quick question about port length and adding a 90 degree bend

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Nigel
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quick question about port length and adding a 90 degree bend

Postby Nigel » Mon Jun 09, 2008 4:20 pm

Okay, well it's been a while since I've built a box and a buddy of mine asked me for some help.

The enclosure he's looking at building will be:
49" wide by 22" deep by 16" high external dimensions
the port he wants to build will be 14.5" high by 10" wide by 19" long

to accomodate the port he will need to put one 90 degree bend in it. Now can I just make to port go 11.25" long in one direction then make a 90 degree and go 7.75" further or does the bend affect it? I seem to recall reading somewhere that the bend adds length onto it...

any help appreciated.

Nigel
'03 S-dime ext. Cab.
Dropped 3" all around
Shadow billet grill
De-Trimmed/De-Badged
18" Beyern Type Five's in Hyper Silver
Nexen N3000 245/45ZR18 rubber
Eurolite clear tails
clear corners
Truxedo Truxsport Tonneau Cover

Alpine CDA-7998 source unit
Clarion SRS 1686 6.5" comp set up front
OZ Audio ME 12.4 in a 2 cube ported box at 34 Hz
JBL GTO 601.1 II Sub amp
JBL GTO 75.2 II comp. amp
12 sq. ft. Dynamat Extreme on roof
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KU40
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Postby KU40 » Mon Jun 09, 2008 4:52 pm

just remember that you measure port length down the center of the port. So when you get to the corner, the effective port length will continue past the first port board until it gets halfway between that port board and the back wall. then it will make its 90 degree bend, where it is then halfway between the port board and the wall, so it has to go a little bit before getting back even with the port board. bad explanation, I know.

So, in effect, when making a 90 degree angle the width of the port will be the amount added for the corner. So after your 11.25" first port board, you'll need the second port board to be......well.....-3". ha. (after accounting for front baffle 3/4" thickness and adding that onto the first board's length) You're kind of in an awkward mathematical position. Might have to adjust something.
-Nick aka NS1 aka mopey aka speedbump
Alpine CDA-9827
Crossfire CFQ5.2
JL Audio XR 650csi components, Planet Audio TT2300
Crossfire VR404 on factory rear 6x8's until I get them upgraded
SI 15D2 + 3.8 @ 30 hz + Crossfire BMF 1000D = :)
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Nigel
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Postby Nigel » Mon Jun 09, 2008 5:17 pm

the 11.25 was accounting for the front board. and going down the middle to the midline would only be half the width correct? meaning it would be 11.25 + 5 (1/2 of the width) = 16.25" so I would need to go another 2.75" after the bend correct for a total of 19" right?

Nigel
'03 S-dime ext. Cab.
Dropped 3" all around
Shadow billet grill
De-Trimmed/De-Badged
18" Beyern Type Five's in Hyper Silver
Nexen N3000 245/45ZR18 rubber
Eurolite clear tails
clear corners
Truxedo Truxsport Tonneau Cover

Alpine CDA-7998 source unit
Clarion SRS 1686 6.5" comp set up front
OZ Audio ME 12.4 in a 2 cube ported box at 34 Hz
JBL GTO 601.1 II Sub amp
JBL GTO 75.2 II comp. amp
12 sq. ft. Dynamat Extreme on roof
User avatar
KU40
Posts: 6461
Joined: Sun Mar 16, 2003 12:02 pm
Location: Lawrence, KS
Contact:

Postby KU40 » Mon Jun 09, 2008 7:49 pm

But remember that when you get halfway through the corner in one direction and make the 90 degree turn, you're only halfway through the corner in that other direction then. Does that make sense? Might have to draw a picture.

