From the H-D website:
Boom!™ Audio Bass Booster Tour-Pak® Port Kit
This Bass Booster Port Kit provides significant improvements in bass response without adding a subwoofer, and has no impact on usable Tour-Pak® carrier cargo capacity. When installed, the open port permits the speaker pod to "breathe", reducing the compression effect that limits speaker cone travel. You hear a 21% increase in bass response for better sound with a fuller bottom end, and an 82% reduction in peak distortion for accurate bass reproduction. Noticeable improvement is gained with both Original Equipment and Boom!™ Audio High Performance Speakers. The kit contains black-anodized aluminum ports, stainless steel audio-transparent grills, and unique Sound Sealing technology to give you that Boom!™ you were looking for.
76000013
Fits '06-later FLHTCU, FLHTCUTG, FLTRU and FLHTK models. Not compatible with '11 CVO™ FLHTCUSE6. Also fits models equipped with King Tour-Pak® and Boom!™ Audio Rear Speaker Pod Kit. Does not fit with Boom!™ Audio High Output Sound System P/N 76418-06 or 76418-06A. Dealer Digital Technician software update is required. Dealer installation is recommended.
MSRP US $79.95