Ever seen one ?

It's called a port tuning nomogram. I have this graph saved in my PC, and every time I go to build a new box, I print one or two (or more, depending on how many I screw up, and use for scratch paper )

But basically, you take the optimum cubes for your sub(s) and the desired tuning frequency (always available from the manufacturer), and simply draw a line, using a straight edge, across the two, until it intersects the line labeled Lv/Sv.

From there, you draw a line straight across the graph.

Now, you have to figure out your sq inches of port area, (the curved lines on the graph) and it probably won't work exactly with any of the existing curved lines, but that's okay. You can simply draw one in very close to where it should be. {ex: my two, 6" ports work out to 56 sq in. so I drew in a curved line, just slightly below the line labeled 50.3}

So, wherever your horizontal line intersects your curved line, follow it straight down to the bottom of the graph, to determine the length of your port(s), for your desired tuning frequency.

Sure, all of the latest, greatest box building programs will work fine, as long as you plug in the right numbers, but in reality, they are all based on this old school port tuning nomogram.
And I find it fun, to play with these graphs anyway, tweaking the numbers and such.

My current project is a 7 cu ft (net) enclosure, for two 15"s. The nomogram shows that I will need two, 6" diameter ports, 20" in length, for a tuning of 28 Hz.... Hoping for a low frequency monster

This old school nomogram has never failed me. My latest enclosure will be about my 10th one using it

Peace,
Fish

 

This looks sweet ha, I might have to try this out when I build my new box. I'm gonna try to get my 15 in the back with the back seat still in. I'm thinking seat lift with majority of the box under the seat and the speaker in front of it. So glad I only need 3.5 cu feet to fit this sub at is optimal ported box size ha.

 

Cool... never even heard of this before. I've used WinISD, Bassbox Pro and some excel based programs before.

 

Right on.

Hey Melo, I've used some of those programs too.... But then what I do, is that I go back and use the old nomogram, to "check" that I used the box building program correctly. A few X's, I was amazed at how nearly spot on the old nomogram figured the enclosure, compared against these modern high tech PC programs. But a couple other X's, the numbers were completely different. After a lot of head scratching, I found out where "my mistake" was, and corrected it, before the first piece of wood was ever cut

Peace,
Fish