Perfect For Your Hearing-Impaired Guests. Vibration-Activated Signal Alerts Guests To Knocks At Their Door With A Super-Bright Strobe Light. Adjustable Sensitivity Switch. Attaches Easily With A Mounting Bracket (Included). Runs On Four Aa Batteries (Not Included, Sold Separately On Page 492). Overall Size: 3.25 Inch W X 3.125 Inch H X 2 Inch D. Ul-Listed. Meets Ada Requirements. 1-Year Warranty. Ship. Wt. 1 Lb.