Can someone explain how to find the square inch per cubic feet when designing port?

[osiris10012]

What am I doing incorrectly for the tuning? I used the car stereo.com calculator set to 35Hz with the net volume after the sub displacement was accounted for and measured center to center making the port about a .25" longer than the length the calculator gave me because it was easier. Any help would be greatly appreciated. I really want to get this right and know how to do it properly.

[Joe X]

I don't use that calculator but even that would give a way longer port than you state:

[osiris10012]

I was trying to figure out the way you mentioned changing it to the net volume after the port displacement was subtracted and recalculate the port length but realized you could never get the port to stop getting larger. Every time you adjust the port for the now lower net volume it is going to automatically make the port larger. This is where I got confused. Because in theory I could extend the port out to 37.26" long and this would now make the port .48 cu ft dropping the net volume to .82. So when I put that in the port now needs to be 44" long. sorry if it seems like I am arguing because I am not. I just don't understand how I am suppose to keep recalculating the port for the net volume that keeps getting lower. I feel like eventually the whole box will end up being a port. lol. Am I misunderstanding something?

[Joe X]

Lol no, you are not arguing, what you are saying is just a fact, the reason I don't use the calculator above is that it can't figure displacements.

Anyways what you need to do is cut port area but only just enough, figure out the port displacement you need to have 1 cube net which is what the sub calls for, then you have to do a port in that volume that is tuned to what you want to but without cutting port area below 16 square inches, if you can do more area better, if you can't meet the 16 square inches of port area then loose volume by upping tuning a bit.

The volume of a port like that is calculated like Vp = (port height) x (port width + 0.75) x (port length)

[osiris10012]

What program are you using? I was playing around with winisd and it was saying around 27"L for a 3.5x5.25 port tuned to 35hz? But that program is confusing too. I watched a video on it and the graphs on mine just don't seem right so I don't know if I am doing something wrong. lol. If you don't mind would you be able to help me out in figuring this whole port length thing out? Sorry my brain is starting to hurt. lol. I just can't seem to figure this out. I know I basically have 1.3 cu after the .1 cu for sub displacement is subtracted. So in theory I have .3 cu ft. to work with for the port. I think my port area is close to the 16 sq in also. Heck I don't know. lol

[Joe X]

Try a 3W x 5.25H x 29L port, area will be 15 3/4 square inches which is near enough, that should leave you around 1 cube net @35Hz

[osiris10012]

Awesome, Thank you! I hope you don't mind me picking your brain a bit more but is there anyway you could break down how you figured that out?

[Joe X]

Don't take my word for it, update to the sketch and see how near it really got.

[osiris10012]

I don't know how you did that but I redid the sketch up with the new port and found its displacement is .30 cu. ft. Then checked the car stereo calculator and it said according to the new port dimension and using the 1 cu ft after all displacement was subtracted the port needed to be 29.87. Should I add that tiny bit extra or just leave it as is? Here is the new sketch up.

[Joe X]

I gave you rounded numbers, the difference is a fraction of a Hz so the 0.87 is irrelevant, add it if you want. Lastly you may want to round any sharp edges inside the port:

And also the external end edges should be rounded like previously discussed in the thread. I think that's about it!