The first of its kind: the Lifetone HL™ uses a 520Hz square-wave sound pattern proven by the Fire Protection Research Foundation to be more effective at alerting and awakening people with hearing loss, seniors, children and heavy sleepers than standard fire alarms. No need to change your existing smoke or fire alarms. The patented Lifetone HL™ sensor listens for your existing T3 smoke alarms and then broadcasts its low frequency, 520Hz square wave sound pattern. For an additional alarm, there is a water-resistant bedshaker included. The Lifetone HL™ is both a state of the art fire alarm and an attractive , easy to use alarm clock. There is a battery backup that will provide emergency alarm signals for 7 days or more in a power outage. It will alert you to an AC power loss and low or no battery. It will also alert you to an unplugged or defective bedshaker. Measuring 8.5” L x 5”W x 4”H, the HL fits anywhere. Its attractive white finish complements any decor. Two year warranty. Be protected. The Lifetone HL™ helps ensure a life-saving environment for you and the ones you love.