You have to figure out the Port volume for your concepts. Every sub is made differently. eg. A 12" Vega Series needs a 4in Diameter X 10inches long in a 2 CU FT box to be tuned to 35 HZ. A 15" CV Stroker needs 2 6In Diameter X 18inches long in 5.4 CU ft box to be tuned to 35 HZ.
If you have the spec sheet for your subs it should say it on there. Volume and port volume or Diameter and length
