Sound waves can travel through solids as well as through air. When a sound wave reaches a solid it makes the solid vibrate, and vibrations in a solid can make sound waves in the air. These conversions work in both directions. A loudspeaker cone vibrates and produces sound waves, while sound waves hitting the diaphragm of a microphone make the diaphragm vibrate.
The ear works in the same way. Sound waves travelling through air reach the ear drum and make it vibrate. The vibrations pass to other parts of the ear, such as the small bones in the middle ear, and this causes the sensation of sound.
These conversions only work over a limited frequency range. The ear drum and the other parts of the ear can only vibrate in step with sound waves inside this range. Waves with a frequency that is too low or too high cannot make them vibrate properly. This restricts the limits of human hearing. The range of normal human hearing is from 20 Hz to 20 kHz, so 20 kHz is the upper limit and 20 Hz is the lower limit.