Quick reference guide to slot ports with 3 common walls

[20 hz bass machine Premium Member]

I use BB6 to design subwoofer enclosures. The biggest problem with the program (in my opinion) is the lack of a slot port option. This has you doing all sorts of math going back and forth making changes until everything is right.
BB6 does have a rectangular port option but the program assumes you are building the port straight, and in 4 different pieces (Top, Bottom, sides). In the case of a slot port without a bend, the math to find the actual port displacement is rather easy as the length doesn't change. In the case that you have a slot port with a turn, there are other factors that need to be accounted for and calculating an accurate port displacement is more difficult.
I took it upon myself to do the math and make a "cheat sheet" for calculating proper displacement.

The example I used when building the cheat sheet is as follows:

30" Internal port length
12" Internal height
2" Internal width
1.5" baffle
.75" port wall thickness
3 common walls
1 turn

Inputting the data into bassbox will give you a internal volume of .82 cubes and an overall port length of 31.5" (1.5" added from the baffle)

In reality, the actual displacement is .612 cubes

In short:
BB6 calculated the port displacement 134% larger than the actual port displacement. In contrast, the actual port displacement is 74.6% smaller than BB6's calculated displacement.

Id get into the math on how I arrived at those values but ill spare you the time.

Additionally, when using a slot port with a turn, the actual port length needs to be modified. This is due to the port length being measured from the middle of the port rather than the combined length of the 2 internal port pieces. The calculation for this one is easy.

Inputted internal port length+ baffle thickness+ internal port width.

In our example, an internal port length of 30" would actually be 33.5"

Lastly, if your slot port doesn't have a turn, the length stays the same but the actual displacement % comparison is different.

BB6's displacement stays the same .82 cubes 143% bigger than actual
The actual displacement is .573 cubes 69.9% smaller than BB6's calculated displacement.

In conclusion;
If the software you're using doesn't have a slot port option, you can use the values listed above to calculate actual port displacement and port length from whats provided by the program very quickly.

Hopes this proves helpful to someone.
Thank you.

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[Buck 5,000+ posts]

I make all of my own volumetric calculations. I use several different methods to see what size I want a box and what the port area needs to be and how far small or large I can go with any parameter. I figure out the cubic feet, port area and tuning I optimally want, then I manually use my spreadsheets to calculate volume very precisely. Very very precisely. Different styles of slot ports take up different amounts of space. Center slot ports, if they bend, the port becomes 1/4 of the width of the port longer vs displacement because of how the turn at the back wall happens when it splits in two. It depends on the size of the box, but for like a 1 cube box I'm within a few thousandths of a cubic foot of what I tell you the airspace is and with larger boxes, say like 5-10 cubes, I'm within a hundredth usually of the airspace I actually need. When the volume of a box changes, literally relative port area and tuning change as well. You gotta be on point with your volumes.

 

[Buck 5,000+ posts]

I'm really hesitant to share these, but here's one of my spread sheets for a 2 18's series 6th order wall

 

[Buck 5,000+ posts]

Every box that I do has it's own spreadsheet where everything is accounted for, volumetric-wise. I have to do that. I can't not know exactly what the net volume is of any sub box that I do.

 

[Firewalker]

Personally buck I would do it via pm and make him agree to not share it. Otherwise everyone will end up using your hard work.

 

[20 hz bass machine Premium Member]

Personally buck I would do it via pm and make him agree to not share it. Otherwise everyone will end up using your hard work.

Buck shared his collection of spreadsheets with me awhile back. I have no intrest in sharing someone else's work.

 

[20 hz bass machine Premium Member]

I make all of my own volumetric calculations. I use several different methods to see what size I want a box and what the port area needs to be and how far small or large I can go with any parameter. I figure out the cubic feet, port area and tuning I optimally want, then I manually use my spreadsheets to calculate volume very precisely. Very very precisely. Different styles of slot ports take up different amounts of space. Center slot ports, if they bend, the port becomes 1/4 of the width of the port longer vs displacement because of how the turn at the back wall happens when it splits in two. It depends on the size of the box, but for like a 1 cube box I'm within a few thousandths of a cubic foot of what I tell you the airspace is and with larger boxes, say like 5-10 cubes, I'm within a hundredth usually of the airspace I actually need. When the volume of a box changes, literally relative port area and tuning change as well. You gotta be on point with your volumes.

Couldn't have said it better myself. The numbers I provided only work with ports that have 3 common walls. My ratios are specifically useful when using BB6.

 

[Buck 5,000+ posts]

In some of these big walls, let's say you mess up and get your port wall .75" messed up because you forget an extra baffle that adds to port displacement inside of a box. Sometimes on a big wall, a small mistake can be like a .25 cubic foot mistake. Having your port volume off because you think the port is .75" shorter than it is can make your box .25 cubes smaller. When you add braces, 45's, the port, how ever many sub baffles, the sub displacements, tons of sub holes if you have tons of baffles, all of that you have to account for. Every detail makes a difference.

 

[20 hz bass machine Premium Member]

In some of these big walls, let's say you mess up and get your port wall .75" messed up because you forget an extra baffle that adds to port displacement inside of a box. Sometimes on a big wall, a small mistake can be like a .25 cubic foot mistake. Having your port volume off because you think the port is .75" shorter than it is can make your box .25 cubes smaller. When you add braces, 45's, the port, how ever many sub baffles, the sub displacements, tons of sub holes if you have tons of baffles, all of that you have to account for. Every detail makes a difference.

I think accounting for displacement/calculating proper net volume is the most important factor when designing a box and often overlooked or done in correctly. As you said, 1 little thing changes 3 others.

 

[Buck 5,000+ posts]

I'll tell you one thing that gets confusing is when you have a series 6th order with 2 slot ports in 2 different chambers. So the rear chamber port wall is a slot but the rear port exit is also fully surrounded in the front chamber as it is in the rear, which in the rear is what gives the slot it's extra lenght vs a more free-hanging port/ open ended port. Then if the front has one too, you sort of have this weird dynamic of how much invisible port length do I really have here? Oh man, you change the front port of a series 6th order, the whole box changes. In some walls, the slot port is not rectangular but shaped to the vehicle's shape, and has cut out for the wheel wells or where the roof tapers in, whatever it is. Sometimes it's really hard to tell exactly how much port length you have with 480 in^2 front slot port that has like 10 sides to the surface of it.

 

[Firewalker]

Buck shared his collection of spreadsheets with me awhile back. I have no interest in sharing someone else's work.

No problem, I just know buck works his butt off for his designs so just helping him is all since he has helped me in the past and vice versa.