i mean the one from police and firetrucks and ambulances. is it a recording or is there actually something making the sound? if its a recording, what created that sound originally?
An electronically generated waveform is amplified.
Here’s a simple circuit diagram.
Of course emergency service sirens are more complicated, with varying settings, etc, but the principle is the same.
There’s a horn with a rotating disk inside. The disk has holes punched in it, so that when air passes through it makes the siren noise. Furthermore, the sound is actually a bunch of discordant notes, kind of like pressing a hand’s width of piano keys all at the same time.