i found a port calculator on the internet and started playin with it. my friend wants me to build him a box for his expedition for 3 twelves tuned to 35 hz. i started with a small slot port like 3 inches x 12 inches and went up from there. one port i found that said it would work is 17x6 inches with the length of 19 inches. i dont think that can be right.17x6x19=1938 sq inches of port.

can this be right? will it sound ok?

 

Your dimensions are wrong. L*L*L=L^3. So your 1938 is cubic inches of space, which is irrelevant for the question at hand. A 17"x6" port is 102sqin of port area. The length of the port is a variable dependent on the port area and net volume of the box through a series of equations.

For a tune of 35Hz, that is a 8.2cuft net box. Perfect for 3 12's, if not a little big. Gross will be like 9.5cuft plus driver and wood displacement. What kind of 12's are they?

Download WinISD. Its anecho response graphs are perfect world and therefore, often inaccurate. Some of the other information in WinISD can be helpful, though. Most importantly, it is a user friendly port calculator. You can set the net volume and change any of the other variables (length, tune, port size) and it will calculate all of the other variables for you.

I'd explain how I design boxes, but It'd be a short novel with a lot of algebra and a bit of calculus. Math is a wonderful tool in the speaker world. It allows you to maximize and optimize volume, port area, and tune with the amount of space available. I used to charge people for box plans over on CA.com. Then people saw that I was making some pocket change from it and started doing the same. The difference is I put a lot more time into it to optimize the box for their application. However, people who need people to draw them box plans usually dont care about all that junk so my competition beat me out of the business by getting the job done, reguardless of how well it was actually done.

/ramble

 

x2

There are a lot of people who just go out and buy generic sub boxes, throw a sub in, attach it to an amp and then drive around looking and sounding like a cheap white rap star.

Even those guys that build their own usually do it all wrong. The sub box needs to be built to suit the sub, and THEN make it fit into the application (trunk or supercab....just an example).

The sub box I have was made to spec as per what each sub needed in terms of cubic inches. Then it was configured to fit into the trunk of my car. Since then I've modified the outside of the box to fit behind the seats in my truck, however the interior cubic space hasn't changed. The amp was tuned to meet the new dimensions and interior cab space of the truck. I like to hear my music, but there's no need for the guy in the next car to.

Just design the right box for the subs and tune the amp to work with the sub specs and you'll be o.k.

 

the 3 subs are 12 inch alpine type r.