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Transmission line


michael hurd
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On 2/2/2005 4:16:09 PM michael hurd wrote:

Has anyone here farted around with a transmission line as a subwoofer? I read an article a while ago in a home theater magazine, where they used a 13" woofer in a piece of sonotube as a subwoofer. If you built one say to reproduce 20 hz, you would need 13 3/4 feet of length, correct? (1/4 wave length)

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Well it's actually a little bit over 14 feet, but whatever. The transmission line will need to be that 14 feet, but this can be bent all over the place. I think the only important factor is that the diameter never gets smaller than that of the driver? The transmission line works on the same concept as having a baffle of 14 feet diameter around the driver...At the frequency that corresponds with the distance, the waves from the front and rear of the driver are in phase, which when implemented correctly increases the extension of the driver. I believe there are some variations on this where the TL is also a port tune to a frequency slightly lower than that of the TL which even further increases the extension. All the designs are bit more complicated than that, but it's the basic starting point.

This is probably the best resource you'll find anywhere on the net:

http://www.quarter-wave.com

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Guest Anonymous

13 feet would be pretty long, in theory you should be able to use 1/8th the wave length and get similiar results not that 6.5 feet or so would be much better... i am not sure though

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Thanks for the replies! I have been doing a little poking around, and I also operated a JBL Gt1801 ( 8 ohm car sub with a listed 28.8hz fs ) in one end of a piece of 375 mm sewer pipe 41.5 inches long. This is for my diy subwoofers for the apartment, just have not mounted the other endcap with 4" flared port, and done any tuning of the port length.

So, this is a straight tube 41.5 inches long, with a driver mounted in one end with the other open. The driver radiating area is 55.02 inches squared, and the tube crossectional area is 106.139 inches squared. Surprisingly, even though it is not a closed or vented system, there still is quite a bit of control over the excursion of the cone, until the very lowest frequencies.

I know that it is techincally not long enough to be a transmission line, but I did this as a whim, and I was quite surprised at the results. Typically a transmission line length is tuned by the 1/4 wavelength of the fs of the driver. This would result in 9.809 feet for this woofer ( would have to verify fs first, could be off by quite a bit ).

This tube is only 3.4583333 feet long, which would correspond to a 1/4 wavelength of 40.84 hz.

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You might be interested in visiting www.passdiy.com for the "El-Pipo". It started with a 12 foot long pipe and two 18 inch drivers. Then there was a smaller version.

Fortunately he had a 12 foot ceiling. My neighboors are fortunate in that I have an 8 foot ceiling. Of course it could be set on edge.

Gil

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I realize this is way off topic,

But I'd bet Cornwall components in a good transmission line would really sing!

The old documentation from Speaker Lab had some info on LARGE transmission lines. I believe these may have been featured in some of the early publications like (but not necessarily in) Speakerbuilder.

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michael,

at first i thought your first post read:

"Has anyone here farted around a transmission line of a subwoofer"

that made me initially think the sub-interconnect could pick up on "bowel interference"3.gif

after i re-read it.....el-pipe-o

i love the name...ha, thanks for the link, an interesting read.

as a svs guy, the idea of a tube that big get me all sweaty.

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