Yeah I stole this from someone...thanks Used2Know) but I have used it a few times and I thought it would be useful here. I will add more as I find them. Kind of a compilation of many threads from other sites.
Also free google sketchup download is great for drawing up designs, boxbuilder pro6(not free), winISD once you figure out how to use it.
- figure out your workable cubes (space you have to fit your enclosure, the max dimensions minus 1.5 inches from external measurements (hxwxd), giving you your gross internal volume assuming you are using 3/4 mdf)
- choose an alignment given your speakers parameters or use an accepted real world tested enclosure size and tuning that will fit via your workable cubes
- use the first port length formula to figure the first unadjusted port length
- calculate port(s) displacement and any other internal displacements i.e. speaker(s) or any bracing and then subtract that from gross volume
- use the other port length formula to figure out tuning
- adjust length, thusly affecting your volume obviously, until you are satisfied with end result
You most likely will need to only recalculate twice or sometimes trice.
- height * width * depth / 1728 = Ft3
- (outside diameter/2)^2 * 3.14 *(length of port inside said enclosure)= in^3
- then obviously convert that to ft^3
- don’t worry about the internal flanges of an aero as they are minuscule in the whole scheme of things
Multiple Port Diameters
- SQRT(A^2 + B^2) = total combined diameter
Slot Port Displacement
- calculate the internal volume inside the port as well as the volume of the wood making the vent wall(s) for the port and then subtract that from the internal volume of the enclosure
- length * width * height / 1728 = Ft3
Slot Length to Width Ratio
- length/width = ratio
- keep it under 9:1
- most manufacturers include this in their parameter specs, but if not use this following formula, not 100% accurate but works none the less
- 4 * 0.33 * 3.14 * (depth from back of basket to rear of magnet)^3 * 0.5 * 0.6
- shoot for 5% of speed of sound (</ 17 m/s) and you wont have any problems
- 12-16 square inches of port area per cubic foot of volume is the generally accepted guideline
- area of a circle is pi*r^2
- area of a rectangle is h*w
- dV > 39.37*(Fb*Vd)^0.5
- dV is minimum useable port diameter
- Fb is enclosures tuning frequency
- Vd is the volume displaced by the speaker at its full excursion (peak-to-peak)
- convert min useable diameter to square inches if going slotted
- Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2]-0.823* SQRT (Av)
- Fb = 0.159*SQRT[(Av*(1.84*10^8)]/(Vb*1728*[Lv+(.823*SQRT(Av))])
- Vb is true net volume (all displacements taken into account)
- Fb is the enclosures resonant frequency (tuning)
- Av is the cross-sectional area of the port (port area)
- (area of a circle is pi*r^2)
- (area of a rectangle is h*w)
- Lv is the length of the port
From devil driver(ROE):
The most common materials used for an enclosure are as follows:
2. particle board
3. marine grade, void free plywood
MDF in 1/2", 3/4", and 1" sizes is perhaps the most common choice as it is a great price/performance material. Particle board is not a quality material to use for buildling enclosures, as it will swell and flake if it gets wet and will buckle even at modest pressure levels. Of the three I have mentioned, it is the one I recommend the least. Lastly, we have the plywood category: please note that I am not referring to your average plywood. The usual, inexpensive plywood you will find in large quantities at a hardware or renovation center will be only minimally better than particle board. Plywood is typically anywhere from 3-7 ply (3-7 layers), and can perform rather poorly at high pressure levels, requiring you to double or even triple the thickness of your material. On the other hand, a typical 13 ply baltic birch or apple ply wood will give very favourable results. Of the three materials I listed above, I would only recommend options 1 or 3.
The goal of any enclosure is to keep "enclosure talk" from occurring. Enclosure Talk refers to the flexing of the enclosure's panels to introduce unwanted distortion or frequencies at an audible level. There are two approaches to preventing this:
1. Making the enclosure stiff enough that any resonance will be higher than the frequencies played by the speaker/enclosure
2. Making the enclosure heavy enough that any resonance will be lower than the frequencies played by the speaker/enclosure
MDF is typically better at adding mass, which will lower resonance. Void free plywood is typically stiffer and will raise the point of resonance. It is also very easy to add stiffness by using simple braces inside the enclosure. Speaking in generalities, it is easier to stiffen the enclosure to raise resonance than it is to add mass to lower resonance. Marine-grade Plywood will be lighter and will not be damaged by contact with water, whereas MDF will be heavier and can be damaged by water. So it certainly seems that certain types of Plywood will create a more favourable result than MDF; but this is without considering price. Void free, marine grade plywoods can cost upwards of $50 a sheet, whereas MDF is usually $15-$25 a sheet. As you can see, the difference in price is very noticeable.
So the question you need to ask yourself is: am I on a tight budget? If not, it will likely be worthwhile to purchase some quality plywood. If so, than MDF will work well for you. Keep in mind that any speaker with large levels of output will require thicker sized sheets, but 3/4" is the most commonly used size.
I have come to realize lately that a lot of people dont know how how to find the net volume of an enclosure in cubic feet. i thought this might help some people that are having a hard time.
I am using the example that Aroc gave me.
So now we have
14.5H X 32.5W X 12.5D=5890.625 cubic inches
5890.625 divided by 1728 = 3.4 cubic feet
now you take all the dimensions you have and minus 1.5"(if your using 3/4" MDF on each side of the enclosure)
so are new dimensions are:
divided by 1728=2.565 cubic feet.
now, if you were going to add a port to this, i would shoot for 32" of port area(width of the port x the height). a good rule of thumb is to have 12-16" of port area per cubic foot(this box will have a little more but its just an example).
to achieve this, you will have a slot port that is 2.5" wide and 13" tall(13" is the internal height of your box) this gives you 32.5", pretty close eh?
now lets just say the port is 25" long. to find out how much space this port takes up, you take the port width and add the thickness of your wood to it(that is the outside of the port). so now we have 3.25". times that by the height(13") and then by the depth(25"). lets put it into a formula.
3.25x 13 x 25=1056.25
divided by 1728=.611 cubic feet.
subtract this number by the internal volume of your box
2.565-.611=1.953 cubic feet internally.
now you have to subtract sub displacement. lets say we have a pretty beefy 10" woofer(hdc3 10" for example) and it has a displacement of oh maybe .2 cubes?
1.953-.2=1.753 cubic feet after all displacements.
i hope this helps you guys out. this box went from 3.4 to 1.753 cubes pretty quick didnt it? with some braces this example box would work great for a decently sized woofer like a hdc310. dont forget to subtract the displacement from braces if you using them.
oh and i thought i would add how to find the displacement of round ports:
use this formula, Pi x R^2 x D/1728 (3.14 x Radius x Radius x Depth of Port/1728)
i will use a 6" port that is 25" long as my example
3.14 x 3 x 3=28.26"
28.26" x 25"=706.5
706.5/1728= .408 Cubic Feet
just as you did with the slot port, minus this from the internal volume
edit I put this in the box section, I didn't realize something similiar was in the tutorial section