THE SONIC GHOST tm Full range audio tactile transducer (speaker) Attach to behind ceilings and walls (inside) for a truly INVISIBLE home theater experience No unsightly speakers visible anywhere - these speakers are completely hidden behind the wall. Mount to plywood, fiberglass, or a number of other materials to propagate sound through the air Will even propagate sound through water (can hear sound through the water - mount on outside of pool or spa wall) Unit may be submerged in water for limited periods of time. Applications:  Home Theater (use in conjunction with subwoofer), museum exhibits, displays, spas, bathtubs, message therapy, plate reverb projects, ceiling tile PA, tactile therapy, and many more limited only by your creativity. Comes with full 1-YEAR Product Warranty Key Specs - Frequency Response: 100 - 17,000Hz (depends on mounting surface)  - Impedance: 8 ohms - Max power: 50W - Dimensions: 3" diameter, case depth 1", with attachment bracket, depth is 1 1/2". - This auction is for 4 units and mounting plates / hardware.