The high and loud sound that is unpleasant to hear.
Shrillness is usually not a good thing because it can hurt people's ears.
Examples:
Her voice has a shrillness that hurts my ears.
The shrillness of the alarm woke everyone up.
I don't like the shrillness of the whistle.
The shrillness of the cat's meow made everyone laugh.
The shrillness of the scream was so loud it hurt my ears.
The shrillness of the bird's song was like a loud bell ringing in my head.