Abstract: An alarm system for sensing an alarm condition within a relevant alarm area includes one or more portable alarm units for sensing an alarm condition. Each alarm unit includes a radio frequency transmitter for identifying the alarm unit and a secondary, audio, transmitter for defining the relevant alarm area. The system also includes a plurality of transponders. Each transponder includes a primary receiver for receiving a radio message from an alarm unit and a secondary receiver for receiving an audio signal from the alarm unit. A real-time clock in each transponder provides the elapsed time between receipt of a primary and receipt of the secondary alarm signals. The alerted transponders then transmit to a control unit a transponder address code, an alarm unit identification code and an elapsed time indicia. Based on the address code or codes and the minimal elapsed times detected by the control unit, the relevant alarm area can be determined.