This is a vent exit radius for the "MiniMax" subwoofer project by Zarbo Audio Projects which uses a 2" inner diameter ABS vent. This part screws into your cabinet with (4) four #6 screws, black oxide in this case. It is meant to reduce turbulence or "Chuffing" as the fast moving air inside the vent meets the exit of the cabinet. It does help, but does not eliminate the chuffing altogether. You can search for "MiniMax" on the Parts Express Tech Talk forum for more information on this project or visit the Zarbo Audio Projects Youtube channel.