The ELK-100 Siren Driver is a great choice for residential and commercial security installations. It draws less current than most siren drivers, and offers two volume levels (example: one indoor, the other outdoor) simultaneously. Our exclusive MilliMiser option allows further reduction of current draw for controls that have limited alarm or auxiliary output power. The ELK-100 has two inputs, Yelp & Steady. The Steady sound overrides the Yelp. The Yelp input has our unique ComboTrigger feature, especially designed for controls that have a single alarm output with a pulsing option for fire or burglary. When pulsed, this channel produces a distinctive third sound pattern.