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Old 28th January 2014, 05:09 PM   #1
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Default Port location in/on a sub

An oft-repeated rule of thumb is that the port should be some distance away from an inner edge, e.g. one port diameter away. Is there some real substance to this tribal knowledge? If valid, what is the effect when the port is located too close to an inner edge or wall?

I'm working with a small design and it is quite difficult to get the right spacing around the port. The port is external but the the opening is almost adjacent to a wall.

Pics to come.
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Old 28th January 2014, 05:20 PM   #2
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Too close and the local boundary condition[s] makes the vent acoustically longer, larger, so lowers tuning and why shelf vents can be so short even though they are typically larger than need be to keep from compressing at high power. In your case then, a shelf or round, square or triangular corner vent is the way to go.

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Last edited by GM; 28th January 2014 at 05:23 PM.
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Old 28th January 2014, 05:56 PM   #3
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it is not just the location, but the port it self. I would use just this kind Precision Port 3" Flared Port Tube Kit | 268-350
and I would put it on the back.
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Old 28th January 2014, 09:26 PM   #4
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Shelf vents should be used for all vented subwoofers, end of story .

The big advantage they bring, apart from large port area, is that the "shelf" serves as additional bracing for the box, something that traditional tube vents do not provide.
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Old 29th January 2014, 03:46 AM   #5
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Quote:
Originally Posted by Brian Steele View Post
Shelf vents should be used for all vented subwoofers, end of story .

The big advantage they bring, apart from large port area, is that the "shelf" serves as additional bracing for the box, something that traditional tube vents do not provide.
They also make construction more complicated. Also, tuning the port after the enclosure is assembled is more difficult.
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Old 29th January 2014, 03:46 AM   #6
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Image attached. The short vent is a 3" SCH40 PVC pipe, the other one is 4".
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File Type: png assy.png (150.2 KB, 92 views)
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Old 29th January 2014, 04:07 AM   #7
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Originally Posted by 454Casull View Post
They also make construction more complicated. Also, tuning the port after the enclosure is assembled is more difficult.
How so? Only one more or two more panels are needed, and these can be cut from the same strip that's used to produce the front and back panels. And all can be cut with the same circle saw. Try cutting a hole in a box with a circle saw .

As for tuning, if you want to adjust it, just make part of the internal panel removable. Tune by trimming the panel, then glue it permanently into place when tuning is complete (or leave it removable to allow future tweaking).

Oh, and I forgot - using a shelf vent is usually cheaper too - no extra vent to buy.
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Old 29th January 2014, 04:33 AM   #8
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Originally Posted by Brian Steele View Post
How so? Only one more or two more panels are needed, and these can be cut from the same strip that's used to produce the front and back panels. And all can be cut with the same circle saw. Try cutting a hole in a box with a circle saw .

As for tuning, if you want to adjust it, just make part of the internal panel removable. Tune by trimming the panel, then glue it permanently into place when tuning is complete (or leave it removable to allow future tweaking).

Oh, and I forgot - using a shelf vent is usually cheaper too - no extra vent to buy.
I don't cut straight pieces. The more pieces I need from the same board, the worse the assembly gets.

Anyway, this box has external vents for a reason. I need them to stick out the front as much as possible.
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Old 29th January 2014, 12:11 PM   #9
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There are many variables that effect port tuning. Best to build, measure and adjust if necessary. With a reflex cabinet which had 2 port tubes I had fairly accurate sim results with Hornresp
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Old 29th January 2014, 03:57 PM   #10
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Not sure what you're doing there, but you should be using a slot port in any compact design. It is the most space efficient, making it much easier to accomidate a larger surface area. Having the port interact with the wall is a positive thing, it make the port act longer, thus it can be sized larger and shorter.
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