Port dimensioning help

[TheOtherSide 10+ year member]

Alright, to answer a few questions. Yes I am good enough with power tools to make what I am asking about. Yes, I am very good at math so if your answer is an equation, I can handle it. No, I am not crazy.

Here's the scoop. I have a Silverado Ext Cab. I am going to be putting a box under the seat with a 1.5" lift. I am looking to get about 2.25 net tuned to 28Hz. I may have to settle with 2.0 net though. By net, I mean gross volume minus port volume. My problem lies here though. The box can't be a typical rectangle. Sure, it'd make things easier and I wouldn't have to ask but I need all the space I can get to reach 2.0+ ft^3.

Now the question:

When figuring port length on a port that is just straight in a rectangular box, i.e. the 20 sq in of port area is equal at the output of the box and the internal input to the port, is it all based on volume of air moved through the port? If this is the case and the port dimensions change (both height and width), can you just alter the dimensions to make the available port area the same at the input (internal port) and output (the part you see on the outer surface of the port)? Also, can I just calculate necessary port volume and make the port long enough, wide enough to fill that given the 20 in^3 of port area?

See this drawing for an idea of what I am talking about:

Assume port lenth P(L) is 28 inches.

Port Area on the outside is given as P(Hi) {height} and P(Wi) {width}.

I need P(Wf) and P(Hf). The final port height is going to be variable though due to incident angle of the bottom of the box.

I am using 3/4" MDF.

{By the way, I just made these #'s up too. But they aren't totally random. I want 20 in^2 of port and the external dimensions are close but not exact.}

Click to read more...

 

[Trixter 10+ year member]

use the formula in my sig....

the port needs to be square all the way through...meaning that if the opening is 10" x 2" (to get your 20) the inner opening needs to be 10 x 2 also. the formula will help you out a lot. just remember that the length you get, you will need to calculat it's volume and add this to the gross internal volume. also, since your using the wall of the box as a wall of the port, you will need to add 1/2 the port's wodth to it's length for the correction factor. when finding the port's volume (displacement) you need to account for the wood it's made of also. but don't count the wood on the box side or the front 3/4" because you only need to account for the internal measurements. i gotta go to a meeting....need anything else, lmk.

 

[TheOtherSide 10+ year member]

use the formula in my sig....
the port needs to be square all the way through...meaning that if the opening is 10" x 2" (to get your 20) the inner opening needs to be 10 x 2 also. the formula will help you out a lot. just remember that the length you get, you will need to calculat it's volume and add this to the gross internal volume. also, since your using the wall of the box as a wall of the port, you will need to add 1/2 the port's wodth to it's length for the correction factor. when finding the port's volume (displacement) you need to account for the wood it's made of also. but don't count the wood on the box side or the front 3/4" because you only need to account for the internal measurements. i gotta go to a meeting....need anything else, lmk.

Unfortunately it's not that easy. As you can see, the height available to the port at the rear of the box is about 2 inches less than at the front. Thus, my dilema.

I will try to use the formula but being square all the way throughout, not even remotely possible. It will end up being more trapezoidal, basically a rectangle with a triangle added to the bottom.

Basically, if you can let me know if port size and length is determined by port volume then I can extrapolate the info I need.

 

[Trixter 10+ year member]

port length is determined by the port area and box volume. port volume means nothing except to find it's displacement. it needs to be square. what i would do in your case is have the port as tall as the back of the box. yeah, the opering at the front will not be all the way from top to bottom, but at least you maintain squarness. you have 8.5"H at the back. add 3/4" mdf for the bottom of the port and you have a 7.75"H port.