A siren is a loud, high-pitched sound that is used as a warning, especially on emergency vehicles or in warning systems for dangerous situations like storms or air raids.
Sirens can make people feel scared or worried because they usually mean something bad is happening.
Examples:
The ambulance had a siren on top.
When you hear the siren, you need to move out of the way.
The siren is very loud and hurts my ears.
The siren sounds like a very loud cat!
The siren was so loud, it woke up the whole town.
The siren blared as the storm approached.