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Old 29th August 2013, 01:30 AM   #1
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Default Port Question

I have a question regarding internal port clearances.

Is there a general rule of thumb to follow when it comes to how close a port can be to a rear baffle? If I have a 2" dia flared port that is 1.5" from a rear baffle would that have any negligible effects on tuning or port noise?

I was thinking it may be best to be at least one port diameter away from a wall.

Essentially I have a box that is 8" deep and would like to use a 2" port that is 6.5" long.
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Old 29th August 2013, 02:26 AM   #2
xrk971 is offline xrk971  United States
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I think that you want to make sure that the cross sectional area of the port (3.142 sq inches) equals the circumference x the gap between the port and the back wall. The circumference happens to be pi x dia or 3.142 x 2in. Thus the gap is 0.5 inch. So a gap of 1.5 inches is 3x the cross sectional area so you should be fine by a factor of safety of 3. At gap of less than 0.5 in the air would accelerate from the enclosure into the port and make more noise. I think you are safe.
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Old 29th August 2013, 01:04 PM   #3
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Really? A common rule of thumb, that is successful in practice, is to keep the port end at least one diameter away from the wall.



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Originally Posted by xrk971 View Post
I think that you want to make sure that the cross sectional area of the port (3.142 sq inches) equals the circumference x the gap between the port and the back wall. The circumference happens to be pi x dia or 3.142 x 2in. Thus the gap is 0.5 inch. So a gap of 1.5 inches is 3x the cross sectional area so you should be fine by a factor of safety of 3. At gap of less than 0.5 in the air would accelerate from the enclosure into the port and make more noise. I think you are safe.
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Old 29th August 2013, 04:31 PM   #4
chrisb is offline chrisb  Canada
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2" sounds like standard plumbing pipe very commonly used for ports - you can always use elbows to fold the port away from back panel
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Old 29th August 2013, 06:09 PM   #5
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by fastbike1 View Post
Really? A common rule of thumb, that is successful in practice, is to keep the port end at least one diameter away from the wall.
The ROT is indeed good as it provides a ~2x margin for the area needed. The minimum area derived by equating the duct CSA to the area formed by the perimeter x gap spacing is really just a restatement of mass conservation assuming in-compressible flow (true for low Mach numbers).

1/4 x pi x D^2 = CSA of round duct

pi x D x L = Area of perimeter against wall formed by gap of dia D and depth L

equate the two, you get :

1/4 x pi x D^2 = pi x D x L

pi cancels, and solve for L you get

L = D/4 as minimum. So for a 2 in dia, the minimum is 0.5 in gap. If you really want to get technical, the flow formed will have a necking called the vena contracta that is about 0.6x the gap so really, to be safe gap should be

L = (1/0.6) x D/4 or

L = 0.42 x D

So setting gap at D is more than 2x the min required and seems like a good ROT to follow. In a pinch you can go to 0.42X the dia and probably be OK - it will look tight to your eyes, but the air flow won't know the difference.
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Old 29th August 2013, 07:03 PM   #6
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Everything I've seen or read says min. 3 inches! I have used elbows with good results.

Larry
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Old 31st August 2013, 02:10 PM   #7
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Quote:
Originally Posted by chrisb View Post
2" sounds like standard plumbing pipe very commonly used for ports - you can always use elbows to fold the port away from back panel
We are actually using the purchased 2" pipe with the flair on both ends, does that mean i don,t have to use the inner flaired end, to increase the distance from the end to the baffle
ps the port is in the rear baffle
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Old 31st August 2013, 02:17 PM   #8
xrk971 is offline xrk971  United States
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The flare significantly reduces the pressure losses in the tube and will require the port length to be longer than for an equivalent non flared port - if you want the tuning to be what you think it is with a straight tube. The flare reduces port noise though. If your box design program accounts for flared ports then use it.
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