View Full Version : Help with my measurements!

01-21-2013, 12:07 PM
Hi guys,
Im new to the forum and was wondering if anyone can help me. I have got an alpine swr 1242d type r 12" woofer and i want to build a slot ported box for it. I have read loads of posts about making boxes and working out port length and area. From what i have read its optimal to have a box between 2-2.5 cubes tuned between 30-35hz. I have used various enclosure and port calculators and have come up with some numbers that id like you all to verify for me. The outside dimension's are h15" w26" d16" with mdf .708" thick. I have worked this out to give me a gross volume of 2.818ft^3. Now i have worked out the port to be 13.6" tall 2.5" wide with a length of 28.5"(not sure if length is right for 32hz tuning?). I have worked out that the volume of the box minus the port will be 2.257ft^3. So i worked out the volume of the port wall to be .218ft^3 so a final net volume of 2.039ft^3 with hopefully a tuning of 32hz. Can someone please tell me if these figures are right or if i have just wasted half a day trying to figure this out?

Many thanks

01-21-2013, 12:21 PM
Don't forget to take into consideration that other things take up space. As little as it does factor in sub displacement and any other bracing included. 45s will eat up volume as will 2x4 bracing etc etc.

01-21-2013, 02:17 PM
Thanks for your reply im sure i did take into consideration the woofer displacement in my calculations that i have written down but forgot to post it up, The displacement of the woofer is .071ft^3. When im at work tomorrow i will check what i have written down and correct any mistakes i have made in my previous post.

Thanks again

Firebird 1991
01-21-2013, 03:41 PM
the displacement of a type R is not .71!! Maybe you ment to type .17 cu.ft displacement...

01-21-2013, 03:44 PM
I don't understand how the port only takes up .218 cubic feet. Doing some quickies it should be more like .75 cubic feet.

01-21-2013, 03:59 PM
Thanks for the replies. The displacement of the woofer is .071ft^3 must of missed out the 0. Also the .218 is the volume of the port wall inside the box. The volume of the port is .560ft^3 so .778ft^3 including the internal port wall displacement. My net volume will be 1.969 ft^3. Now im not sure if my port will be the right length?

Firebird 1991
01-21-2013, 04:06 PM
.071 sounds to small lol, who knows. I know an sa-8 is 0.1 displacement and im just guessing that a type R is the same or more...

Firebird 1991
01-21-2013, 04:10 PM
you may want to consider having someone design the enclosure for you and then you can build it...just a thought

01-21-2013, 04:40 PM
I have just used the slot port calculator on the12volt.com and have come up with a port length of 26.94 so after i add the port wall volume inside the box it adds up to .75ft^3 as buck said. So will give me a net volume of 2.068ft^3 without taking into consideration any bracing. So if i make the box exactly 2 cubes net i will have 117.5in^3 for bracing which i assume will be more than enough? The other thing is do i need to take away the volume of the bracing when i work out the port length? Thanks again.

01-21-2013, 04:48 PM
Firebird i got the displacement from the alpine website so i don't think they are wrong. I would love someone to design me a box but designing it myself is better for me as i will then have the knowlege for the future so i can design and build more boxes correctly. I have made atleast 4 boxes in the past but have mainly been sealed, easier in comparison to a ported enclosure.