Port size

[anarchyaudio 0]

What difference if any does the width of the port make? I understand the tuning and freq changes and all that but lets says you have 2 boxes size and build appropriate for 33 hrz. for 2 12's, if you could build the box with a 2 in wide square port, a 3 in flared port, or what is the down size to using a smaller port area say a 2 in round or a 1 in wide sq port. I realize that it would reduce air flow, but if you have the box built and the port tuned properly does it really matter?

Edited December 9, 2008 by Anarchyaudio

[johnecon2001 82]

certainly.. you'll start to choke the sub without enough port, and you'll experience port noise.

[seanbw 1]

my port is a little small for my box size (i have about 28in sq for one 15 in 4ft cu) and it makes a whooshing/chuffing noise at very high volume on low frequencies. i've heard anywhere from 8-14 in sq per cube as a rule of thumb but i'm pretty sure there are a lot of factors that determine how much port you need. i'm sure the application, woofer, power and volume of box all contribute to the type and size of ports to use.

[anarchyaudio 0]

I am trying to figure out how to determine what is a good port size then. What are the factors that you take into conssideration? I am new to box building and would like to learn more about port size and calculation. I have played with the box software, but I can build a side port box with a 1" wide port the hgt of the box about 12" long. I was under the impression that was a good port. So what should be taken into considersation when designing a port?

[seanbw 1]

Just use any of the box modeling programs on the net. It doesn't matter or change the tuning frequency if the port is inside or outside of the box. Just be sure not to exceed the ratio of 1:9 for your cross-sectional area of the port if it is rectangular (i.e. a 1" x 8" port is OK, but a 1" x 10" port could be a performance compromise).

i think between a 2 or 3 inch wide port that runs the height of your box (a slot vent) will work fine. adjust port size and box tuning with your box calculator to find a design that gets the right box volume and tuning frequency for your woofers and enclosure space limitations. the problem you will run into with too small a port is port noise. i'm waiting for someone more knowledgeable to chime in but it has something to do with the velocity of air in the port area. it will depend on the woofer you choose, the manufacturer should have enclosure recommendations for each woofer size and type, following these guidelines and using a modeling program should keep you in the clear with most woofers.

[KU40 173]

Really it's just about port area. You just make it as wide as you need to get the port area you want. With normal slot ported boxes, the port runs the entire height of the enclosure, so to get the port area you want you just adjust the width. If you have a 15" inside height enclosure and want 30 sq. inches of port, make it 2" wide. If you want 60 sq. inches, make it 4" wide. As long as you keep the ratio of height to width below 1:9, you're fine.

As for how much port area you need, that depends on the setup. WinISD has a port velocity calculator. I believe it wants you to keep it below .14 m/s on the beta version (not sure exactly, it's been probably 5 years since I've used beta). The alpha version just has a graph that it shows you, covering the port velocity over all the frequencies. But .14 m/s still applies. Alpha is better in that you can see right away what the velocity will be at any frequency. If your ported box is tuned to 35 hz and you have a subsonic filter at 30 hz, the port velocity at, say, 20 hz doesn't really matter. And remember that usually port velocity goes up as frequency goes down. So while you may get .2 m/s of velocity at 20 hz and it seems too high, your port still may be fine if you only have .1 m/s at 30 hz, since that's closer to where you'll actually be playing.

Generally 12" subs need at least 15-20 sq. inches and 15" subs need at least 35-40 sq. inches or so, with your generic slot port. At least from my limited experiences. Aero ports cut down the needed area quite a bit. A single 4" port (~12 sq. inches) for a 12 and a single 6" port (~28 sq. inches) for a 15" work fine.

[95Honda 486]

Really it's just about port area. You just make it as wide as you need to get the port area you want. With normal slot ported boxes, the port runs the entire height of the enclosure, so to get the port area you want you just adjust the width. If you have a 15" inside height enclosure and want 30 sq. inches of port, make it 2" wide. If you want 60 sq. inches, make it 4" wide. As long as you keep the ratio of height to width below 1:9, you're fine.

As for how much port area you need, that depends on the setup. WinISD has a port velocity calculator. I believe it wants you to keep it below .14 m/s on the beta version (not sure exactly, it's been probably 5 years since I've used beta). The alpha version just has a graph that it shows you, covering the port velocity over all the frequencies. But .14 m/s still applies. Alpha is better in that you can see right away what the velocity will be at any frequency. If your ported box is tuned to 35 hz and you have a subsonic filter at 30 hz, the port velocity at, say, 20 hz doesn't really matter. And remember that usually port velocity goes up as frequency goes down. So while you may get .2 m/s of velocity at 20 hz and it seems too high, your port still may be fine if you only have .1 m/s at 30 hz, since that's closer to where you'll actually be playing.

Generally 12" subs need at least 15-20 sq. inches and 15" subs need at least 35-40 sq. inches or so, with your generic slot port. At least from my limited experiences. Aero ports cut down the needed area quite a bit. A single 4" port (~12 sq. inches) for a 12 and a single 6" port (~28 sq. inches) for a 15" work fine.

Yes, yes and ...... yes....

[anarchyaudio 0]

Very cool, I did not consider noise. I was going to make a slot port, but to keep with the 1:9 ratio that wont work, or at least not the way orgianally intended. Thank you guys. I learned something very vaulable out of this.

[KU40 173]

what was wrong with your planned slot port? Was it going to be really long and skinny, like along the entire bottom of the box?