an alarm system for a building has a plurality of audible alarm generating units disposed at various locations throughout the building . each of the units is within audible range of at least one of the other units . each alarm unit includes an audible alarm signaling device , a circuit for driving the audible alarm signaling device , a detector for abnormal conditions such as the presence of too much smoke , a detector responsive to the audible alarm signal produced by the other alarms in the system , and a triggering circuit for enabling the alarm driving circuit in response to detection of either the abnormal condition or the audible alarm signal of any other of the alarms in the system . detection of the abnormal condition at any location in the building will result in activation of all the alarms in the system , providing enhanced warning capacity .