varying port diameter

Hi folks,

I was wondering if there is any rule of thumb or web site you'd recommend for understanding how a varying port diameter effects the tune of a port/box?

e.g. having a 4" port with a restriction down to 3" in it or the other way round, a 3" port with a 4" dia bulge in it.

Even understanding what would a 2 stage port diameter would do would help (4" dia for x lenght then 3" dia for y length)