Props to fzaba @ sounddomain.com
All of my calculations come from Winisd ProAlpha using internal dimensions with .75” MDF walls. If you use the updated Winisd version that’s floating around the net, you have to select “One Flanged End” to get the same numbers as shown below.
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READ THE INCLUDED HELP FILE! There is just too much useful information to pass up. It’s a goldmine! It will greatly help you in understanding this thread.
Elemental Design's slot ported box for the E12a is shown below. Go ahead; take a look at it now. All numbers used will come from this design.
***Finding Internal Dimensions and Volume***
To get to internal dimensions subtract 1.5” from each individual external dimension.
External Dimensions – 25” x 16” x 14.5”
Internal Dimensions – 23.5” x 14.5” x 13”
To find the volume of a 3 dimensional box in cubic feet, you use the formula (# x # x #) / 1728. Put the internal dimensions for ED’s design into this formula to find the total gross internal volume.
(23.5 x 14.5 x 13)/1728 = 2.5635CF (CF is short for cubic feet)
Start up a new project using the Elemental Designs E12a. You’ll come to a graph, and a floating menu that shows the sub’s make/model. Select the Box tab on the floating menu and type in 1.5ft^3 under volume, then 26Hz under tuning frequency. Next, go to the Vents tab and click the circle to change the shape to square. Now go ahead and type 1.75” X 13” for the vent diameter. Again, all numbers come straight from the design shown below. Look at the vent length in Winisd; for simplicity’s sake, we’ll call it 56.25” just like the design. Also notice the cross area- that’s just the product of the port dimensions. (1.75 x 13 = 22.75sq inches) Ideally, you want as much cross area as possible, but there's a catch. The more you increase cross area, the longer the port will have to be to keep your desired tuning frequency. :/
When coming up with port dimensions for your custom box, use the “Rear Port – Air Velocity” graph from the pull down menu. First, you have to enter a power amount in the floating menu on the Signal tab to see the graph at different power levels. Along with other side effects, having too much air velocity leads to port noise. Increase your cross area to keep air velocity, and subsequent port noise, at a minimum. In some cases you may end up with a nasty whistling from the air moving too fast around your ports boards. So you obviously want as little port noise as possible, but in some cases you have no choice because of space limitations. If you have a trunk, you can get away with higher levels of air velocity because the seats will filter out most of the port noise. Still, the more cross area you have, the better. Without a trunk, shoot for a maximum of ~18m/s with tunings around 35Hz. With lower tunings, say about 28Hz, you can bring that maximum value up to around 21m/s to keep noise down.
Getting back into the calculations now; the goal is to find the gross internal volume
so you can determine the internal dimensions needed for your box. To do this, add together the net volume from Winisd
(1.5CF), the port’s displacement
(shown below), the sub’s displacement
(.1CF for E12A), and any other bracing
(none here) you may have.
*** Calculating the port’s displacement***
Let's go through using the numbers from Winisd.
Take that effective length given -56.25”
Subtract .75” because it is part of the front baffle and will not count against the port's displacment.
Take that multiply by the port height of 13”.
Take that and multiply by the port width of 2.5” (You add .75” to the width because of the MDF’s thickness used as the port’s walls [1.75" + .75"])
Finally, divide it all by 1728.
Put it all together now- Port Dimensions- 1.75” x 13” x 56.25”
(55.5 x 13 x 2.5)/1728 = 1.0438CF
<- The port’s displacement.
NOTE: This is NOT a 100% accurate way to calculate your port displacement. This is just a good way to get a ballpark figure. The only way to get an accurate number is to draw up your design, then calculate the port’s displacement from that. When you start from scratch, you can’t just draw up a design at this point because you don’t know the needed dimensions. If you don’t want to go too much further into math, then you can just use this figure all the way through. In reality, you won’t notice a difference in the sound one way or the other. The WORST
that can happen is your box may be +.1CF and -1Hz from the intended design. If you must know why it’s not accurate, PM me and I’ll gladly explain.
