Abstract: A siren unit including control means for controlling activation of a siren loudspeaker (8) at a selected frequency based on a frequency control signal, and input means for providing the frequency control signal to the control means. The input means further provides a volume control signal and a duration control signal. The siren loudspeaker (8) is activated in response to a trigger signal received by the input means which may be a plurality of predetermined data bits. The siren loudspeaker (8) can be activated on the basis of a data communications packet received and provided by the input means. The siren loudspeaker (8) is also activated when the data communications packet is not received.