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School me on T-Line

14K views 45 replies 17 participants last post by  SkizeR 
#1 ·
So I have always wanted to experiment with building a T-line box but really don't know much about them other than they can be loud. I have this ID8v3 D4 I'd like to experiment with. In someones post they were building to the speakers Fs and people chimed in to note that it wasn't the parameter to go off of. Is there a really good calculator out there? If someone could educate me on the science behind this it would be very appreciated.
 
#2 · (Edited)
heres a little something. i think this is from Brandon Buck


DESIGNING YOUR T-LINE
This will be a quick and simple primer on how to design a consistent dimension ¼ wave t-line. There are a variety of different ways to build t-lines but this will be an explanation of what has been, for me, the easiest method. For the example, we'll use a single, hypothetical 15' driver.
First thing we need from our sub is Sd or, usable piston area. To find that we need the diameter of the driver, which is measured from the apex (center) of the surround on one side of the driver to the same point on the opposite side. For most car audio 15s, that measurement is 12.75".
Then we take that measurement and half it: 12.75 ÷ 2 = 6.375
Now apply the formula πR² to that number: 3.14 * 6.375 - 6.375 = 127.6
For our purpose, that number can simply be rounded to 127in².
Sd will be out line area. Line area is what would be called port area in a conventional vented enclosure but since our box is nothing but one long port or, line... it's called line area. This number can be modified for a couple of different reasons. First off, if your design is strictly for sound quality, I'd leave the line area equal to Sd but if you need to save space or you want a bit more of an SPL oriented box you can reduce that number by up to 40% with no serious affects on sound quality. What you will lose, in my experience, is performance on the top end of the sub bass region (60-100Hz). But, that loss is minimal, especially in a vehicle cabin. Aside from the aforementioned benefits, reducing line area also seems to increase cone control, which is really important for us bassheads since we tend to abuse our subs on a daily basis. smile emoticon
Next we have to calculate the tuning frequency of the line. For that you need to know the speed of sound in feet. That varies with elevation and other factors but for my area and conditions it's usually 1130 feet per second so, that's what we'll use. You can find it for your area if you like but for the purposes of what we're doing there is no need of that much accuracy. We'll tune our line at 30Hz, which is the most common tuning I use for daily builds. The formula is speed of sound ÷ tuning ÷ 4. This formula tells us the length of a sound wave at the tuning frequency we chose and then we take ¼ of that length for out box... hence ¼ wave transmission line.
For our box: 1130 ÷ 30 ÷ 4 = 9.41'
To convert that to inches: 9.41 * 12 = 112.92 or, round it to 113".
Now that the easy part is over we have to figure out a way to contort our 9' long square tube around and into the trunk or back seat of a vehicle. That's where line area reductions become pretty handy because even though the line won't get any shorter, its bulk will be lessened. If you have a large footprint but not a lot of height, I would recommend a snail shell styled box with the sub(s) in the center facing upward and the line terminus (port) facing out the side or rear, as in a trunk. If you have a wide but narrow space for the box, like the back seat of a truck or behind the back seat of a truck, a folded design with sub and terminus up works pretty good. Either way you're going to have some 90° and/or 180° turns to make and below I'll explain how I calculate the length of those turns.
The formula for circumference is πD or 3.14 * diameter.
For our box we'll assume the line dimension is 14" wide by 9" tall. I arrive at that measurement by dividing our needed line area (127in²) by the mounting diameter of our typical 15" driver (14") for a height of 9". Knowing that the line is 9" tall is important because that's the number we'll use to measure the length of our bends. Next we'll ad the thickness of our construction material to that number, which is normally 3/4" so... 9.75 is the diameter of our radius bends.
Now apply the formula to find the length of any 90° or 180° bends....
9.75 * 3.14 = 30.615 or rounded to 30.6. That's the circumference of our radius.
For 180° bends, divide that by 2 for: 15.3".
For 90° bends divide by 4 for: 7.65"
So now all you have to do is draw the line out and enter those numbers into any bends you have and then add them all up (along with the straight lines) to get the total length of your line. For a simple box I normally take my two most critical measurements and then start arranging my lines and bends through the center point of the line height until I find an arrangement that'll work.
Lastly, when figuring wave guides (45°s) for the bends, just measure from the pivot point of the radius at a 45° angle out to the desired line height and then measure across the line 90° perpendicular to get the width of your wave guide.
 