By calculating the port displacement manually using the drawing, ED’s design comes out to 1.467CF @ 26.3Hz. Using the formula above we get this:
2.5635 Gross internal volume [(13 x 14.5 x 23.5)/1728]
1.0438 Subtract Port Displacement [(55.5 x 13 x 2.5)/1728]
- .1 Subtract Sub Displacement
1.4197 Cubic Feet NET
Enter 1.42CF back into Winisd and you get 26.73Hz for tuning while keeping the port length the same at 56.25”. This shows that the real design is +.047CF and -.43Hz from what we would get using only Winisd. Not a big difference. Audible; not at all- .22db variance in SPL at 30Hz doesn’t exactly scream out at you. The formula above is more accurate with less bends in the port.
***Finding Gross Internal Volume***
Just add your net volume, port displacement, and sub displacement to get the gross internal volume in cubic feet. Now that you have it, you must change your internal dimensions around until it fits this amount cubic feet. Use (# x # x #)/1728.
Here’s the trick: Measure your car to get your max width and height; convert to internal dimensions. Take your gross internal volume and multiply it by 1728; you can then divide that number by the product of your 2 internal dimensions (width x height). This will give you the internal depth needed for the box. It will most likely be an off the wall number, so round it up to the nearest .25” or .5”. You can then add bracing to your design to bring the volume back down to the correct amount of needed cubic feet. After all of that, just add 1.5” to each individual internal dimension to get the external dimensions.
***Finding Physical Port Length***
Winisd gives you the effective port length with end correction already factored in (56.25” in this case). Effective length is used when tuning the box, but you need to know the physical length because that is how long the actual MDF will be inside the box. To get at this number, just take the effective length and subtract end correction (which means half the port’s width).
Using ED’s box- Port Dimensions of 1.75”wide x 13”tall x 56.25” long.
Take 56.25” (length from Winisd-effective) and subtract .875” (end correction-half of 1.75). This gives 55.375” of physical port length; the length of MDF needed inside the box.
So what’s the reason for end correction anyways? Good question. You are using 3 of the enclosure’s walls, plus the added port wall, to act all together as the port. The final port board in this box ends after 8 7/8”, BUT the 3 enclosure’s walls continue on. Because of this, even after that 8 7/8” has ended, the port acts as if it is still bound by all 4 walls. The EFFECTIVE length is now half the port’s width longer (.875”). Physical length + end correction = Effective length.
***Measuring Vent Length***
When planning the box, you also have to know how to measure the port length down the middle of the vent. The tricky part is calculating the vent length when turning the corners inside the box. Here’s a great diagram for getting the feel of this:
Here’s how to do it (remember- down the middle of the port):
.75(baffle) + 12.75 + 1.75(corner) + .75(MDF) + 12.75 + 1.75(corner) + .75(MDF) +12.75 + 1.75(corner) + .75(MDF) + 8.875 + .875[end correction (half of 1.75)]
All of that come to 56.25” – The effective port length like the diagram states.
Why add 1.75” when turning the corner? Broken down, it is just half of 1.75 up and half of 1.75 over to stay in the middle of the vent. Half and half come back to 1.75”. Knowing this, after the first turn you have another port board of 12.75”, but it is part of the second turn. You know the turn is 1.75” so why not subtract that from the 12.75” board? Well if you subtract 1.75” from 12.75” you’ll just have to add it back up again. 12.75” – 1.75” = 11”. 11” is the length of second board without factoring in the corner. Factor in the corner and you’re back at 12.75”. Just keep it at 12.75” and you’re set.
Just keep reading over this section until it clicks. You’ll eventually get! For first timers there will be a whole lot of confusion, that’s how you learn.
Also, you can see the physical length inside the box is 55.375” (56.25 - .875). You physically need 55.375” of MDF in the box to get the needed effective length of 56.25.