What is the audible range (hearing range) of humans?

For human, audible range of sound in Hertz is:

Which of the following is a mechanical wave?

What is the name of the part of the ear that collects sound waves?

What is the name of the phenomenon where two waves combine to form a single wave?

A worker was working on the rail track. A boy at a distance holds his ear near the rail track. This boy was able to hear sound of the blow of workman twice. It is because of :