Sound waves are

The pulse rate of a man is $80$ beats in one minute. Calculate its frequency. (in $ Hz$)

Why do astronauts talk to each other through radio telephones?

An echo is heard after $0.8 \, s$,when a person fires a cracker $132.8 \, m$ from a high building. Calculate the speed of sound.

In $SONAR$, we use

Sound made in front of a tall building $18 \, m$ away is repeated. Name the phenomenon and briefly explain it.