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Old 2nd April 2007, 03:44 AM   #1
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Lightbulb New Compact TL Project

My plan: replace my un-scientifically designed (but good looking) rear surround speakers with new carefully designed ones. The present ones are a Voight pipe kind of guess work mis-mash that I concocted a few years ago. Without any good reason, they sound half descent. For us who are never satisfied, that's not enough though. Given the excuse to start today with everybodies favourite material: baltic birch plywood, I jumped in.
First, I'll show what I am replacing. The drivers are Vifa P13WH0008 and Seas 19TAF. A great combination IMO.
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Old 2nd April 2007, 03:54 AM   #2
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What I have in mind is the same size box, but a tapered line inside:
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Old 2nd April 2007, 04:29 AM   #3
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The line is about 65 inches long, with roughly a 3:1 taper. This line length closely matches the quarter wavelength of the driver at Fs. Using Martin King's models, I have the approximate frequency response:
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Old 2nd April 2007, 04:42 AM   #4
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Internal volume will be ~14 litres when the line divider, centre brace and drivers are accounted for.
Thoughts? Suggestions? I want this to be the best it can be given the drivers I have.
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Old 2nd April 2007, 05:31 AM   #5
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What's the highpass frequency for these surround speakers? Also, what's the wall placement? If you want them to play any kind of bass, and they're not right in a corner, then they're going to require serious correction and will not be very efficient.
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Old 2nd April 2007, 05:33 AM   #6
owdi is offline owdi  United States
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I haven't built any TLs, but IMO, the modeled sealed response looks better than the TL. Why not just go with a sealed enclosure, if you are not going to boost the low end with the TL. Maybe I am missing something.

Dan
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Old 2nd April 2007, 12:09 PM   #7
cs is offline cs  United Kingdom
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Quote:
Originally posted by MJL21193
The line is about 65 inches long, with roughly a 3:1 taper. This line length closely matches the quarter wavelength of the driver at Fs. Using Martin King's models, I have the approximate frequency response:
What is the fs of the driver ?
65" sounds a bit long to me.

You may find you get better results with a shorter line, say around 50", with a stronger taper, say 10:1. Also, you may need a larger cross-sectional area at the driver end.
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Old 2nd April 2007, 01:58 PM   #8
bjorno is offline bjorno  Sweden
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MJL21193,

Quote:
....I want this to be the best it can be given the drivers I have.
Ofcourse.

owdi,

Quote:
...but IMO, the modeled sealed response looks better than the TL.
Agree, but its possible to convert to an aperiodic MLTL too.

b

1(2)
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Old 2nd April 2007, 01:59 PM   #9
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2(2)
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Old 2nd April 2007, 07:16 PM   #10
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The purpose of a transmission line, in my opinion, is not to augment the bass- its purpose to to effectively deal with internal resonances and standing waves. This improves midbass and lower midrange clarity, and makes the driver behave more as if it were in an infinitely large sealed box.

Transmission lines can, with the right drivers, be tuned to give bass augmentation comparable to a vented box- but as their performance approaches that of a vented box, so does their behavior. There's a fuzzy line between transmission lines, mass loaded transmission lines, and ported boxes. To me, the critical thing is that throughout the bandwidth of the woofer there is not a strong standing wave that can be excited, and there is no major reflection from the inside of the box to store energy.

B&W's Nautilus makes use of terminated tapered tubes to effectively absorb the rear wave of the drivers, allowing them to behave very much as though they are in infinitely large sealed boxes.

If you're not expecting bass, and you build your walls well so that everything is solid, then this looks like a fine design and should be very non-resonant.
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