by port opening and lengths the londer the lower th hz the shorter the higher the hz but you must alsp apply the amount of power and air velocity expected out of the port (how much air comes out of the port) and if its too high make the port wider to accomodate
by port opening and lengths the longer the lower th hz the shorter the higher the hz but you must also apply the amount of power and air velocity expected out of the port (how much air comes out of the port) and if its too high make the port wider to accomodate (if you dont youll get port whistle)