View Full Version : check box specs

the_flatlander

12-07-2005, 06:58 PM

this is my first box I have designed and wondering if my math would be correct. 12" XXX box is crx style 4^3 tuned to 29hz, 52.5^2 of port area and port length is 36" here is a very crude MS Paint pic that I drewhttp://img370.imageshack.us/img370/8558/box4tt.th.png (http://img370.imageshack.us/my.php?image=box4tt.png)

and it's all going in a '95 integra hatchback

swimfreak26

12-07-2005, 10:46 PM

I just checked it over REALLY quickly, but I get that the box is about 2.54 cubes tuned to 36-37Hz assuming a sub displacement of .2 cubes

Now, I didn't do hand calculations, and actually I just redid my excel calculator today so there could be bugs, but I think it's right.

Your box is 4.03 cubes gross. Now, what that means is that if it were a sealed box it would be 4 cubes. Being ported however, the NET volume is what matters. Net volume is that after all displacements, meaning the port, port wall, and driver.

From another quick calculation, to get a 4 cube box tuned to 29Hz, with that port dimension (3x17.5) you're looking at a box that is about 5.6 cubes gross.

swimfreak26

12-07-2005, 11:13 PM

Ok, this is not drawn to scale AT ALL, but the dimensions should be correct. I'll check them over tomorrow to make sure, but for a real quick draw up, I think it's correct.

http://img525.imageshack.us/img525/120/dsc001034ke.th.jpg (http://img525.imageshack.us/my.php?image=dsc001034ke.jpg)

the_flatlander

12-08-2005, 01:41 AM

I don't get your dimensions, they are like the same as mine

swimfreak26

12-08-2005, 08:29 AM

They are not the same as yours ;)

You did the port calculation correctly, for a box that was 4 cubic feet. The problem is that your box was not 4 cubes, it was ~2.5 because of port displacement, port wall displacement, and sub displacement which you did not take into account.

My port is the same as yours because that calculation was correct, with 52.2" of port area you need 36" in a 4 cube box to get a 29Hz tuning.

I just did all the math that takes some thinking for you, and extended the box as to fit your desired size/tuning. Now, I'm not sure that will fit in your application, may need to re-"shape" it, but that's an example of a 4 cube box tuned at 29Hz.

the_flatlander

12-08-2005, 12:07 PM

Now I understand, thanks

the_flatlander

12-08-2005, 01:13 PM

could I add 5.5" to the depth to keep the length at 31 and height at 15"?

the_flatlander

12-09-2005, 10:59 AM

anybody?

swimfreak26

12-10-2005, 12:42 PM

If nobody else gets around to this I'll grab my calculator later on this evening and let you know.

the_flatlander

12-12-2005, 11:53 AM

I'm still confused on how you got all of the displacements

swimfreak26

12-12-2005, 12:02 PM

God I keep forgetting to check this thread.

For the total (gross) box volume, subtract 1.5 from all external dimensions and do L x W x H/1728 which will give you the volume in cubic feet of the enclosure.

Port displacement is a bit trickier. First you have to determine what length of the port is actually inside the box. For instance, the first .75" is usually the front baffle, and therefore not included in the displacement calculation. Once you determine how much of the length resides in the box then lwh and you've got the displacement.

The port wall displacement requires a true drawing of the box. You've got to take the actual volume of the wood that you use for the walls of the port. Again an easy calculation once you determine the lengths of any walls.

Driver displacement is a specification provided by the company that makes your sub. In general you can expect a 15 to displace somewhere around .2 ish, but should always check to make sure - if you want a really accurate box.

So what you want to do over all, is find out how much total displacement you're going to have, and "add" that to the volume you want your box to be. Then calculate dimensions that will give that new, increased, volume.

