Box help again

[robby512]

Changed a little again Making a box for a soundqubed hdc3 15 and am going to have 6 cubic feet before displacment I tried to follow the soundqubed design and I think I did but what I came up with is

I have a 3.5 inch port and total port length is 21 inches

My main question is?

is that port to small?

and I'm guessing I have it's tuned in the 36-40 range?

Box dimensions are 40.5 x 14.5 x 21.5 (length x width x height)

[ham4864]

i may be wrong but that seem kinda big. But i'm sure JoeX will check for you.

[bbeljefe]

You have 5.86ftⁿ gross if your material is 3/4" and if you don't have laminated baffles or side walls.

Do you mean you have a slot port that's 3.5" by 20"? If so, you have 70in² of port area and you need ~134.

If you have a 3.5" round port... well, you have a sealed box with a leak.

[robby512]

Yes it is a slot port

Haha a sealed box with a leak

[robby512]

So I'm going to go threw my numbers again staring with cubic feet

Inner dimensions 39 x 13 x 20.5= 10393.5/1728=6.01 cubic feet

Now to get net volume I take cubic feet and multiply it by .82 which is 18 percent

.82 x 6.01= 4.928

Now we take the net and multiply by 16 square inches So 16 x 4.928= 78.848 sq" of port area

Now need to find the width of the opening for the port cause we already have height so we take

78.848/22= 3.584 which we round to the nearest .25 so 3.5

And my port width is 3.5 all the way around and I have first part is 17 inches down then 4.5 inches and it is an L port

So to get the total port area I have 17x3.5= 59.5

5 x 3.5= 17.5

59.5 + 17.5 = 77 total square" of port area with a port that is 22 inches long

I followed the directions right of sound qubed website.

And while anyone is helping those outer dimensions are all the make outer dimensions they can't be any bigger

[bbeljefe]

If your outside dimensions were correct in the OP, then your latest inside dimensions are incorrect. They should be 39 x 13 x 20, which is 5.86ftⁿ.

I don't know what net volume formula you're using there but how do you know ports, drivers and bracing will take up 18% of your enclosure? I'm referring to the .82 number you mentioned, which is 82%. Case by case, ports, drivers and braces may only take up 10% of your gross volume, in which case the multiplier would be .9... or they may take up 33% of gross volume, making that multiplier .67.

As for port area, 16in² works fine in low power boxes but you're using a driver with 35mm of xmax. That thing is going to displace a LOT of air when it's running hard and you can't rely on 16in² per foot with it. If you do, you'll have port noise when it's slamming.

Here's a calculator that will help you without having to remember or use the complicated formulas for port area. For your driver and tuning combination, you need 137in² of port area.

Just as an FYI, I find out what my gross available space is first. Then I model (in WinISD Beta usually) my driver(s) in a box volume that should fit in that space... then start with making it fit. But first, I have to know what kind of port I'll be using so I can figure exactly what it's gonna displace. If my port makes the overall box too large, I re-model the box at a lower volume and start the whole process of figuring out how big my box has to be again.

If you want to assume the port will take up ~20% of your available space and you know you have ~6 cubes to work with, then start by modeling a box that's 4 cubes.

[robby512]

Ok so I think I understand after your help and talking to a local shop let me try this again

I need a total of 137 sq" of port area.

Now I need to make a width of port I choose 5 inch wide. Correct me if I'm wrong but I can pick any number just about as long as total port area is correct numbers within reason about 3-8 inches wide would be fine?

So I will begin my port with a 15.5 inch long piece of wood going down but still have 5 inches between the end of the box and that wood. So since that obvisously won't be 137 sq" I need to add another one to make it a L port

So I choose a 11.25 piece of wood cause i still have .75 from the down of the port to add into so all together it will be 12 inches .

So when you do the math I have

15.5 x 5 = 77.5 12x5= 60 77.5+60= 137.5 total sq" of port

The total port length seems a bit long 27.5 inches long should I use a wider port won't the longer the port the lower the tunning? Shouldn't I be between 18-24

So then I will be using about 23 inches of port per cubic foot

[bbeljefe]

The mouth of the port where it exits the cabinet is the only place you measure for port area. What happens inside the box is irrelevant, so long as the port neither narrows or widens.

So if you have 15.5" of height, you'll need almost 9 inches of width to maintain 137in² of port area. You can fudge it a little, but just remember that the smaller you make the port, the more inclined it will be toward being noisy.

Port are is determined by xmax and tuning.... it really has nothing to do with box volume. And essentially, when your port is too small, it's like trying to move 10 gallons of water per minute through a pipe that can only move 5 gallons of water per minute. It just won't happen. And the harder you try, the more violently the water comes out. Same with air... when you pressure it up and try to force more air out (or in) than the port will allow, you get all sorts of noise, turbulence and other ill effects. Tuning will go out the window, driver efficiency will suffer and overall, you won't have a good sounding box.

In the end, if you don't have the space for that port and box volume at that tuning, you're much better off reducing the box volume and/or raising the tuning frequency to a point where everything works in concert. A sub with enough port to breath in a box tuned to 40 Hz will sound better than a sub starved of port area in a box tuned to 32 Hz.

[truthsayer]

http://www.the12volt.com/caraudio/boxcalcs.asp#rec Just go to this site, this will give you answers to every question you have on box size, type, ports, etc.

[fecupe2001]

137SQ of port for a single AQ 15" sub??? that´s dumb!