A sound, subliminal more than a real noise, alerted them.

The loud sound created will alert anyone at the building or site that there is a fire so they can escape safely.

The sound of footsteps alerted him that someone was coming.

The sound of our motor must have alerted the flying guard.

A similar sound to the one Sarah had heard alerted him.

Finally, the sound of a slamming screen door alerted them both.

It was a kind of silence, and unfamiliar sound would alert me.

But if there were any guards left, the sound of his fall would have alerted them.

The sound of voices coming back alerted him, and he wondered how long they'd been gone.

Maybe the sound of my breathing alerted her that I was there.