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View Full Version : Calculating and Designing Boxes (my logic)



rpfuror
02-06-2007, 09:46 AM
I have seen hundreds of question in regards to box design, I wanted to put out some basic fundamentals so that if you really want to design something yourself you will have a starting point, which is if you are willing to dig thru my logic.

First, I am not going to cover calculation of port length and width. I have described this a couple times on the forums (so you can search my nick) and there are plenty places to get the calculation you need (jl's site, carstereo.com many others).

I'll try to lead you in the right direction, as it can be a cumbersome process as there are many way to calculate the same thing, I will give you an idea on how I started to do it with basic math.

You determine port length by use of internal box height, desired tuning frequency and target Net volume. Now this volume is never going to be "fixed" as when you add of take away length you change the volume (if you basing this value on final external L x external W x external H of the enclosure).

I’m only going to speak to a “slot port design”.

Ok, here is the fun stuff; you have to work with internal volumes not external.

You will need the following to determine tuning and box dimensions.
eH = Target external Height of your box (needed also for port calculation)
eW = Target external Width of your box
eL = Target Box External Length
wT = Wood Thickness
Lv = Calculated port length (I’m not going to touch this here)
pW = Determined port width (I’m not going to touch this here)
Vb = Target net volume (needed also for port calculation) in cu/inches

Note: add your extra internal displacements in cu/inches to Vb

Calculate the internal height of the port/box; I will call that iH (internal Height).

iH = eH – (2 * wT)
This value gets plugged into the equations for tuning.

Calculate the internal width of the box; I will call this iW (internal Width).

iW = eW – (2 * wT)

Now you can keep changing numbers until you get somewhere around the values you are looking for or you can stick with 2 dimensions, to make things easy use height as one variable as to not change the length of the port, to solve for the other. In this example I will be solving for Depth based on Length and Height

You will have to add the thickness of one wall into this equation; I will call this ipW (internal port W).

ipW = pW + wT

Next determine total internal volume displaced by the port, I will call that pV (port Volume).

pV= Lv x iH x ipW

Now you need to add the internal port volume to the net volume you are targeting, I will call this giV (gross internal Volume).

giV = pV + Vb

Now you have all you need calculate your correct external Depth of your box to fit your target port and net volumes, I will call this iD (internal Depth).

iD = giV / (iH * iW)

Finally calculate the external depth; I will call this eD (external Depth).

eD = iD + (2 * wT)

You have just designed your box eW x eH x eD !!!

PV Audio
02-06-2007, 10:21 AM
That's okay, but you need to use actual audio terms and not your own. It makes zero sense otherwise.

bdawson72
02-06-2007, 10:26 AM
All the boxbuilders should get together and make a sticky

azbass
02-06-2007, 10:27 AM
no because ill never follow it. :)

rpfuror
02-06-2007, 10:37 AM
That's okay, but you need to use actual audio terms and not your own. It makes zero sense otherwise.

This is why I stated ďmy logicĒ. As the logic is good, but is the way that I am using variable and commenting on the variables. If someone wants to equate them to ďAudio LingoĒ I will change then to follow a standard.

I would also be willing to add the port tuning to it if it is necessary, but it will be much more cumbersome then what I have already documented.

Please make comments and test the logic to see what you get;)

j3bus2k3
02-06-2007, 12:57 PM
I'm sorry but I find that whole process to be even more cumbersome that the whole design process itself...

No offense, but I think once new box builders see this, there just going to get more confused; not to mention, all this does is computer external dimensions and deals nothing with port calculation...

rpfuror
02-06-2007, 02:42 PM
I generated this based on a response I was going to put in another forum post. I thought it would benefit more people having it be it own forum then being buried in an anther.

If you’re not using an application to spit out values you would have to do something very similar with a pencil and paper... again if you think it would be beneficial I can post the calculations in dept from the start.

Also, I’m still looking on input about variable translation to a standard.

j3bus2k3
02-06-2007, 04:05 PM
Port Length is Lv and Net volume is Vb...

