View Full Version : port and sub displacement

hzsogood

11-10-2005, 02:25 AM

I ve been looking at a few sites on how to calculate enclosure size and im still not getting how to factor in sub and port displacement .. For example if i want a box that s 2.5 ^3 tuned to 30 hz net volume , the external volume would obviously have to be much larger and im just really confused on a simple way to figure this out. Just as an example a box 32x14x14 is 3.63 cubic feet (gross) now how would i figure out sub and port displacement for 30 hz with a sub displacement of .14 , any help or links would be cool , thanks

MrSector9

11-10-2005, 04:46 PM

You subtract the sub displacement so

3.63 - .14 = 3.49

So for the port figure how long of a port you need,figure out the displacement of said port and subtract that from your 3.49.

So say for a single 4" port using 3.49 you need a port length of 7.85, The volume approx of that is .06CF

So then 3.49 - .06 = 3.43CF NET

The only thing is once you figure that in your box is going to be tuned a little differently from the internal volume getting smaller form the port,but with a port that size it owuld only be like 30.15hz tuning instead of 30hz

For webapges

www.the12volt.com

www.carstereos.com

hzsogood

11-16-2005, 12:05 AM

thanks for the help. but just for futurre reference what is the formula for figuring out what ur tuning will be if u no what size u want ur port to be

seth350

11-16-2005, 05:33 PM

What I have always done is this................

Start off by messing with the measurments until you get the needed amount for your sub(s).

Ex: 34"x13"x18" = 3.56cuft internal.

How you get the cuft is take your measurments subtract 1.5 from each one and mulitply them all and then divide by 1728.

So, 34x13x18 becomes 32.5x11.5x16.5 = 6166.875, then divide by 1728 and you end up with a net volume of 3.56cuft.

Once you have your net volume you then calculate your port displacement.

Ex: Port is 18" tall x 3" wide X 30.5 long and that ends up being about 33.66hz.

What you do to calculate disp. is this...

Take the length of your port (30.5) and subtract .75 from it = 29.75"

Then multiply that by your height. (18)

Then take the width of your port (3) and add .75 = 3.75 then multiply that.

You end up with 1874.25 and once you have that, you then divide by 1728 to get your cuft.

Which is 1.08cuft.

Now once you have all of that, you then take your net volume (3.56cuft) and add your port disp. (1.08cuft) and then add your sub disp. and whatever that eqauls is how much cuft your box needs (gross volume) to end up with 3.56cuft after you add your port and the sub.

So then you take your original dimensions and adjust them to fit your gross volume.

Try not to adjust the height tho, cause then you will tamper with your tuning of the port.

But after all that you should have your measurments and everything. :)

Hope this helped!

hzsogood

11-16-2005, 11:20 PM

Yea thank u that did help out alot , cuz someone told me to only take off .75 for your internal measurements, but 1.5 makes more sense when i actually look at it.. But in ur port example how did u end up with the 33.66 hz tuning by just figuring out the port size ?? is there a formula to that or is it just easier just to use a port calculator ?? Thanks again

Matt

seth350

11-17-2005, 03:04 AM

I used WinISD Pro to calculate the port. A very handy program indeed. Just google it and you will find it. :)

hzsogood

11-17-2005, 11:38 PM

What I have always done is this................

Start off by messing with the measurments until you get the needed amount for your sub(s).

Ex: 34"x13"x18" = 3.56cuft internal.

How you get the cuft is take your measurments subtract 1.5 from each one and mulitply them all and then divide by 1728.

So, 34x13x18 becomes 32.5x11.5x16.5 = 6166.875, then divide by 1728 and you end up with a net volume of 3.56cuft.

Once you have your net volume you then calculate your port displacement.

Ex: Port is 18" tall x 3" wide X 30.5 long and that ends up being about 33.66hz.

What you do to calculate disp. is this...

Take the length of your port (30.5) and subtract .75 from it = 29.75"

Then multiply that by your height. (18)

Then take the width of your port (3) and add .75 = 3.75 then multiply that.

You end up with 1874.25 and once you have that, you then divide by 1728 to get your cuft.

Which is 1.08cuft.

Now once you have all of that, you then take your net volume (3.56cuft) and add your port disp. (1.08cuft) and then add your sub disp. and whatever that eqauls is how much cuft your box needs (gross volume) to end up with 3.56cuft after you add your port and the sub.

So then you take your original dimensions and adjust them to fit your gross volume.

Try not to adjust the height tho, cause then you will tamper with your tuning of the port.

But after all that you should have your measurments and everything. :)

Hope this helped!

Cool thanks, last question tho.. Y did u subtract .75 from ur port lenght example and then add .75 to the width .. Sorry bout all the questions im just tryin to catch on . thanks again

DBfan187

11-17-2005, 11:40 PM

because it uses the walls of the enclosure itself as the port.

seth350

11-17-2005, 11:45 PM

No worries man, if you dont ask you wont know ;)

But yes as BDfan said, it uses the wall of the box