Box Design: A few questions

[Maxim]

Oh god. It's 3 in the morning and I'm still up trying to figure this thing out and something just hit me in the face and I realized I still have no idea what the fuck I'm doing...

For future reference, any time I refer to a "calculator", I'm referring to this: http://www.the12volt.com/caraudio/boxcalcs.asp

Anyway, when I try to calculate the port length for a box, it has me input the width and length of the port, as well as the box volume and desired tuning frequency. The calculator then spits out a number based on some calculations I do not know. I have a couple questions about this.

1: is the port length the calculator spits out for an external port? I keep double-checking myself with this, and getting more and more confused. If the port length it gave me was for an internal port, would I then have to subtract the volume of the port from the box's volume and re-calculate the port length again? Or is the value for an internal port, and no additional calculation is required on my part?

2: What the hell kind of calculations does this program do? I took AP Physics and AP Calculus last year, so I should recognize some of the formulas. I just don't know how the program computes the values because I don't know the formulas. Also, I'm majoring in Computer Science, so I'd like to build myself a little program for my own personal use, so knowing the formulas would do me some good there.

Once again, thank you guys for all your help!

[LordArcane]

well once you figure it all out and build the program send to me i'll for sure give it a few good trys! so many box idea's in my head for myself and for friends and family that i got alot to think about lol

[Maxim]

Okay. I think I've finally got this sonofabish figured out. the12volt's calculator, when used to calculate port length, gives you the port length for an internal port. I figured this out by designing a box for 4 RE-MT 12's and using it as my guinea pig. Anyway, I built it using the12volt's calculator, then went and compared it to the results I got off another calculator, found HERE. They both worked out to be the same, a box tuned to 34.07 hz. Pretty close to the 34 I was shooting for

Anyway, moral of the story is that I think I finally figured this thing out. I'll post a couple pics of the box to show what I was up to. It wound up being only about 6.6 cubes after displacement, which I know is small for 4 MT 12's, but it was simply for learning purposes, and I figured smaller was better for a start. I'm still not sure how L-shaped or aero/flared ports work, but those are for another day. Thanks to everybody who helped me out with this thing. I know I learned a few useful things, and I hope I helped a few others out as well. Time to get some sleep though. :lazy:

Figured I might as well post a pic or 2 of the box. I spent a while on it, even if it's not the prettiest thing ever =)

[andym85]

im not sure if the 12volt calc subtracts the port volume from the box volume u enter or not...which is why i use my own..and trust me, there are still a few gray areas i encounter...

bot looks good, but port looks kinda small...remember, u want 12-16sq" of port area per cubic foot of box....so for a 2cube box, port area would be between 24-32"...give or take a few depending on things..

[Sanitarium]

im not sure if the 12volt calc subtracts the port volume from the box volume u enter or not...which is why i use my own..and trust me, there are still a few gray areas i encounter...

No it does not.

Heres some ways to work around it.

1. Only use 2 set in stone measurements for your box.

Example: 24" by 48" area, don't care about height. you want 10 cubic feet, 150 inches of port tuned to 35 hz.

so you figure a port of 15x10, 1" thick wood throughout, double faceplate (2" thick), no shared walls for ports

and say 1 cube for displacement of subs. and port coming out of the face plate.

So you figure the port first. you need 17.89 inches of port. round to 17.875 for ease of use (7/8")

port displacement = 17x12x15.875 (see previous post if you don't know how i got those numbers) = 3238.5 cubic inches.

now figure total room you need.

speakers = 1 cube = 1728 cubic inches

box chamber = 10 cubes = 17280 cubic inches

port displacement = 3238.5 cubic inches

total interior volume needed = 19336.5

take what we know and work it together to get what we don't know.. basic algebra

24x48xH=19336.5

19336.5/(24x48)=H

H=16.785 (round to 16 3/4")

2. instead of varying yoru box dimensions, guess at port length, adjust chamber size to compensate and check the calculator if the tuning is where you want it. << someone needs to come up with an excel spreadsheet for this one.

[andym85]

i have a spreadsheet that automatically does those things....but its gets a little in depth with slot ports, and probably isnt that user friendly for most people.

may be able to come up with an extremely simplified version though...

[Sanitarium]

i have a spreadsheet that automatically does those things....but its gets a little in depth with slot ports, and probably isnt that user friendly for most people.

may be able to come up with an extremely simplified version though...

All it would need to do is allow you to give it port length and box internal volumes then feed out the final chamber volume. then you would just change port length and see what the chamber volume changes to. then punch the new numbers into your favorite calculator.

[andym85]

it gets a little complicated, because u can put a port on the top, side, or front....which would change the shape of the slot port (because of the different depths) and therefore change displacement.

plus, port length is measured down the middle, and spitting out a port length doesnt really mean much, because you dont know if they are talking about the length of the wood, or the measurement down the middle of the port....or if takes into account end correction.

this is why i find it difficult to make a very basic calculator, and why mine might seem a little complex to some...which its not...u just have to know how to use it...

[Sanitarium]

it gets a little complicated, because u can put a port on the top, side, or front....which would change the shape of the slot port (because of the different depths) and therefore change displacement.

plus, port length is measured down the middle, and spitting out a port length doesnt really mean much, because you dont know if they are talking about the length of the wood, or the measurement down the middle of the port....or if takes into account end correction.

this is why i find it difficult to make a very basic calculator, and why mine might seem a little complex to some...which its not...u just have to know how to use it...

So an input box for "Number of common walls" would be easier for the end user, but probably harder on the programer.

[andym85]

yea, that wouldnt be too bad...that actually wouldnt be too bad at all come to think of it

the only thing i dont like about most calcs is that u dont know if port length is the physical length of the wood, or the measurement down the middle of the port....which really is only a problem with "L" shaped ports...because its multiple pieces of wood, but one measurement, and the actuall length of the port that is measured down the middle is going to be quite longer than the wood used to make it...