How does a siren work?

Siren

A siren is a kind of whistle which makes a loud noise as air is blown through holes in a spinning cylinder or disk.  It is a wailing noise

When a fire truck roars through the streets, its loud siren warms people to get out of the way.  An ordinary air siren has two cylinders, one inside the other.  The cylinder contain a number of holes.

When the fireman presses the button, an electric motor spins the inner cylinder and air is pushed through the holes.

When the holes in the inner cylinder are lined up with the holes in the outer cylinder, puffs of air escape and cause vibrations.  When the vibrating air (sound waves) reach our ears,  you hear the familiar ooooooOOOOOOEEEEEEeeeeee noise of the siren.

The faster the inner cylinder spins, the greater number of puffs of air, and the louder the siren.  A newer kind of siren makes its wailing sound electronically.  It does not have a moving parts like the older types. – Dick Rogers