ported box help

[Cobalt camaro]

Well i'm building a new box for my 15, i want to go to a ported box. My question is how big should my port be for a 4 cubic foot box, tuned to 35hz. I need like the length width height dimensions. I used Winisd to calculate how big the port needed to be, and it said my port needs to be 3 in wide by 6 in tall by 8 inches long. Now i've had this sub in a ported box before, and the port size in my old box was alot different, and was ported to the same frequency, so i'm just trying to figure out what is the correct dimensions before i mess it up and have a whistling box.

[Bonemaro]

Re: ported box help

I'll need the thiele-small perimeters of your sub to tell you that.

[Cobalt camaro]

Re: ported box help

RE SX 15

15"
Electrical Q Value -Qes: 0.38
Mechanical Q Value -Qms: 4.7
Total Speaker Q Value -Qts: 0.38
Free Air Resonance -Fs: 27.8 Hz
Equivalent Compliance -Vas: 126.3 liters
One-Way, Linear Excursion -Xmax: 22 mm
Efficiency -SPL 1W/1m: 90.4 dB SPL
Effective Piston Area -Sd: 810 cm^2
DC Resistance -Re: 1.9 ohm
Nominal Impedance -Znom: Dual 2/4 ohm
Thermal Power Handling -Pe: 1000 W
Force Factor -Bl: 21

[96firebird311]

Re: ported box help

Port dimensions can change between designs, but still remain at a 35Hz tune. For example, you could make a 2"x2"x0.5" port for a 35Hz tune.... lol.... but 11"x4"x18.6" would probably give you a little less port noise. lol


EDIT: Is that 4 cubes before port and woofer displacement? If thats 4 cubes net volume, then the dimensions I gave would be great. If you still need to displace the port and sub volume out of that 4 cubes..... 11"x4"x25.6" would be closer to a true 35Hz tune.

[Cobalt camaro]

Re: ported box help

11*4*25.6 seems huge, winisd only gave me a port size of 6*3*8 before displacement and port size. I'm trying to just understand how there are so many different opinions on how big the port should be. I've never even heard of a port that big before, and it seems like it would be playing alot higher than 35 hz. But if you say that's right, i guess that's what it's gonna have to be. Oh and the box is before port and sub displacement.

[96firebird311]

Re: ported box help

I put the net airspace at 3.1cuft. The total port displacement is probably going to be over .5cuft by itself (don't forget to measure the OUTSIDE of the port to include wood thickness) I'm sure the air displacement of the sub is a little chunk as well. I ballparked it and went with 3.1cuft for my calculations.

Don't forget to click on the "signal" tab and input an average RMS wattage figure. (Think I ballparked it at 1kW) On the graph, then click the rear port air velocity tab.

Click back on the port tab. Pick a height and width that you can easily build on your box. Most people choose the height of the entire box as the height of their port. Then you can use one of the walls of the box to double as a wall of the port. The absolute minimum width you should go with is, say about, 2 inches. Super skinny ports with create turbulence across the edges of the port itself, often creating audible port noise.

You'll notice, while you're playing with the height and width figures, the length changes accordingly (you can now see how all these different combinations still give you a 35Hz tune) ALSO the port velocity graph moves. This gives you a ballpark idea of the amount of air pressure that will passing through the port at any given frequency. Anything higher then roughly 25-30m/s will cause audible port noise that can be heard in a hatchback. If this was stuffed in a trunk.... you might be able to go a little higher.... but alas, it's not.

[Cobalt camaro]

Re: ported box help

thanks alot man, your explanation does make sense. thanks again.

[95Batmobile]

Re: ported box help

The old school way, with a calculator :P
http://mobile.jlaudio.com/support_pages.php?page_id=165

[Cobalt camaro]

Re: ported box help

I just wanted to thank everyone that gave me advice and some direction. The box turned out great. I went with 4 cubes before sub and port displacement, and tuned to 35 hz. In the end the box volume was probally 3 cubes. It sounds great, no port whistle, and is really loud. I'll get some pics up when i get home later tonight