#4 ·
Yeah, that's waaaay too easy, and concise, to understand. :D
Thanks SkizeR!

I remember trying to build one, when I was about 16.
There was literally no math involved. :blush:
I just tried to copy a picture of one, I saw.
I had a pair of Fosgate 8's, and figured "it can't be that hard"....
It sounded absolutely terrible.

Oh well, it was fun, and all I wasted was a sheet of particle board. So about 10 bucks.:laugh:
 
#5 ·
This thread needs stickied!!!!!

A point that was not touched on about the Fs, you don't want to tune below that is what I was told but that it is also the optimum tuning point for sq and output.
 
#6 · (Edited)
I have a feeling you're talking about my thread. Those guys were FOS, so I stopped replying to them. As the person above me said, tuning to the Fs is the optimal tune for a T-line regarding SQ and output.

It's not all that hard, I can summarize that long winded post above:

T-line = quarter wave transmission line box

The "line" has the same area as the subwoofer's Sd and the same length as 1/4 the wavelength of the Fs or tuning frequency.

At the bare minimum you will need to know the woofer's Sd and Fs.

To figure out the length of the line, you will need to divide the speed of sound by the Fs. The speed of sound is 1128feet per second, so for 28hz, 1128/28 = a 40foot wavelength. Divide that by 1/4, so one quarter of 40feet is 10feet, so a 28hz 1/4wavelength line will have to be 10feet long, and have the same area of the Sd of the speaker.

Once you know the line length and area, simply all that's left is designing the box how you want. It's pretty simple and the outcome is more than worth it.



This is the giant 25hz T-line I'd have to build for my Eclipses. Might have to build it since everybody wants to recommend box design instead of driver selection like I asked... sheesh.
 
#9 ·
I wasn't asking for either. I was asking for recommendations on a driver with a low Fs, and I want it tuned to the Fs because I do care about SQ.

Essentially, the goal of the transmission line is to minimize acoustical or mechanical impedance at frequencies corresponding to the driver's fundamental free air resonance. This simultaneously reduces stored energy in the driver's motion, reduces distortion, and critically damps the driver by maximizing acoustic output (maximal acoustical loading or coupling) at the terminus. This also minimizes the negative effects of acoustic energy that would otherwise (as with a sealed enclosure) be reflected back to the driver in a sealed cavity.


That is the whole point of tuning a T-line to the Fs of the driver.
 
#8 · (Edited)
I actually found that to be a good read. It got a bit muddy at the end though. Where is the 9" tall line measurement come from? If the chamber is 14" x 9" that's more area than the calculated 127 in2. How do you determine the depth of the speaker compared to the start of the line? Why would the measurement be 9.75" if using .75" build material. Shouldn't it still maintain its 9" measurement? Why not save space with 1/8 wave? (not sure its possible but just asking)

There was no talk about the tapered line or horn or what ever its called when you increase the width of the line till exit.

Yes, Rollin I was referencing your thread but didn't want to jack it with my curiosity. I like your simplified explanation though.
 
#10 ·
Here is a document with everything you will ever want to know about designing a T-line.
http://www.quarter-wave.com/TLs/Alignment_Tables.pdf

Better yet, here's a link to a T-line modeling program. Enter the specs of your driver, and see first hand how all the variables contribute to its response.
Download | Leonard Audio

In summary, don't listen to anyone who says you must tune a TL to the driver's FS to get the best results. They are simply regurgitating a loosely created "one size fits all" rule of thumb that has spread around the internet long enough that it's become lore.
 
#13 ·
!



Thank you for the links. That was easy to get lost in the info. :D I was playing with the software but got stuck when trying to generate a model with the speakers parameters. I'll have to contact the group to see if someone can help me along. Maybe I'm viewing the software wrong? The impression I had was I could input the speakers pararmeters and a design model would be displayed with theoretical performance data.
 
