Originally Posted by Illini2000
It was his build thread that got me thinking about the 21 inch drivers. They look great with the Titans. I would be happy to use the size you laid out above but it looked like there was a benefit to going larger to be able to get the port speed down and resonance up. I think 48 wide by 42 high is the biggest I want to go. I still need to keep the depth to 18 inches. I'm making progress with winisd but I can't the port lengths done seem to line up with the designs on the forum. I think I read something about the type of port you designed effectively lengthening the port but I don't know how to model that. Any recommendations?
for a slot port, the height of the port is added to the physical length, then a further half a hertz or so of adjustment is typical.
so if you want to design a cab with a 18hz tune, set the winisd value to 18.5hz.
then, if the port is 4" tall and winisd tells you to build it 42" long, subtract the 4" height and build the port 38" long.
after all is said an done, it will end up around 18hz.
sometimes there is a little divergence, but typically very small, less than a hz or so.
the tricky part comes in when the port is close to an internal wall of the cab, because that will effectively lengthen it further. no such adjustment is needed with your design though because the port ends up firing more toward the middle with plenty of space before any interior elements (bracing or wall).