Standard Smoke Alarms are not always adequate in alerting those with hearing impairments.
"The Lifetone HLAC151 uses patented monitoring technology—perfected over 10 years of development—to actively listen for the high-frequency sound of your smoke alarm. When it detects a smoke alarm, your HLAC151 wakes you up and alerts you with 4 different signals: a loud, low-pitched (520 Hz) T3; a powerful vibrating bed shaker; instruction in a baritone voice (“Fire! Get out!”); and FIRE in large text against a flashing orange backlight." - LifeToneSafety.com
The diagram below outlines how the device works in conjunction with your current smoke alarms.