#11 · (Edited)
All I know is that t-lines used to be hard to implement in the automobile since the older modeling software assumed a near anechoic environment with regards to their response plots. Now factor in the cabin gain typically common in the automobile and there is a large margin for error when designing and implementing a t-line in the car.

OTOH, I'd use a t-line inside my home or for PA in a heartbeat. I just think the trial and error aspect for the mobile environment isn't worth it, nor is the size tradeoff typically required for a t-line. Then again, that is just me. With regards to car audio, I'm all about taking the easy path these days because I just don't have the will or the time to dedicate to perfection in an imperfect audio reproduction environment.
 
#14 ·
I value my cars cargo space so T-line is certainly not going to be permanent but more of a fun experiment. Sure I have enough projects, but one more isn't going to kill me. Once I gain the knowledge and feel confident I know what I'm doing I would like to make some house speakers. That's really the motivation to this project. I been using a Philips 5.1 in the house for the past 10-12 years and always wish I had something good.
 
#15 ·
I've done two different versions over the years.
1st was a two 10" version about the size of a washing machine in a Toyota 4 runner. Finally settled on some 10w6v1's. Thing got seriously loud with relatively little power. @ 450 watts. Folks were shocked to hear 2 10's that loud. Of course this was the late 90's.

2nd one was a single 8" sundown. A long narrow one. Again with about 400w. Think I tuned it to @ 35hz. Pretty loud, but didn't do much above @ 60hz. This was in my impala SS.

If you have the room they are a lot of fun. With power being cheap nowadays, and small encolosure subs the norm, they are kinda outdated. The two ten inch box I had could of easily been replaced with a modern duel 15" encolosure with a class d amp.
 
#16 ·
I built a transmission line with a couple of those Adire Shiva clones that Parts Express sells. (I think they're called "DVC 12s?")

I wasn't real thrilled with the box - here's why:

Hoffman's Iron Law is going to dictate how much output your box generates. IE, you might be able to increase the efficiency of a box by a few dB by building a transmission line, but you can also increase the efficiency by simply using more boxes.

So it boils down to one question:
Do you want to buy four subs or do you want to buy one?

If you're cheap, you CAN raise the efficiency, but keep in mind that an array of sealed boxes will always win because they have more voice coils, more power handling, and more displacement.

Basically a TL is a way to squeeze more output out of a single woofer, but an array will win.

Here's some examples of what I mean:


Here's the output of four sealed twelves in a box that's 87.9 liters. (3.1 cubic feet) The woofers are diyma twelves.


Here's the output of TWO diyma twelves in a transmission line that's 87.9 liters. (Same size as the sealed box, but with half as many woofers.)


Here's the output of the two boxes overlaid. You can see that the response curve is very very close. This is Hoffman's Iron Law - the box size is going to dictate your output, not the alignment.

Now, obviously there are a lot of reason you might want to build a transmission line:
1) Main reason is cost. You can get more output from a single woofer.
2) TLs, FLHs, BLHs and THs allow you to get more output with the same power. This isn't a huge deal in an era of $300 kilowatt amps. But it *can* give you the edge if you're limited by your alternator. A real-world example of this is that every other concert I go to lately has a power outage, because the buildings aren't capable of delivering enough amps to power the amps.
2) The TL is going to reduce excursion, and if you design it real carefully, you can use that to increase output at high power. (The sealed box will still win, due to lower power compression, higher power handling, etc.)
3) One huge factor is that you can use these weird alignments to get F3s that would otherwise be impossible. For instance, I once built a tapped horn that played down to the single digits. It's pretty easy to play a full octave below resonance with a TL, TH, back loaded horn, or bandpass.




Here's the hornresp model if you want to play with it. Hornresp can model all of these alignments. Note that both boxes get the same wattage, but the *voltage* is different because the sealed box is 4ohm, and the TL is 2ohm, due to the voice coils being wired in parallel for the TL and series-parallel for the sealed. Bottom line : same wattage, same box size, same output (for the most part.)

