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Port length

Old 09-30-2009, 08:15 PM
  #1  
50 Watt CAFz'r
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Port length

Hi

I bought an 8" subwoofer and followed the specs that came with it for a vented box. I made the box exactly 0.6 cubic feet like directions say.

My question is this. It tells me to use 2.5" diameter port that is 8" long.
My box is only 9'" deep. This means that there will be a clearence of only 1" between the end of the port and the back of the box.
Is there a rule of thumb as to the minimum amout of inches between the end of the port and the back of the box? I could not have made the box any bigger or it would have had a volume greater than 0.6 cubic feet. I made the height and width as small as i possibly could in order to get it to be 9" deep.

Thanks for any help.

J
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Old 09-30-2009, 09:54 PM
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is your box .6cu.ft exactly..or did you allready calculate the lost space for the displacement of the port?.

Also if you are using 2.5" diameter piping (usually PVC for central vac setups) you can buy a 90 degree bend and give the port a turn inside the box.
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Old 09-30-2009, 10:12 PM
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Originally Posted by Sikk Nation View Post
is your box .6cu.ft exactly..or did you allready calculate the lost space for the displacement of the port?.

Also if you are using 2.5" diameter piping (usually PVC for central vac setups) you can buy a 90 degree bend and give the port a turn inside the box.
it is 0.6 cu. ft. exactly. it would seem that i did not calculate the displacement of the port. do you suggest i use 8" of tube with a 90 degree bend or just use a 6" straight tube or something like that. typically, how much space should there be between the end of the port and the back wall?

J
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Old 09-30-2009, 10:16 PM
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if your box is exactly .6cu.ft it's going to be .49cu.ft after port displacement even smaller after woofer displacement.

The smaller the box, the longer the port has to be to get the same port tuning frequency so you're fighting an uphill battle making the box smaller, which makes the port have to be longer, which makes the box even smaller...

I'd reccomend building the box to .73 cu.ft then using the "reccomended" port if you really want to stay in spec.
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Old 09-30-2009, 10:41 PM
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What subwoofer are you running?
How much power are you feeding it?
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Old 10-01-2009, 06:33 AM
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Originally Posted by jalat View Post
What subwoofer are you running?
How much power are you feeding it?
thanks for the suggestions

its a JBL GTO 804
feeding it 220 Watts RMS at 4 ohms
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Old 10-01-2009, 10:11 AM
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0.8 cubic feet tuned @ 38hz

Top & Bottom - 18.5"x13.5"
Front - 15"x9.5"
Back - 18.5"x9.5"
L Side - 12.5"x9.5"
R Side - 11.75"x9.5"
Port 1 - 10.5"x9.5"
Port 2 - 13"x9.5"

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