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Old 3rd December 2003, 11:33 PM   #1
P901 is offline P901  United States
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Join Date: Nov 2003
Location: Mansfield, CT
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Lightbulb Modified Jordan VTL`s

Anyone out there might be willing to run some numbers and provide me with a go-no go commentary on a set of four
Jordan VTL`s that I am planning to build.
This design will be based on the design from the EJ Jordan site,
www.ejjordan.co.uk/enclosures.html
I plan either to make the TL tube a single port with multiple tapered baffles to produce a length in the 98" range, or a single center tube that splits at the bottom of the enclosure to another set of baffles and out to two separate ports.
Thoughts, comments?
Thanks, andy.
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Old 4th December 2003, 05:58 PM   #2
Brendon is offline Brendon  Canada
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Join Date: Nov 2002
Location: Calgary
Default You may want to reconsider...

Hi P901,

You may want to reconsider the length. 98" is waay too long for this driver.

You should be looking somewhere between say 30" and 75" in length, depending on the cross-section of the line. For a straight pipe, no taper, you can take the speed of sound, divide by 4 for the 1/4 wavelength fundamental of the pipe, and then divide again by the fs of the driver. This will give you about 72" (speed of sound about 13500 in/s).

Again, no taper. If you start the line at 4*Sd of the driver, and then taper to 0.5*Sd, then you should multiply the result of 72" by about 0.65 to shorten it, maintaining about the same fundamental.

What I've mentioned here will give you tuning of the pipe at approximately the fs of the driver. Some have argued that the tuning of the pipe should be higher than the fs of the driver, and I don't disagree. You may want to consider 1.3*fs for the target pipe tuning.

Keep in mind that as you bend the line, you do two things. You roll off the upper pipe harmonics, which is good. But you also attenuate the fundamental. Also, if you go for lower tuning than higher pipe tuning, you'll lower the overall amplitude. So you have to determine if you want extension or amplitude of the resonant pipe.

The next thing is you need to figure out where to terminate the pipe. The floor, I think is a good recommendation as you'll get more gain from the floor boundary. On the front I think is the worst choice since you'll hear the upper harmonics that are not filtered due to stuffing, or due to the line baffling. The pipe output would sound 'pipey' and not like a pleasant bass note. Talk in a drain pipe and see what I mean about pipey.

Lots to think about. I would recommend for this unit that you aim for tuning around the 50 Hz mark. Then start the line at 4 to 5 * Sd, then alternate the line back and forth, emptying at the floor with a 0.5*Sd area. You can do this with 4 switchbacks. I've done it myself this way with success, though my line is tuned too low right now. But with my box's removable back panel, I can shorten the internal baffles easily increase the pipe fundamental.

There are software tools and look-up tables to help compute all this, but nothing replaces experience of success or failure.


Brendon
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