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Why 2+ ports??


k58.cross

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I created an account just to ask this because it's really bugging me!

Why do so many people use more than one port in their box? From everything I've seen in various port length calculators and box planning software, two ports of X/2 length DO NOT equal one port of X length. In fact, it's worse because each port has to be double length to get the same frequency.

And what about port area? You can't assume that two 3 inch diameter aeros of X length are the same as one 6 inch diameter aero of X length (you actually get half the tuning frequency in that case).

So please help me understand why I see so many pictures of people with multiple ports.

 

Thanks

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There's a logistical bit that doesn't always lend itself to having a singular port

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if you needed a 6" round port but your box was only 5" wide

all you need is to split that port in to 2 smaller ports with the same port area. Keep the same length and effectively you have the same port.

a 6" port has about 28sq in. of port area. Two 4" (roughly) ports that are the same length as your 6" port will perform pretty much the same.

note: those numbers are pretty rounded...

 

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On 8/31/2018 at 1:15 PM, k58.cross said:

 

And what about port area? You can't assume that two 3 inch diameter aeros of X length are the same as one 6 inch diameter aero of X length (you actually get half the tuning frequency in that case).

You are adding the diameters instead of adding areas that's completely wrong, You need four 3" ports to roughly equal the area of a single 6" port. Get Torres calculator or just state what you intend to do to get help.

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Sometimes I do it just for symmetry, long box. Sub is centered, looks goofy with 1 port on the side. So I do 2 ports, 1 on each side of the face so it looks symmetric. 

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you're basing an assumption without knowledge of enclosure design. a correctly tuned box has several factors that are being taken into account but also have factors that are not being heavily considered. maybe someone is looking to reduce port velocity or looking to tune lower. one is not better than the other on a grand scale. while a single port is ideal b/c it's suppose to be slightly more efficient than multiple ports, it's not silver bullet. you have to keep in mind that a properly designed enclosure is all about compromises. I also see in your initial post that you are saying that the port has to be twice the length, it's not a continuous length. example, if you use 1 6" port at 17" in a net 4 cube box to get to 30 hz. you can use 2 4" ports at 17" to achieve the same. the difference will be that the 2 ports will have a slighly higher port velocity and a slightly lower tuning frequency. it's not a linear swap. it's design dependent and what that person is willing or able to do.

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On 9/4/2018 at 3:41 AM, Joe X said:

You are adding the diameters instead of adding areas that's completely wrong, You need four 3" ports to roughly equal the area of a single 6" port. Get Torres calculator or just state what you intend to do to get help.

Good point here thanks. I did some calculations to understand this better:

 

A single 6" diameter port area = 28.3 sq.in

Same area, but two ports would be 14.15 sq.in each. --> each port diameter would be 4.25".

Putting that into a port calculator for a 2.5 cu.ft box tuned at 35 Hz, the calculator says I need a port length of 21.75". Now is that PER PORT, or total of all ports combined? Because if it is total of all ports combined, then you are correct. If it is per port, then using two ports as opposed to one would not make sense.

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That is per port unfortunately so depending on what subs you are working with a two 4" ports should be a better bet and those be shorter as well. Best thing to do if you have enough space is going with a slot port with a port area custom to your needs.

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