Port question

[Dammed 12]

I'm going to make a enclosure that will use almost all of my trunk space, so I'm going to make the bottom out of fiberglass so that I can use the space from the spare wheel well. The port have to be so long that it will close off 1/3 of the opening of the spare wheel well.

Few pictures to illustrate the problem:

from the side

From above:

Will this be a problem? And how will this affect the subwoofers?

[KU40 173]

It will be fine.

[Dammed 12]

good.

About the tuning, I used a program that calculated that I needed a port that's 25" long, when the opening is 2" x 40". But I have read a couple of places that the opening should not have a larger ratio then 1:9 between the width and height, and I have a ratio of 1:20. Is the tuning correct? The maximum size I can make the opening in the front baffle is 2,75" x 40"

And regarding the air velocity through the port, how should I calculate it?

Info about the system: two 18" Fi Q's powered by a Hifonics Colossus II amplifier that gives 3200W rms in a 10.25 cu ft enclosure filled with Fiberfill.

Edited January 26, 2010 by Dammed

[rmzsuzuki89 61]

good.

About the tuning, I used a program that calculated that I needed a port that's 25" long, when the opening is 2" x 40". But I have read a couple of places that the opening should not have a larger ratio then 1:9 between the width and height, and I have a ratio of 1:20. Is the tuning correct? The maximum size for the opening in the front baffle is 2,75" x 40"

And regarding the air velocity through the port, how should I calculate it?

Info about the system: two 18" Fi Q's powered by a Hifonics Colossus II amplifier that gives 3200W rms in a 10.25 cu ft enclosure filled with Fiberfill.

There is a good chance that you will get a lot of port noise and turbulance with that opening ratio.

[Dammed 12]

But why, if you increase the port area shouldn't it reduce the air velocity so that the air traveling through a tight gap will move slow enough so that it doesn't make any noise?

Any way I can counter this effect? Make rounder edges / smoother walls?

And how should I calculate the tuning or is it correct?

[KU40 173]

That port opening is going to be bad. 40 square inches per 18 is not sufficient for no port noise in a "good" slot port, let alone one with a ratio such as that. I would suggest maybe switching to some round ports if you are having this problem. You calculate tuning using any box building program or any of the tuning equations on the net. But I get the tuning to be about 25 hz with a 10.25 cube box with 2x40 port x 25" long.

[Dammed 12]

hmm, I can make a 110 square inch port, would that be enough? If not, how big port opening should I have?

I used this calculator: Public - Windows Live Made by a guy named Torres on the S_M_D forum.

Data:

height x width x depth

23.622" x 41.34" x 21.654"

port:

height x width x length

2" x 40" x 25

Subwooferne displacement: 0.5 cu ft

This is a slot port and it has a bend and wood thicknes: 0,75"

gives me 27.92hz

edit; Regarding the 1:9 ratio:

"The ratio of W to h should not be any more than 9 to 1 to prevent tuning shifts introduced by excessive friction between the rapidly moving air and the port's surface. It should be noted that this is not a hard-and-fast rule, merely a rule of thumb to help you prevent a mis-tuning. "

source: http://mobile.jlaudio.com/support_pages.php?page_id=165

Edited January 26, 2010 by Dammed

[Dammed 12]

I found out that with a small redesign I'll have room for a 133.9 in2 port. (3.54"x37,8") in a 10.24ft3 box, that would be 13.08in2/ft3, which is within the 12-16in2/ft3 limit that people here and makers of subwoofers recommend.

But regarding the ratio, which is still something I believe to be kinda strange, Since I can make a port that's more then 3" from side to side, why would it produce more noise then a 3" aeroport if I keep the air velocity low?

And regarding the tuning, if I use fiberfill inside the box what volume should I use to tune the box, the physical volume or the volume I get after the fiberfill has been added?

Could someone please help me out here, I don't have any experience with vented enclosure and I don't want to waste material making something that will not work.

[KU40 173]

3" wide slot port and 3" round port are waaaaay different things.

If you know about air or water movement through an opening or channel you know that the air on the outsides moves slower than the air in the middle due to resistance against the wall. If you have a 10" square hole, there is a lot of room in the center of that, away from the walls, for the air to move quickly through and not be affected by the slowdown caused by the walls. In a narrow port, by the time you get away from one wall's resistance you're almost to the next wall and running into its resistance zone. Since there's more perimeter area around a long rectangle than a square of the same area, there's more resistance by the walls. Any port velocity calculator only takes into account the overall area of the port, not any characteristics caused by port shape. So it may show you that the port area is enough with your long and skinny rectangle, but in the real application it will not be.

[Dammed 12]

a slot port and aeroport is different just like the resistance provided by a canal to water and a port to air is different considering the density of water and air molecules. (air travels much easier then water through tight gaps)

But my point was if I made twenty 3" x 3" square ports witch has a area of 9in2 x 20 = 180 in2, how is that any better then a square port that is 3" x 60" with the same area? The space between the 60" sides is 3" same as the space between the sides in a quadratic 3x3" port, and even better, you have only 4 sides instead 80 sides. The air traveling through the 3x60" port must have a better air speed considering you have a lesser side area that contributes to slow down the air.

Now if there are other factors that comes into play here like turbulence or something else then I would understand it, but if the only argument is the resistance the "tight" gap provides, then I'm unsure if the statement about the ratio is correct.

I will ask my professor in fluid mechanics as well to see what he thinks, but this "rule" sounds a bit strange to me.

Edited January 28, 2010 by Dammed

[KU40 173]

You are comparing the long narrow port to the wrong thing. You don't want to compare it to 20 3" square ports as that adds even more resistance because that's a lot more perimeter wall. None of us would recommend 20 3" square ports over a single rectangular port. Again, a 3" x 60" port has 126 inches of perimeter. 20 3" square ports have 240 inches of perimeter. However, a 13.5" square port (which has approximately the same 180 sq. inches of area) has only 54 inches of perimeter.

If you want to find out if air has similar resistance properties than water, find a bunch of straws and stuff them in your mouth and breathe through them. then take the straws out, and find another straw or pipe that's as large in diameter and length as all of those smaller straws, put it in your mouth, and breathe again. See which one is easier.

[Dammed 12]

yeah a 13.5" square port would be best, but my problem is space. I could throw out my back seats to make a bigger enclosure and then I could have a more square like port, but I'll rather not do it. It's good to have back seats in the car.

My space in the car (the measurements are in cm):

The enclosure I was planning:

(the wall in the middle will not be built, I'll use some threaded rods instead)

As you can see the space is quite limited, are you sure that there will be port noise? What if I flared it at the openings and made the walls smooth with some filler and paint?

I'm also open to suggestions on how I should build the enclosure, if someone has a tip or two.

Edited January 28, 2010 by Dammed

[KU40 173]

Well if you want to try it, go ahead. I can't tell you for sure that it will have port noise.

Have you thought about upfiring?

[Dammed 12]

it will be upfiring, As you can see in my first two pics the port is facing upwards and the subwoofers will be facing the same way.

I have talked to some other people, and one of them has built a port that was 1,2" x 19,7" and he didn't get any noise. So I'm pretty sure it's doable without getting port noise, he said just flare the ends and build the bend with a PVC pipe and it should be fine. Then he said that a 133,9in2 port was way to much, since I had to build it 31,5" long to get a tuning of 28hz, he advised me to build a 77,5in2 that it would be enough for two 18" and 3000W rms.