Calculating Slot Port length

[shizzzon 461]

I want to make sure I calculate this correctly because apparently i am slightly off, either that or other manufacturers are off.

When calculating port length, let's say it says to use 26" long and you are, for example, using 12.56sq in of port which is for a 4" diameter port. Radius is 2"

I thought to calculate slot port length was to take the 26" depth and subtract from that the sqin radius by converting the sqin value to a circle then finding the radius, which in this case is 2".

That would then make the length 24" but when looking at manufacturer's giving you specs like a baby, either their calculations are wrong or i have been told wrong!

So, what do i need to know to get tuning one is looking for?

[audio-neon 34]

I want to make sure I calculate this correctly because apparently i am slightly off, either that or other manufacturers are off.

When calculating port length, let's say it says to use 26" long and you are, for example, using 12.56sq in of port which is for a 4" diameter port. Radius is 2"

I thought to calculate slot port length was to take the 26" depth and subtract from that the sqin radius by converting the sqin value to a circle then finding the radius, which in this case is 2".

That would then make the length 24" but when looking at manufacturer's giving you specs like a baby, either their calculations are wrong or i have been told wrong!

So, what do i need to know to get tuning one is looking for?

IIRC you only take the half of the port height for slot ports. If you are using pvc piping, it's not really a slot port. Just remember to keep it atleast a port radius away from any other object (the interior opening).

[shizzzon 461]

No, that's not entirely accurate.

I still haven't found the actual formula but JL audio mentioned something about it. I believe they actually have the formula and i was just too tired to do the math.

Anyway i found a calculator that will do it for you and yes, if you change the height and width and maintain the same sq in value, the length does change so it must be accurate.

http://www.carstereo.com/help/Articles.cfm?id=31

[shizzzon 461]

came across something "odd".

This calculator that i posted above, let me give you guys the info and answers it's givin me because somethin is "odd" about this-

3.8 cuft

tuning - 40hz

1 slot port

Dimensions - 16.25" high x 3.75" wide

Required length - 18.69"

As mentioned earlier, the height of the slot port effects the acoustical length so i reversed the dimensions-

1 slot port

Dimensions - 3.75" high x 16.25" wide

Required length - 12.44"

The port for both calculations is using 3 walls of the box for the port...

Both calculations are the same really, it's just the port is turned to it's side in the 2nd one...

I don't understand...Is this accurate?

My internal box dimensions are -

16.25" high x 16.25" deep x 34.75" wide

So you see, both scenarios max out height and depth(this is width for the port in 2nd calculation)

[BKOLFO4 29]

3.8 cuft

tuning - 40hz

1 slot port

Dimensions - 16.25" high x 3.75" wide

Required length - 18.69"

As mentioned earlier, the height of the slot port effects the acoustical length so i reversed the dimensions-

1 slot port

Dimensions - 3.75" high x 16.25" wide

Required length - 12.44"

Calculate that box without slot port checked, and you will see it uses 1/2 whatever you put in for width. That is the calculator I use.

What I have found building boxes and actually measuring the tuning frequency is that once you get over 4-5" in width, the slot effect changes in real life.

A port is tuned based on resistance. The slot effect is nothing more than the resistance changing due to the added friction of the wall extending beyond the actual port. When the port gets wide enough, that addition resistance had less effect due to so much port area away from the wall.

If I build a box with a port more than 6" wide, I calculate standard port length and then subtract 3". Works pretty good so far. . .

Brian

[audio-neon 34]

No, that's not entirely accurate.

I still haven't found the actual formula but JL audio mentioned something about it. I believe they actually have the formula and i was just too tired to do the math.

Anyway i found a calculator that will do it for you and yes, if you change the height and width and maintain the same sq in value, the length does change so it must be accurate.

http://www.carstereo.com/help/Articles.cfm?id=31

Actually what I said was entirely accurate.

You take the port height (from the port wall to the box wall) divide by 2 and subtract that from your port length. That was from the JL Audio formula.

As noted JL audio's method does not take into account for large ports.