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understanding ports, a lesson in dimensions vs efficiency.


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i think this would be the best place for my question about an 18 inch subwoofer port recommendation

i was told the port area should be 40 square inches and 10 inches in length to get a tune around 32 hertz with the sub on 5000 watts rms in a 6 cubic foot box (with driver displacement)

Some people have told me the port area seems small and length seems short


Area of the port, volume of the box and length of the port determine the tuning of the box.

Larger area = Higher Tuning
Smaller box = Higher Tuning
Shorter port = Higher Tuning

Smaller area = Lower Tuning
Larger box = Lower Tuning
Longer port = Lower Tuning

seems like the smaller area of port would be lower tuning plus shorter port would mean higher tuning so does it make sense thats what i would want to do for this enclosure?

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  • 2 weeks later...
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PSi recommends 12.5" per cube, although technically the size of the box has nothing to do with the tuning frequency of the port. I have had a good experience with a box for 2 15's that I built, it was 10 cubic feet and had 120 sq. in. of port (2 slot ports 4" x 15")

To answer your question, yes a smaller port lowers tuning because it is more restrictive. A shorter port raises tuning because it is less restrictive. (maybe restrictive isn't the correct term, but that's how I picture it in my head). Why doesn't everyone just run a small, short port? One problem is that as the port gets smaller, the speed of the air coming out of the port increases. At a certain speed (read Krakin's post on 1st page or go to http://www.subwoofer-builder.com/port-flares.htm) the air is fast enough to cause a chuffing noise as it exits the port. You can see from the tables on the site linked above, flaring the port allows higher port velocity without chuffing. WinISD and Jeff Bagby's Woofer Box Designer spreadsheet will both calculate port velocity for you if you plug in all the other numbers.

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I have heard/read that 12" - 16" of port area per cubic foot of box is a good guideline. PSi recommends 12.5" per cube. I have had a good experience with a box for 2 15's that I built, it was 10 cubic feet and had 120 sq. in. of port (2 slot ports 4" x 15")

To answer your question, yes a smaller port lowers tuning because it is more restrictive. A shorter port raises tuning because it is less restrictive. (maybe restrictive isn't the correct term, but that's how I picture it in my head). Why doesn't everyone just run a small, short port? One problem is that as the port gets smaller, the speed of the air coming out of the port increases. At a certain speed (I have read 17 m/sec, 20 m/sec and 25 m/sec) the air is fast enough to cause a "chuffing" noise as it exits the port. I have also read that a sufficient flare on both ends of the port will allow you to double the port velocity without chuffing. So with a nice flare, up to 34, 40, or 50 m/sec port velocity is acceptable. WinISD and Jeff Bagby's Woofer Box Designer spreadsheet will both calculate port velocity for you if you plug in all the other numbers.

Most of everything you have read has been greatly oversimplified, the port per cube guideline is rubbish, and your estimation on flares and their threshold on chuffing isn't accurate as the velocity to make chuffing apparent varies on frequency.

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I would have replied earlier, but I was measuring the output of my amp with a yardstick . . .

What you hear is not the air pressure variation in itself

but what has drawn your attention

in the two streams of superimposed air pressure variations at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space

Everyone learns to render the 3-dimensional localization of sound based on the individual shape of their ears,

thus no formula can achieve a definite effect for every listener.

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I think density of the air is the actual factor, though since air is fairly "static" in its density in how we use it, frequency tends to be the apparent modifier.

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I have heard/read that 12" - 16" of port area per cubic foot of box is a good guideline. PSi recommends 12.5" per cube. I have had a good experience with a box for 2 15's that I built, it was 10 cubic feet and had 120 sq. in. of port (2 slot ports 4" x 15")

To answer your question, yes a smaller port lowers tuning because it is more restrictive. A shorter port raises tuning because it is less restrictive. (maybe restrictive isn't the correct term, but that's how I picture it in my head). Why doesn't everyone just run a small, short port? One problem is that as the port gets smaller, the speed of the air coming out of the port increases. At a certain speed (I have read 17 m/sec, 20 m/sec and 25 m/sec) the air is fast enough to cause a "chuffing" noise as it exits the port. I have also read that a sufficient flare on both ends of the port will allow you to double the port velocity without chuffing. So with a nice flare, up to 34, 40, or 50 m/sec port velocity is acceptable. WinISD and Jeff Bagby's Woofer Box Designer spreadsheet will both calculate port velocity for you if you plug in all the other numbers.

Most of everything you have read has been greatly oversimplified, the port per cube guideline is rubbish, and your estimation on flares and their threshold on chuffing isn't accurate as the velocity to make chuffing apparent varies on frequency.

How specifically do you determine port area for your box designs, if I may ask? Vance Dickason's formula? iRNr35Q.jpg

Or this slightly modified version? mYOYCZJ.jpg

Or do you have a different process?

"My" estimation on flares and their threshold on chuffing is lifted directly from http://www.hometheatershack.com/forums/diy-subwoofers-general-discussion/38752-port-velocities-cinema-subs.html. Thank you for clarifying that it is inaccurate. I should edit my post to keep this stickied discussion on point.

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  • 4 months later...

Great, great info, thanks to all. Good read, I don't understand it all though. Is this thing sticky yet? lol

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  • 2 months later...

My very first time inthe world to be writing anything in a forum. Thank you for the time, I own an original Rockford Fosgate "full range" box. My question is: the ports are on the top of the box and i have the "face" pointing inside the car with a super tight gap around it to the back seat frame it was like if would have been a custom built, the ports of the box where supposed to be facing? To the inside of the car just like the subwoofers or to to inside of the trunk thank you

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  • 1 month later...

I made a box which I thought would be tuned to 34 and doesn't play very low, is this normal

Need more info. What subs? What are the specs of the box and the port.

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