Need help with port dimensions

[bumpinne0n 10+ year member]

I'm using this formula: LV=[(1.463 x 10^7 x R^2) / (Fb^2 x Vb)] - 1.463xR.

Where:

R=4.1889

Fb=32

Vb=5616 cu. in.

I end up with 38.5in.

I used this http://subwoofertools.com/forum/ported-box.asp?Ew=30&Eh=13.75&Dia=6&Xmax=32&Vd=.19&NumW=1&Vb=3.25&Fb=32&Qts=&Vas=&Fs=&CE=0

to see if I figured the port right. I did, but I don't understand why they're listing 38.5in. in length and then in the board dimensions, they list 19.5 x 12.25 in. for one piece and 12.25 x 12.25 in. for the other one, which comes out to 31.75 instead of the 38.5 which was what I thought it was supposed to be. I'd appreciate it if someone would explain this to me, I'd like to be able to design a box without using an online calculator. Thanks in advance.

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[PSturmer 10+ year member]

getting mixes up with tuning frequencies. and also you measure the middle of the port.

 

[bumpinne0n 10+ year member]

getting mixes up with tuning frequencies. and also you measure the middle of the port.

So the port pieces need to equal 38.5in. in length? Does wood thickness come into play with this (Where the port connects to the box, where the opening is, does that add 3/4in to the length of the port? And where one port piece connects to the other port piece, does that add 3/4in more to the port length?)?

I don't understand what you mean by measuring the middle of the port..

Go easy guys, I'm new to buidling boxes and I'm trying to learn all I can.

 

[xluben 10+ year member]

this should help:

http://mobile.jlaudio.com/support_pages.php?page_id=165

 

[bumpinne0n 10+ year member]

this should help:
http://mobile.jlaudio.com/support_pages.php?page_id=165

Ok, so the port should be measured down the center, I think I understand that. Here's something I drew in Sketchup:

The two port pieces are 19.5 x 12.25in and 17.5 x 12.25in, 37in total. I accounted for the wood thickness of the front piece of the box and the thickness in the port piece, so it's 38.5 in. long. Does that sound right?

 

[xluben 10+ year member]

you need to add the length through the corner and half of the port width (for end correction).

 

[bumpinne0n 10+ year member]

you need to add the length through the corner and half of the port width (for end correction).

Ok, I think I've got it. Thanks for helping me and sorry if this has been aggravating for you, it's been very confusing to me.

This time I tried a different box:

External Dimensions:

30"W x 13.75" H x 24.50" D

3/4" Wood thickness

Port:

4"W x 12.25"H

Length=37.20 so I rounded that to 37.25

Here's what I came up with in Sketchup:

*hopes it's right this time*

 

[xluben 10+ year member]

the length needs to extend 1/2 the port width past the end of the port (inside the box). looks good other than that.

 

[bumpinne0n 10+ year member]

the length needs to extend 1/2 the port width past the end of the port (inside the box). looks good other than that.

So I need to add 2" to the 15 1/2" length of the port, for a total of 39.25"? Or do I need to decrease the size of the 12 3/4" port piece down to 10 3/4" and keep the total port length 37 1/4"?

Thanks for the help //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif.

 

[bumpinne0n 10+ year member]

I think I've got it this time (again). Here's another drawing:

The port width is 4", so that should show 2" end correction, hopefully.

 

[xluben 10+ year member]

looks right!

good work, i hope you learned a little bit along the way. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[Trixter 10+ year member]

I'm using this formula: LV=[(1.463 x 10^7 x R^2) / (Fb^2 x Vb)] - 1.463xR.

bout time somebody actually has enough kahones to use it and not an online calculator.

 

[bumpinne0n 10+ year member]

looks right!
good work, i hope you learned a little bit along the way. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Yeah I learned a lot //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif. Now I just hope I can get my cuts straight.

bout time somebody actually has enough kahones to use it and not an online calculator.

I'm all for taking the easy way out but I'd rather understand how something works then have an online calculator do it for me. Those online calculators may not always be there, this way I'll know how to figure it if they aren't.

 

[Trixter 10+ year member]

I'm all for taking the easy way out but I'd rather understand how something works then have an online calculator do it for me. Those online calculators may not always be there, this way I'll know how to figure it if they aren't.

dam straight

you can lead a horse to water but teach him to fish and ....you get the idea.