This sounds like an alternative to the general "air raid" siren. Would it really add anything? Would the ambulances cover any area that isn't covered by a general siren (and the radio)? Would pedestrians even recognize that there is something different about the sirens?

I think that "air raid" sirens would be more effective and cheaper.

It would work like this, ambulances would have one specific tone, and fire trucks andother, and combined, they would produce a distinct sound.

FWIW, an traditional air raid siren emits two distinct notes, which when combined, produced their sound.

There are many minor disasters requiring both fire engines and ambulances. A major attack such as that by a terrorist requires a separate alarm system not to be confused with the daily sirens heard in such cities as New York many times per day.

I see this becoming useful should the sirens activate on all dispatched vehicles, simultaneously. It would be a jolt to everyone, wondering why all the emergency vehicles are making unusual sounds. Perhaps city and utility vehicles could be included.