How about:

Fb=.159sqrt([Av*1.84x10^8]/[Lv+.823sqrt{Av}])

End correction factor k=1/2(width of port)

Or for round ports:

Lv=[(8466.4R^2)/(Vb*Fb^2)]-1.463R

Fb=Tuning Freq
Av= Port Area
Lv=Port length
R=Radius of the port

Those are really, the only 3 formulas you need. If you dont know how to calculate cubic volume, then you have no business designing enlcosures...

rpfuror
02-06-2007, 04:15 PM
Although, you are correct by stating:


If you dont know how to calculate cubic volume, then you have no business designing

The truth is people don't know or understand how it relates to adding or subtracting port displacment. Hence the reason for some may ports all on the same basic question.

That's what I'm trying to get across by putting together the basic math so if someone was interested it would give them a place to start.

PV Audio
02-06-2007, 04:21 PM
Cubic inches / 1728 = volume.

PowerNaudio
02-06-2007, 05:30 PM
Cubic inches / 1728 = volume.

= cuft = Cubic feet

phyphoestilic
02-06-2007, 05:37 PM
ur trying to make something more difficult then it actaully is

sku
02-06-2007, 06:35 PM
In all honesty, if that's for a beginner is pretty complex.

You can make a simple faq for them.

Just simply tell them to:

1) figure out the net internal volume they have

2) figure out the desired tuning frequency and port area

3) plug the data into a web port length calculator

That's all I ever have to do when I build boxes....

ultimate157
02-06-2007, 07:05 PM
According to:


Lv=[(8466.4R^2)/(Vb*Fb^2)]-1.463R

A box with volume 1.2 cubes requires a port length of 26in for a tuning of 35hz, correct?

PV Audio
02-06-2007, 07:12 PM
= cuft = Cubic feet:emb: Shoulda put that on there.

rpfuror
02-06-2007, 07:41 PM
:emb: Shoulda put that on there.

:p:
I guess it was far past a beginner point of view (I due that often), I guess should have stated the the basics.


In all honesty, if that's for a beginner is pretty complex.

You can make a simple faq for them.

Just simply tell them to:

1) figure out the net internal volume they have

2) figure out the desired tuning frequency and port area

3) plug the data into a web port length calculator

That's all I ever have to do when I build boxes....

In that case when the site goes down that spit out your values no more box design fo you. This also suggests people know how to determine / understand anything your refering to. ( not like my logic is any more understandable :rolleyes: )

sku
02-06-2007, 08:25 PM
:p:
I guess it was far past a beginner point of view (I due that often), I guess should have stated the the basics.



In that case when the site goes down that spit out your values no more box design fo you. This also suggests people know how to determine / understand anything your refering to. ( not like my logic is any more understandable :rolleyes: )

Oh c'mon man...how many of those site are out there? You can't be serious about the site thing?

rpfuror
02-06-2007, 08:36 PM
Oh c'mon man...how many of those site are out there? You can't be serious about the site thing?

Actually I was being quite sarcastic.

What I was trying to get across is not to trust blindly something that spits out answers as some of them arenít always correct. (subwoofertools comes to mind)

Learn to do it yourself so that you donít have to rely on the sites.

sku
02-06-2007, 08:50 PM
Actually I was being quite sarcastic.

What I was trying to get across is not to trust blindly something that spits out answers as some of them arenít always correct. (subwoofertools comes to mind)

Learn to do it yourself so that you donít have to rely on the sites.

Gotcha, nothing wrong with learning...

helotaxi
02-07-2007, 08:14 AM
A box with volume 1.2 cubes requires a port length of 26in for a tuning of 35hz, correct?

Maybe. There's a cirtical piece of information missing.

PV Audio
02-07-2007, 08:44 AM
*raises hand* Ooooh ooooh! I know!

rpfuror
02-07-2007, 08:58 AM
^^ :naughty: