The Smartwares RM149 sounds the alarm when it detects smoke. The duo pack consists of 2 smoke detectors, offering twice as much security in your home. The 85-decibel signal will warn you immediately. The smoke detector runs on a 9V battery, which lasts about 1 year and can then be easily replaced. Since the smoke detector runs on a battery, you can mount it to the ceiling without requiring a technical installation. When the battery is empty, the smoke detector notifies you with a beep.