1 Introduction

Smoke alarms provide a standard 3kHz signal, which is proven effective in alerting people to danger when they are awake. However, this frequency is proven not to be the most effective one regarding the occupants that are asleep or people who are hearing impaired. The 2010 edition of the NFPA 72 National Fire Alarm and Signaling Code introduced the term low frequency sounders, which are proven to have the most significant effect in terms of alarming people in sleeping areas, people who have hearing impairment, and individuals under the influence of drugs or chemicals. Areas such as hotels, lodging houses, dormitories, apartment buildings, daycare facilities and hospitals are marked as sleeping areas where low frequency sounders show the best results in terms of waking effectiveness. The frequency of 520Hz square wave signal is proved to be the most effective in terms of waking up the high-risks groups in the event of a fire compared to the standard 3kHz signal. Including the 520Hz frequency signal is typically associated with higher cost since the low frequency requires more power to produce. This generally requires a different sounder element and driver circuit. The smoke alarm can have a single sounder element and driving circuit with alarm tone selected by firmware for an optimized cost design.