The middle ear

After sound waves enter the outer ear, they travel through the ear canal and make their way to the middle ear.
The middle ear's main job is to take those sound waves and turn them into vibrations that are delivered to the inner ear.
To do this, it needs the eardrum, which is a thin piece of skin stretched tight like a drum.

The eardrum (or tempanic membrane) separates the outer ear from the middle ear and the ossicles.
They are the three tiniest, most delicate bones in your body.
They include:

• the malleus , which is attached to the eardrum and means "hammer" in Latin
• the incus, which is attached to the malleus and means "anvil" in Latin
• the stapes, the smallest bone in the body, which is attached to the incus and means "stirrup" in Latin

When an acoustic sound goes into the external ear canal, it vibrates the tempanic membrane.
The vibration is then transmitted to the cochlea via the ossicles.
Therefore, the middle ear plays an important role that converting the sound to the mechanical vibration.
If you shout something at a swimmer who is under the water, he cannot hear the sounds, because almost all the sounds are reflected on the surface of the water. The cochlea is fulfilled with the lymph fluid. Therefore, if there are not the ossicles in the auditory system, the sound cannot be transmitted to the cochlea. However, in fact, because of lever effect of the ossicles and the ratio of the area of the tempanic membrane to that of the stapes footplate, the sound can be transmitted to the cochlea effectively.