understanding graph in WinISD

the red line is a flat response no "peaks" or "dips" and after the response curve crosses the purple line is where the freqencies wont reproduce very well. its a cut off point.

does that help?

flat= good response. stay as flat as possable.

The program will try to produce the flattest response when you input the parameters and click through the dialogue boxes.  But usually this flat response comes at the expense of huge enclosures or unmanageble port sizes.  So keep your install location in mind, as well as the manufacturer's suggested box volume, when you work up a design using the program.  You will almost always have a peak in the response above the 0db (red line) when you design a workable enclosure.

The purple line shows you the F3 cutoff point as menace described.  You have been asking a lot about F3, so there it is.  It's 3 decibels down from 0 on the left column.