the purpose of a PR is that it acts like a port. however, you typically need double the cone area. for example. If you have a 10" sub, you will need 2 10" PR's or 1 12" PR. If you have an 18" sub, you'll need 4 15" PR's or 2 18" PR's. It will save space and give you the best of both worlds as far as sealed and ported enclosures go.