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Old 19th September 2005, 04:50 AM   #21
jim_vt is offline jim_vt  United States
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Looks good!

I'll be building mine shortly...
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Old 19th September 2005, 05:07 AM   #22
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Are you going to use GMs design or another design?

Josh
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Old 19th September 2005, 05:24 AM   #23
jim_vt is offline jim_vt  United States
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Just going to be using that design...with a round port.
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Old 20th September 2005, 03:10 AM   #24
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GM,

Thanks for the design and help.

I was wondering if you could explain to me your 9:1 ratio.

total volume is 761.58"^3
1/9th that is 9.4"^3
volume of your calculated port is 10.48"^3
volume of my port 8.2473"^3

So I'm guessing you calculate port volume as being greater than a 9:1 ration of box volume to port volume? Why is that? and why not exactly 1/9th total volume? I've read MJKs papers but I don't remember this information. Also I've been wondering why TLs should be 4xSD cross sectional. I don't remember either being in MJKs website.

Thanks,

Josh
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Old 20th September 2005, 03:59 AM   #25
GM is offline GM  United States
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Greets!

You're welcome!

The aspect ratio of the vent's length to height. Yours is 7.43/0.5 = 14.86:1, so is theoretically somewhat aperiodic due to all the added acoustic resistance.

TL's don't have to be 4 x Sd. TLs are just another vented alignment in that net Vb is the main factor, then it's just a matter of dividing the length into it to get the CSA.

GM
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Old 20th September 2005, 12:52 PM   #26
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I'm still a little confused about the vent ratio, can you go into more detail about how you calculated 2.28" . My understanding of what you have said is that my port should have been more like 3/4" instead of .5"?

Also, I don't have time right now but I will try and find it, I think on Bob Brines's website under the TL how to tutorial it says that MLTLs should be 4xSD.

Thanks,

Josh
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Old 21st September 2005, 01:17 AM   #27
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Well I was able to do a little searching and on this page Bob has a chart comparing straight, TQWT, and "conventional" TLs. He also says that port length is .5xSD. But I don't know if that is for that speaker only.

Also, if I remember correctly in MJKs mathcad sheets when you download it has his models from his Focal speakers, and the box was 4xSD. I'm not sure, but I think that is how they came.

I'm trying to understand TL design a little more.

Thanks,

Josh
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Old 21st September 2005, 02:58 AM   #28
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Quote:
Originally posted by edjosh23
it says that MLTLs should be 4xSd
Keep in mind that using Sd is just a convention that was established with classic TL design. In actual fact Sd has no correlation* with the size of a TL. It is Vas that largely detrmines this.

*(except for the general trend that the larger a driver tends to have a larger Vas)

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Old 21st September 2005, 03:51 AM   #29
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Thanks Dave,
but is there a good way to calculate port volume?

Thanks,

Josh
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Old 21st September 2005, 07:01 AM   #30
GM is offline GM  United States
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Quote:
Originally posted by edjosh23
I'm still a little confused about the vent ratio, can you go into more detail about how you calculated 2.28" . My understanding of what you have said is that my port should have been more like 3/4" instead of .5"?

Also, I don't have time right now but I will try and find it, I think on Bob Brines's website under the TL how to tutorial it says that MLTLs should be 4xSD.

Thanks,

Josh
Greets!

Please read his caveat underneath the chart.

7.43/9 = ~0.82" height (11/16" is close enough), causing an increase in the vent's length.

What's confusing? The mass of air in the vent acts as a piston of 'x' mass and as a duct morphs from round to square to rectangular with increasing aspect ratio, wall friction and terminus edge diffraction increasingly damps (distorts) its output.

I didn't calc 2.28", I said the length needed to be ~2.82", which is just scaled up from the original vent size of 1.5" dia. x 1.125" long (attained using MJK's WS) to your slot vent's 2.175" equivalent diameter. Since a slot vent terminates on the inside with three parallel boundaries it needs a different end correction than a round one, so the toplate of the vent needs to be somewhat shorter than the scaled 2.82" (if round) to achieve the same Fb.

Since we're mirror imaging the termination's area on three sides this puts it in 1/8 space, so the toplate's cut length is ~2.82" - (8/(3*pi)*SQRT((0.5*7.43)/pi)) = ~1.9", or a bit shorter than my comment that your 2.22" was close enough (I apparently had a 'senior moment'), ergo it's theoretically tuned a bit lower than when a round vent is used.

GM
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