transmission line sub

***** this is a work in progress and will be updated as I go ***** The goal of this thread is to document my all-out attempt to build a transmission line subwoofer, one of the most difficult yet rewarding enclosures. It's worth mentioning a thread started by aznboi3644 and the replies really intrigued me. I hope those interested in a TL box can learn from my experience. There is very limited knowledge on how to properly build one, this is no simple sealed box we're talking about! Nevertheless, here we go. Design goals:

• frequency response from 20hz to 60hz at +-3db
• response must blend seamlessly into component speakers up front, including cabin gain
• high efficiency woofer and enclosure to minimize amp power
• class D amplifier for high efficiency, low heat, small size, and small power cables
• installation must be clean, and enclosure must be production quality

Steps taken:

• measure cabin gain of my vehicle and graph it
• design hypothetical subwoofer to compliment measured cabin gain
• select high efficiency woofer with low Fs to help achieve goal of 20hz and low power requirements
• select amplifier to compliment the woofer in power rating and crossover frequencies
• research to find useful information about building the transmission line (won't be easy!)
• design transmission line with cabin gain, woofer properties, and physical limitations in mind
• build enclosure striving for production quality
• install amp and enclosure, again striving for top quality
• last, measure frequency response, and celebrate!

--- Step the first: measure cabin gain of my vehicle A subwoofer sitting on the shelf in a store will sound different when installed in your car, and it will sound completely different if sitting out in the open or in an anechoic chamber. Ever notice how subwoofers in an electronics store are usually contained in a small room you have to walk into in order to listen to them (hint hint)? The interior shape and size of your car, the materials used to build the car, and any stuff you have inside it will affect the frequency response of the subwoofer. Each car is unique in this respect, and so is every dining room for home theater installations (my other hobby). This effect is called cabin gain. To measure how my car will affect the response of my transmission line, I needed to run a few tests. First, I placed my home theater sub (a Bowers & Wilkins ASW600) on a chair in my backyard, careful to make sure it was not near any large walls or objects that might alter the response. Then I played test tones and measured the response of the sub out in the open, which is as close as I can get to an anechoic chamber which is what all enclosure simulation and calculating software assumes. Then I repeated the test placing my ASW600 sub in my car facing rearward like I plan the transmission line to. I used a Belar spectrum analyzer connected to the best microphone I could find for this test. It is a Sennhieser 421 which has a frequency response rated down to 30hz, the lowest I can get my hands on. This is important to remember, any values below 30hz must be accepted with a grain of salt, they are not as accurate as the higher frequencies. There is a microphone preamplifier between the Sennhieser and the Belar, which should be transparent to these tests. The mic was placed one foot in front of the woofer in my backyard, and was sitting on a small mic stand in my front seat when the sub was in my car. After many attempts both inside the car and out, waiting for the wind to die down and averaging the data over many minutes, I came up with the following two graphs. And here's the result I was looking for. This is simply the difference of the two lines in the previous graph, and it shows how my vehicle affects the frequency response of a sub. So, what does this all mean? Looking at the first graph first, the sub placed in my backyard has a fairly gentle roll off from about 55hz and lower. When placed inside my car the response peaks sharply at about 55hz and rolls off in both directions. The data I want from this test is in the second graph. Other than a peak at 28hz, there is a rise of 8db from 20hz to 60hz. The slope from 60hz and up can be disregarded because I will have a low-pass filter on the amplifier anyhow. Good to know! Now I can keep this in mind going forward, to make sure my transmission line compliments the cabin gain of my car. Step the second: design hypothetical subwoofer to compliment measured cabin gain

keep in mind, this is a work in progress, please don't reply just yet. space reservedMalcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.

Step the second: design hypothetical subwoofer to compliment measured cabin gainLets start with a picture.Or not, I'm not sure what the issue is, I can't see the picture on my preview window here. Anyhow, what I'm trying to show is the measured cabin gain, and the response of a hypothetical sub I want to build. From 20 to 60hz, the cabin gain increases by 8db. So my sub would have to decrease by 8db in order to have a flat response when installed in the car.Having such a low frequency peak and roll off on the high side is not easy to do when building a sub, and very difficult with a sealed or vented enclosure. A bandpass might be able to, but would need a resonant frequency terribly low and the enclosure would grow huge in size. The transmission line however, can extend the response of a woofer well below it's resonant frequency, and be really efficient while doing it. Have you ever heard of a single 4" woofer achieve response down to 35hz!? It's been done, there are a few examples here and here So, expecting a response down to 20hz from a 10" woofer should be no problem, if properly designed!On the subject of cabinet size, it has to fit in my car. I'd like to not sacrifice all of my cargo room for the box, but I know to do this properly will require a decent amount of space. The box must also be removable in case I need the cargo room. After taking measurements in the rear cargo area of my Jeep behind the back seats, I came up with the following: 34" wide, 33" deep, and as short as possible. The height of the box will be influenced mostly by the woofer choice, how large the woofer is, and the internal volume I need for the transmission line. Thus, the height will be a variable I can play with if I need more or less volume for the transmission line.So there we have it. The hypothetical subwoofer will be 34" by 33" and some unknown height, removable, and have a frequency response that peaks around 20hz and slops downward towards 60hz. Of course it must also be production quality and look beautiful. On to the next step.Malcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.

If you want some a nice modeling program go to Quarter Wave it costs some money but its good. only thing is that knowing how the response of the TL will be inside the vehicle is hard to determine. I installed my TL from my thread you mentioned in my friends Oldsmobile Bravada...the difference in the response was crazy. Same sub same amp...the enclosure in my Explorer raped it hands down. Still sounded good but no where near as flat or loud.

Been there... Done that... azn is right, though... Even with a properly constructed TL, your response WILL change. Dramatically. The saddest part about a TL, is there really isn't any tuning, per se, to it. It's all going to rely most heavily on cabin (which you are trying to address) and EQ.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

That's why the best way to "tune' a TL enclosure, in my opinion, is to install it with an open line (whether in room or in car) set up your test gear and RTA and start stuffin'.  Support the12volt.com

aznboi3644: Yep, that's what I'm using as a base to build the box. King's work is really nice, I have a lot of respect for the guy. Expect some screen shots of his worksheets in action! haemphyst: Nice thread! Why I couldn't find it a month ago when I started this project is beyond me... Did you ever post the RTA measurements? That is something I'm very interested in. You say you hit just over 110db using 1kw of power? Or did I read that wrong? I'm aiming for something highly efficient and I'd love to see your numbers. DYohn: I'm building the box right now actually, should have a rough skeleton done tonight. All the stuff I want to write is lagging behind the build though, otherwise you'd see all the tweaking I've been doing with King's worksheets. But yes, agreed, I'm starting with an unloaded line and plan to add stuffing later if needed. The simulation shows a near perfect match to my cabin gain without stuffing, so perhaps I won't need it. We'll see! More posts to follow, but now it's time to earn some money keeping my radio station on the air : ) -JustinMalcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.

yeah I plan on making an SQ t-line soon once I get some time...I plan on making it folded only once like Hams then stuff it to try to get a flat response. Stuffing my T-line is not that easy with the way it is folded.

I wanted to keep writing tonight but it's after 2am and I've got a big day tomorrow including a photography credential for the Sharks game that needs to be abused a little bit. I love my job! Saturday though, I'll be back with lots of writing, build pics, and some first impressions. The wood glue is drying right now. Good night. -JustinMalcom: "This is the captain. We have a...little problem with our engine sequence, so we may experience some slight turbulence and then...explode."

Jayne: "We're gonna explode? I don't wanna explode.