some people will try to call it a 6th, but i figure depends on what part of the 4th you PR
my last enclosure was for a single 15"... sealed part was 2 cu and ported part 3.6 cu tuned to 43 hz... lots of bandwidth with it.. 25 to about 53hz..
one of my new projects i figure same sub but down to 1.8 cu sealed and 3 cu in the "ported" area that will house 2 x 12" PR
this will allow me to adjust tuning on the field to allow me to compete in idbl and bassboxing, i figure.. also it will save me from using lengthy aero ports ..
anyone try something similar before?