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Is is ok if the slot port varies in width down its length?

Old 10-03-2011, 08:47 PM
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Is is ok if the slot port varies in width down its length?

At the moment I don't have access to any hand tools so I had the guys at the store cut pieces of MDF to spec. I'm looking at their cuts and they messed up slightly.

The port is supposed to be 1" in width all the way down but they cut some pieces too short and it starts off as 1.5" on the inside of the box. As for the length, 10" of the port is 1" wide and the other 10" is 1.5" wide. Is this going to make a significant difference? (The box is 1.5CF tuned to 30Hz... the length of the port is about 20").
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Old 10-03-2011, 09:04 PM
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well yes it makes a difference. The area of the port has to be the same. so you have to change the width accordingly to keep the same area as it goes from 1 inch to 1.5 inches. it is also good practice to flare the port exit and also the bends. The air will make noise as it hits the hard corners! where did you ge the design for the box and vent? 1 inch seems small.
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Old 10-03-2011, 10:34 PM
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^^x2...you can get a piece of 1/2" mdf cut the length and width needed and glue it to the 1.5" wide portion to create a 1" wide port.
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Last edited by Denonite; 10-04-2011 at 03:02 PM.
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Old 10-04-2011, 10:44 AM
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Originally Posted by Denonite View Post
^^x2...you can get a piece of 1/2" pdf cut the length and width needed and glue it to the 1.5" wide portion to create a 1" wide port.
Provided this won't badly screw the tuning of the port up, this is a great solution, and will add rigidity. It will take up some of the interior volume of the box though...
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Old 10-04-2011, 11:09 AM
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Right, thanks for the advice.

The subwoofer in question is an Alpine SWR-1042D.

The specifications were as per the manual...

External Dimensions: 12.5" x 14.5" x 17.25"
Gross Volume: 1.3cf
Port Area: 11sq. in. (11" x 1")
Port Length: 22 in.
Vent Displacement: 0.237cf
Net Internal Volume: 1cf
F3, ripple, Fb: 30Hz, 3.5dB, 36Hz

You think it would be better to go with a wider port? More dB gain? I realize this would throw a lot of things off, such as requiring a longer port to maintain tuning as well as a larger box to maintain net internal volume which also makes the tuning peaky.

I normally thought that the rule of 8:1 was the best but this is what Alpine suggests as being optimal.
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Old 10-04-2011, 02:25 PM
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Alpine actually recommended a 11" x 1" vent opening?

The closer to square the better and always try to avoid anything over 6:1
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