The human hearing system consists of the outer ear, the middle ear and the inner ear.
The outer ear (auricle) that we commonly call our ear is only one part of hearing. The much bigger part of the ear is located in, and protected by, the skull.
The three parts of the ear are connected to each other via the ear canal. All acoustic systems run through it.
Tones, sounds and speech we hear are actually oscillations of the air. Before those oscillations are turned into sounds we can understand, they must pass from the outer to the inner ear via the middle ear - as well as all parts of our hearing system via the auditory nerve - to arrive in the brain as a signal.
The area in front of the ear drum, meaning the auricle and the outer ear canal, together form the outer ear.
This is where the sound waves arrive first and from here – much like in a funnel – they are guided inwards through the auricle.
The outer ear canal is shaped in such a way that the oscillating air that arrives is amplified, like in a resonator. The entire outer structure of the ear simultaneously prevents wind and other air movements from causing strong background noise, thus improving our hearing.
The middle ear
The area behind the ear drum is called the middle ear. The ear drum itself is a thin membrane that is hit by sound waves.
From it, minute oscillations are conducted to three tiny little ossicles: Hammer, anvil and stirrup – the smallest bones in the human body. Thanks to their unique location, they are capable of amplifying oscillations 20-fold at this point, guaranteeing proper conduction to the inner ear.
From the middle ear, the Eustach'ian tube extends to the nose and throat area. This tube ventilates the middle ear and equalizes pressure.
The inner ear
The inner ear begins where the stirrup hits the next membrane – known as the oval window.
Located in it are the organ of equilibrium and the cochlear. Approximately the size of a pea, the cochlear actually resembles a snail shell. It contains three canals filled with a liquid. Via one of these canals, the signals conducted into the liquid are directed to the tip and back via a second canal. The central canal is home to the actual organ of hearing, the organ of Corti.
The bottom of the organ of hearing is covered in thousands of tiny hairs – the hair cells. The wave movements in the liquid-filled canals change depending on frequency. The hair cells are only triggered when the amplitude is particularly great. The deeper the tones the further back in the cochlear the tiny hairs move, while high tones trigger the hair cells at the beginning of the cochlear. Wear of these tiny hairs is one of the main reasons for age related deafness.
Signal conduction to the brain
Even when the sound waves have arrived in the inner ear they still have no meaning. Once conducted to the auditory nerve, the sound waves are converted to electrical signals that first hit the brain stem. The signal is conducted to the areas of the brain responsible for emotional assessment. Thus meaning is attached to tones which are then connected to existing patterns in the cortex. This allows humans to understand speech, recognize the voice of a friend and judge hazardous situations.
This means that our hearing only works if the conduction of signals from one station to the other works perfectly and only once oscillating air has been turned into a warning signal, pleasant music or a coherent sentence.
The stations described above show how sound waves reach the inner ear via so-called air conduction.
But sound waves can also reach the inner ear via bone conduction: Oscillating air hits the outside of the skull and makes it oscillate slightly.
Conducted by the liquids in the ear, oscillations also reach the hair cells via this path. Bone conduction, however, is not as effective as air conduction hearing.
Bone sound wave conduction is the reason that we perceive our own voices as strange in video recordings. Hearing them like this, they are only conducted via the air. The bone conduction portion – that we usually perceive at the same time during talking – is missing.