Big boom for boxes with low internal volumes Ports require larger enclosures to be effective and can sometimes produce unwanted noise. When paired with a compatible 10" subwoofer, Kicker's 47KBRW10 10" passive radiator lets you get the punch of a ported box in a small sealed enclosure that can fit in more places in your vehicle. Kicker recommends using a subwoofer with a recommended box volume of 0.5 cubic feet, such as a Kicker CompRT subwoofer . Note: This passive radiator will not produce sound by itself. It must be paired with a subwoofer in an enclosure.