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Old 15th July 2004, 04:29 PM   #1
Nuuk is offline Nuuk  United Kingdom
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Default TL sub - line exit options

I am building a couple of TL subs using 8 inch drivers that I already own (this is until I can afford to build my dipole subs).

I am planning on each TL sub sitting behind my open baffle speakers and have come up with the design below which avoids too many 180 degree bends.

Click the image to open in full size.

As you can see, the line end is in the middle of the box and has to exit through the side. My question is this. What would happen if I cut an exit aperture in both sides instead of one?
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Old 15th July 2004, 06:06 PM   #2
Nuuk is offline Nuuk  United Kingdom
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Where's my picture gone mods?

It's come back again - even more
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Old 15th July 2004, 07:48 PM   #3
Greg B is offline Greg B  United States
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I'd be more worried about the last bend being 180 degrees right in the maximum velocity zone. IME, simpler is better when it comes to TL bends. Can't you get your target line length in a single or double fold? Maybe a spiral pattern would work; like some of the japanese backhorns.

I don't know what drive you're using or what BW you're aiming to reproduce, but a TL that long is likely to act more like an IB than anything, so it may require some EQ depending on the room and placement, etc. (less than a dipole)

GB
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Old 15th July 2004, 08:32 PM   #4
Nuuk is offline Nuuk  United Kingdom
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My'problem' is trying to fit everything in behind the open baffles!

The drivers are 8 inch Seas P21HEX and I am aiming for a line length of 86 (1/4 wavelength @ 33Hz). The subs will fill in up to around the 70 Hz point and I have been advised this makes the usual 'rules' on TL design not so critical.

I could probably just get away without that last 180 degree bend if it makes an improvement.

Thanks for your advice.
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Old 15th July 2004, 09:47 PM   #5
Svante is offline Svante  Sweden
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Default Re: TL sub - line exit options

Quote:
Originally posted by Nuuk
As you can see, the line end is in the middle of the box and has to exit through the side. My question is this. What would happen if I cut an exit aperture in both sides instead of one?
I'm no TL expert, but as I understand it the line would appear a bit shorter, and the velocity at the exit(s) would drop.
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Old 15th July 2004, 09:54 PM   #6
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As long as the total exit area is correct it doesn't matter much. However, it looks from your picture that you don't need to do so- but I can't tell how wide the box is.

The 180 bends will have no effect on the long wavelengths that pass the line and in fact they are beneficial as LP filters against the higher frequencies that you want to damp out anyway.
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Old 15th July 2004, 10:29 PM   #7
JinMTVT is offline JinMTVT  Canada
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i think that you forgot that the wave won't be matched if the distance of the opening and the front of the speaker is not also tunned nah ?

that is why striaght line TL works better than folded type

for the openning i'd say let it open on both side and see how u like it!
sure would look coool
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Old 15th July 2004, 10:40 PM   #8
Svante is offline Svante  Sweden
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Quote:
Originally posted by BillFitzmaurice
The 180 bends will have no effect on the long wavelengths that pass the line and in fact they are beneficial as LP filters against the higher frequencies that you want to damp out anyway.
I thought so too, but did not dare to say so.
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Old 15th July 2004, 11:01 PM   #9
Greg B is offline Greg B  United States
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In theory, sure, it's not supposed to make a difference. In practice, I find that it does. I built a TL once which I later decided needed to be ~12" longer. This was a single fold type. I tried both 90 and 180 degree extensions - either way was going to be dorky looking. The 90 degree one had considerably more output. This was years ago, I don't have numbers on it anymore.

As I implied before, my theory was that this was because its location in a high velocity zone of the line, where aerodynamic drag becomes a factor. That's my theory anyway, I reserve the right to be wrong.

Anyway, I'm going to have to disagree with you gents until someone convinces me otherwise.

Regardless, I strongly recommend sim'ing the TL in MJK's mathcad model, if this hasn't been done already.

GB
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Old 16th July 2004, 03:15 AM   #10
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You might get more output from an unstuffed or understuffed line passing midbass frequencies with 90 bends but below 100 Hz a 180 bend is no obstacle, for the same reason that subs radiate omnidirectionally below 100 Hz.

I see nothing fundamentally wrong with the design; it's actually quite ingenious and very much resembles the inverse case, a coiled folded horn, if you swapped the driver and vent positions.

I don't have P21 HEX specs, but the P21 REX with Qts .37 and Fs 33 Hz would give an F3 of about 66Hz, so it's far from a perfect match unfortunately. If the P21 HEX has a higher Qts things would be better.
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