US Patent 8085147 Security system including audio alarm detection

A building security arrangement includes a sensor for emitting an audible alarm signal. A security system installed in the building detects the audible alarm signal and transmits an electrical alarm signal over a public switched telephone network in response to the detection of the audible alarm signal. A central monitoring station receives the electrical alarm signal over the public switched telephone network and notifies an alarm authority in response to receiving the electrical alarm signal.