Be sure to check calculations at the end though because when you extend certain dimensions sometimes you change the wall displacement.

the_flatlander

12-13-2005, 10:12 AM

ok how about this one... 3.5^3 tuned to 28hz, port length 45.5" port area 52.5"2, dimensions lxDxH external- 31x19.98x15

the_flatlander

12-13-2005, 10:14 AM

how do I figure for bracing

swimfreak26

12-13-2005, 10:48 AM

ok how about this one... 3.5^3 tuned to 28hz, port length 45.5" port area 52.5"2, dimensions lxDxH external- 31x19.98x15

I figure that that would give you a box ~2.44 cubes tuned 33-34Hz

Extend the external dimensions to 38.5 x 20 x 15 and you've got right around 3.5 tuned to 28Hz

EDIT: For bracing, calculate it as another displacement. Determine where you want braces, and of what size...then do the math again. All the calculations I've given you do NOT include bracing.

the_flatlander

12-13-2005, 11:54 AM

what do I keep doing wrong I'm using http://forums.caraudio.com/vb/showthread.php?t=74715&highlight=12%22+xxx+displacement to help me design my box::confused: :wallbash:

tRiGgEr

12-13-2005, 03:49 PM

Use your brain.

Subtract all displacment from the box size. It is that easy. If you need a bigger box increase the dimmensions. It's not really rocket science.

the_flatlander

12-13-2005, 05:55 PM

ok now if this one isn't right I'm gonna :wallbash: now we got port area=45^2 internal vol=3.5^3 tuning frequency=28hz http://img453.imageshack.us/img453/9116/box31zk.th.png (http://img453.imageshack.us/my.php?image=box31zk.png):uhoh: :uhoh:

went over math again and I think internal depth should be 20" and port wall 1 should be 17" and port wall 2 should be 15.6"

the_flatlander

12-14-2005, 01:04 PM

I really hope I got it this time

the_flatlander

12-14-2005, 03:33 PM

could someone plz tell me if I finaly got one right so I can go and build the **** thing today on my snowday

swimfreak26

12-14-2005, 05:03 PM

Almost there :)

If you have the internal depth to be 20, like you mentioned (but wasn't in the pic) then you're looking at right around 3.5 cubes at 28Hz (a little less, like 3.45ish, no biggie). That's with EXTERNAL dimensions being 21.5 x 31 x 15.

The only other issue I saw with it is that your port area is wrong...it's really 40.5 square inches because you have to account for wood on the top and bottom (3 x 13.5).

I didn't check the port walls, but I assume they're correct as long as you get a port length of 34.5" which is what I am getting.

the_flatlander

12-14-2005, 05:31 PM

how deep would it have to be to make the length 31" external this **** is putting knots in my brain

would 29.5*19.5*13.5 work?

swimfreak26

12-14-2005, 05:52 PM

umm, it is 31?

swimfreak26

12-14-2005, 06:03 PM

would 29.5*19.5*13.5 work?

This is 3.5 cubes before any displacement.

the_flatlander

12-14-2005, 06:07 PM

I got 4.5^3 before any displacement

the_flatlander

12-15-2005, 05:03 PM

Almost there :)

If you have the internal depth to be 20, like you mentioned (but wasn't in the pic) then you're looking at right around 3.5 cubes at 28Hz (a little less, like 3.45ish, no biggie). That's with EXTERNAL dimensions being 21.5 x 31 x 15.

The only other issue I saw with it is that your port area is wrong...it's really 40.5 square inches because you have to account for wood on the top and bottom (3 x 13.5).

I didn't check the port walls, but I assume they're correct as long as you get a port length of 34.5" which is what I am getting.

oh I get it I think I just got all screwed up because you but the 21.5 before the 31 in the dimensions :crazy:

the_flatlander

12-20-2005, 11:54 AM

so how does this sound for dimensions to cut

2- 31X21.5

2- 20X13.5

1- 29.5X13.5

1- 26.5X13.5

1- 16.25X13.5

1- 11.5X13.5

the_flatlander

12-21-2005, 07:45 PM

anybody? plz help