases the wave guides were all just scraps that came close to being the correct size and I couldn't complain about the sound quality from them so that's the evidence I have for these comments, for whatever it's worth.

Measuring a line is no different than measuring a port, in that the center line of the cavity is the actual measurement. So, making your wave guides smaller won't change the length but, the worst thing that could happen by doing so (in my opinion) would be that you may create some turbulence in the line. But again, I think that would be minimal if audible at all.

I haven't found the number of bends to affect the SQ of a line but physics kinda tells me that there is a point at which it would negatively affect the sound quality but that's probably a lot longer than we'd ever see in car audio. Not to mention, some early iterations of t-lines in commercial audio are very large and long and they sounded amazing.

Using the sealed volume rule of t-line design as well as the recommendations for driver suitability for a line is in my opinion, pedantic, when we're talking about sub bass lines. Such is the same for obsessing over wave guides, as bass is much less localizable than high frequencies and slight variations from "perfect" performance will be inaudible. This is why I always say that a line is the absolute best alignment for a driver, regardless its particular TS specs. And the only exception to that "rule" is in purpose built SPL applications.

Lastly, here's a thread with pictures of a lot of the lines I've built. I haven't updated it since January and I don't take many pictures of a lot of the basic enclosures I build so this isn't a complete representation of all the lines I've done but there's a good variety represented.

http://www.stevemeadedesigns.com/board/topic/202121-random-t-lines-from-audio-anarchy/?hl=+audio%20+anarchy

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Also, I have a t-line page on Facebook where much of what you mentioned is discussed by myself and others. It may be of some use to you as well.

https://www.facebook.com/groups/990108914350137/

Facebook: facebook.com/audioanarchyllc

Instagram: audioanarchyllc

Youtube: youtube.com/bbeljefe

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