Another way to think about it is to take the length of the port boards + front baffle and add them up. Then take the sum of the lengths of the outer port wall, aka the inside of the box that the port runs down. Then average those two inner and outer port lengths. That will give you the middle port length, which is the one to use.
-Nick aka NS1 aka mopey aka speedbump

Alpine CDA-9827

Crossfire CFQ5.2

JL Audio XR 650csi components, Planet Audio TT2300

Crossfire VR404 on factory rear 6x8's until I get them upgraded

SI 15D2 + 3.8 @ 30 hz + Crossfire BMF 1000D = :)
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Nigel
Posts: 876
Joined: Sat Jan 03, 2004 11:39 pm
Location: Canada
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Postby Nigel » Mon Jun 09, 2008 8:24 pm

I think I know what you mean now. so if on one side my outer wall was 21.25 and the inner was 11.25 then the length for that piece would be 16.25" correct? Now, from that, I'm thinking that if I didn't put a bend in, just left the inner baffle of the port the same distance from the back wall as the width of the port itself, would that then "simulate" the bend (ie. draw a line from the midline of the port to the inner baffle) in theory adding another 5" for a total of 21.25"...

Am I correct in this assumption then? And I think he'll be happy if the port is a little over the 19" than if it were shorter (but I could be wrong).

Nigel

Image
'03 S-dime ext. Cab.
Dropped 3" all around
Shadow billet grill
De-Trimmed/De-Badged
18" Beyern Type Five's in Hyper Silver
Nexen N3000 245/45ZR18 rubber
Eurolite clear tails
clear corners
Truxedo Truxsport Tonneau Cover

Alpine CDA-7998 source unit
Clarion SRS 1686 6.5" comp set up front
OZ Audio ME 12.4 in a 2 cube ported box at 34 Hz
JBL GTO 601.1 II Sub amp
JBL GTO 75.2 II comp. amp
12 sq. ft. Dynamat Extreme on roof
User avatar
KU40
Posts: 6461
Joined: Sun Mar 16, 2003 12:02 pm
Location: Lawrence, KS
Contact:

Postby KU40 » Tue Jun 10, 2008 8:04 am

Yeah, that's what I was thinking.

I just plugged this into winISD, and it says that with a 6.3 cubic foot box (what I calculated this to be, roughly. is that right?), 145 sq. inches of port x 19" long it's tuned to a whopping 46 hz. Is that what you were going for?

I tried tuning to 35 hz but the port just ate up the whole box. Do you really need that much port area?
-Nick aka NS1 aka mopey aka speedbump

Alpine CDA-9827

Crossfire CFQ5.2

JL Audio XR 650csi components, Planet Audio TT2300

Crossfire VR404 on factory rear 6x8's until I get them upgraded

SI 15D2 + 3.8 @ 30 hz + Crossfire BMF 1000D = :)
User avatar
Nigel
Posts: 876
Joined: Sat Jan 03, 2004 11:39 pm
Location: Canada
Contact:

Postby Nigel » Tue Jun 10, 2008 2:49 pm

apparently the 97 by 49 inch sheets of mdf are only 48 inches wide (somebody messed up), so after a few more calculations including a double front baffle (while maintaing the same depth of 22 inches) he will end up with a final box of:

5.61 cubic feet net (after port and sub displacement) tuned to 57.13 Hz

He'll be putting (2) MTX T9512-44 subs in it to use for spl competing. He'll be running them off of a Planet Audio amp that pushes 2250 watts at one ohm.

Thanks for the help.

Nigel
'03 S-dime ext. Cab.
Dropped 3" all around
Shadow billet grill
De-Trimmed/De-Badged
18" Beyern Type Five's in Hyper Silver
Nexen N3000 245/45ZR18 rubber
Eurolite clear tails
clear corners
Truxedo Truxsport Tonneau Cover

Alpine CDA-7998 source unit
Clarion SRS 1686 6.5" comp set up front
OZ Audio ME 12.4 in a 2 cube ported box at 34 Hz
JBL GTO 601.1 II Sub amp
JBL GTO 75.2 II comp. amp
12 sq. ft. Dynamat Extreme on roof

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