View Full Version : I need the math behind port WxH, hz, and length. Help!

06-29-2009, 02:19 AM
Hey guys, I am going to be extending a port on my box that is tuned to 41.5hz, to 31.5 hz. The length is currently 18 inches and I need to get it to ~35.85 inches. For this, I am going to have to bend my port multiple times to fit in my trunk. This means I need to know about how bending affects a port, AND the math behind the relationship between port width, height, length, and hertz. I can't rely on a solid length and then change the port opening size to get a certain hz, becuase I need a different length!

So, if anyone has any information on this, especially the math formula, I was would pleased to hear from you.

06-29-2009, 05:00 AM
just remember also that by extending your port you will lose cuft on the box and that also afects the tuning(hz)

06-29-2009, 11:48 AM
Hmm ok will that make a
Noticeable difference? Also, any idea on how I can manipulate the port without a huge
Equation? Since I know my 3 dimensions already, can I take their volume, keeping their hxw consistant, and bend it as much as I want?

06-29-2009, 04:12 PM
Oh sorry I read cu ft wrong in your post. The rest of the port will be
On the outside of the box so I won't lose any cubed ft

06-29-2009, 06:33 PM
Ok, here's the what you need to do:

-Find the port area

-Find the port length from the center of the port through the whole thing..bends, etc (it's much easier to make your enclosure on sketchup instead of doing this on paper/thinking it out)

-Find the net airspace left inside the enclosure after you take away the port's displacement and the sub(s) displacement...find it in cubic INCHES

After you've found these three pieces of info you can find your tuning with this:

-Square root of the port area

-Mulitply that by .823

-Add your port length onto that

-Multiply this by the net airspace you found in cubic INCHES

-X-Save the number you have now for future use-X-

-Now take 184000000 multiplied by the port area

-Then devide by the number you came up with in the previous steps

-Square root this number

-Multiply by .159

You now have your tuning (if your specs were calculated correctly) :)

Knock yourself out :cool: And remember:
-Longer port length = lower tuning
-Smaller port area = lower tuning
-Larger airspace = lower tuning

Hope that helps :)

06-29-2009, 09:27 PM
The more bends in the port the more turbulence/port noise you'll create. Just build yourself a whole new box.

06-29-2009, 10:12 PM
Thanks for the help saturn! This will come in handy.

To KOL: I'm strapped on cash so I'm trying to make due with what I have atm (college = little money). I'm starting to think pvc tubes are a better choice now because I can just seal my current slot port with wood and bend some tubes up for the inside, while also easily being able to extend the port outside of the box too, if necessary.

If I did do this, would a 6" diameter be enough for each chamber? Or should I double up. I'm running 2.25 ft^3 airspace for each sub right now, which will be 2.5^3 after chopping down the old port. What do I factor in a second port if necessary?

06-29-2009, 11:43 PM
The more bends in the port the more turbulence/port noise you'll create. Just build yourself a whole new box.


if you cant do it right, dont do it at all

06-29-2009, 11:54 PM

if you cant do it right, dont do it at all

No thanks!