Signature Series styling with a cast aluminum frame, aluminum cone, and rubber surround visually identical to the SS18-22 driver. Sandwich Mass Structure (SMS) maintains added mass in a stable, symmetrical position between spider and surround. Passive radiator design delivers extreme low-end extension without the turbulence or cabinet size of large ports. Secure, adjustable tuning via threaded bolt, wing nut, and nylock nut assembly. An ideal companion for the SS18-22 in high-output passive radiator systems