While in theory you can tune the Fh it seems that you're always up against it on the rear chamber volume. You usually can barely fit the driver in the space, so if you need to raise the Fh you're kind of SOL.If you're building a tightly folded horn like the LAB horn it seems that Fs really dictates the feasibility of physically constructing the thing. Whenever I play with Hornresp I always get backed into a rear chamber volume that won't accomodate the driver.