Sound - Natural Vibrations

What do you understand by free vibrations of a body?

Why are the Ultrasonic waves are not audible to us?

Give one reason for Ultrasonic waves use for echo depth sounding purpose.

Two astronauts cannot hear each other on the moon. Why?

State one way of increasing the frequency of a note produced by an air column.

Name the waves used for echo depth sounding

State the ways by which the frequency of transverse vibrations of a stretch string can be increased

What is meant by the natural frequency of vibration of a body?