ports

hi. i have begun building a ported woofer box for 1 of my fusion woofers. Now the spec states that i need a 16.8inch long port with a 4 inch diameter. I cant get hold of any type of port at such a length. i was wandering if i could change this port to a slotted port. this just requires me to build the port but i also want to keep it the same frequency and i still want to get the best performance out of my sub. so if there is anyone out there who knows how to change a port into another port type please HELP!!!!

I also just want u to know that the tuning frequency is 30.1 with the port at 4*16.8inches.

RESPECT!!

One way is to convert the 4" round port into equivalent square inches, then find two side lengths of a rectangle that will make that equivalent number.  Use pi R^2.

R (radius) = 2" and R^2 = 4...............pi = 3.14............so 3.14 X 4 = 12.56"..................so we see that the area of a 4" circle is 12.56".

Now to find two sides of a slot opening that will equal 12.56", you start with any number in inches that you want one side of the slot to be.  Say the slot will be long and narrow, so use a small number for the short side.  Divide that number into 12.56 and you will get the length of the long side.

example:  I want the short side of the opening to be 2".  12.56 / 2 = 6.28".  The slot equivalent of a 4" round port would be 2" X 6.28".  The tuning will remain the same if the length stays the same as you have it at 16.8".  And as long as the net air volume remains the same (see next paragraph).

(pi = 3.14159265 to be exact.)

Note that you should measure the inside volume of your enclosure, deduct all displacements and find actual cubic feet in air volume.  The actual port volume including its walls have to also be deducted from that volume total to arrive at net air volume.  When the port type changes (round PVC port to slotted MDF port) the total cubic inches the port takes up grows considerably because of the thicker port walls you will be using.  Therefore the entire volume of the box has to be bigger if you want to use the port length recommended by the manufacturer, as it is based on the air volume of the enclosure after all displacements have been allowed for.

And here is one port design calculator that you could use to make things easier.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

thanks alot for ur help stevdart, i've begun to build the box with all considerations of wood thickness.

PVC-i couldn't get a port at 16.8inches but i never thought of buying a piece of PVC. nice one mate.