The typical procedure for air raids for Allied forces were: 5 second siren, announcment of the alarm type, proceeded with 30 seconds of siren.
For example, an airraid would be:
Siren - "General Quarters. This is not a drill. Please proceed to your designated area. This is not a drill." Siren.
or "General Quarters.
Emergency such as a fire, incoming missle/shells/motors or unknown threat - "Emergency. Emergency. Emergency. Please proceed to your designated safety area. This is not drill. Emergency. Emergency. Emergency."
Enemy close by: "General Quarters. (Any code-message that indicates an action- i.e. Code STONEWALL - means incoming enemy forces). General Quarters."
I know, I did them for the Gulf war.