 
#17 ·
Over on diyaudio, I updated my thread about my 15hz tapped horn. Makes for an interesting comparison to the TL in this thread.

Read more here : Night of The Living Bassheads - Page 9 - diyAudio

(Read the whole thread if you have the time.)


Being able to hit 110dB at 15hz is quite fun for home theater, and it's even better when you can do it with one 12" and 250 watts.
 
#21 ·
His post was blunt, but he makes a good point. A Tline CAN be used to reduce the Q of the enclosure.

TBH one of the gripes about the Tline that I built was that it was a little too 'polite.'

But I've learned over the years that distortion can actually sound kinda good, and systems with low distortion often sound too 'polite.'

I never ran any distortion measurements on my Tline so I can only speculate.
 
#22 ·
Side note:

Here's a couple of shortcuts that I use when building enclosures that use the back wave to augment the front wave*:

1) tune the enclosure to .707 of the FS
Or
2) model the woofer in a vented box, and use the FB that the computer tells you

Using these rules, you generally find that a 30hz sub wants to use a driver with an FS of about 42.3hz.

* transmission line, back loaded horn, vented box, dual reflex bandpass, tapped horn, etc
 
#29 ·
So I was using that software you linked yesterday and this is what it came up with. Its not working today for some reason. Are you saying I should change the Fs to .707? What would be the reasoning behind that?

BL 7.698
Sd 0.0231
Qes 0.367
Qms 3.572
Rms 3.1221
Re 1.95
Fs 24.7
Vas 0.04389
Mms 0.0718585328870082
Cms 0.000578
Lvc 0.00148
 
#25 ·
Please don't scare off the people who are actually interested in this post.. We gather here to share information, then based on the cumulative we're able to form our own ideals. It doesn't matter if you think you're "more right" than the next guy. Maybe to you, but the rest of us are annoyed by nitpicky argumentative BS
 
#41 ·
Y'all can find some better than average info here: TL Design Basics

But I have to also add this amusing quote from 2008?

I'm not even sure that I know what a "tapped horn" is but here is what I do know:

I've studied LF sound sources my whole life and there are no free lunches. Nothing has ever turned out to be an effective improvement in LF design except the Acoustic Lever and its just plain impractical.

Horns at LF are a joke. There is no such thing as a LF horn, unless its the size of a house. Smaller than that and its just a port and ports have been studied to death; series, multiple, transmission lines, its all a bunch of hand waving because when the wavelengths are ten feet long details simply don't matter.

A horn - at best - has an impedance transformation of the square root of the inlet and outlet areas. A lever goes as the square of these areas. Thus for a 2:1 output to input area ratio the lever is twice as effective as a horn. So much for horns.

I'm sorry, but I'll stick to my belief that subs are simple sources and closed boxs work great. Or if you want to get fancy, use a bandpass to lower the HF distortion and increase the efficiency (a little), but anything beyond that is simple "blowing in the wind".
__________________
Earl Geddes
 
#43 ·
I think the Geddes statement is accurate.

Here's an example of this:

A few years ago, I built a clone of the Auto Tuba. I loved it. Punchy, dynamic, and clean. But it didn't play low at all.

So I measured it, and noticed that it's efficiency advantage was purely above 60hz.

IE, above sixty Hertz it shined. But below 60hz? It basically behaves like an 8" woofer in a sealed box.

That's the conundrum with all of the exotic alignments. You can definitely raise output and lower distortion. But there's no getting around Hoffman's iron law.

I still think they have their place, but I'm not so sure that car subwoofer duty is on the list. Perhaps if you have a lot of space and a small budget. I'm going bandpass for my Mazda.
 
#45 ·
Perfect thread! Been wanting to build a t-line (home) for a long time, forgot everything i knew about them, and all these cool new drivers keep hitting the market!

Would use either Tang Band 8" full range or Dayton 8" full range point source, a small audio research amp to drive them, and I'd expect to greatly enjoy them! Hell i might even go all the way and make 'em look nice - a first for home speaker experiments for me! A tall, narrow cabinet with a single 8" driver looks sick in my head...
 
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