All sound signals can be traced back to movement. The blowing of the wind causes leaves on trees to move, resulting in turn in the movement and vibration of the air molecules. These vibrations are called sound waves and can be perceived by the ear. All acoustic information such as language, music or noise creates sound pressure waves. Slow vibrations (low frequencies) are perceived as low pitch, while rapid vibrations (high frequencies) are perceived as high pitch. The ear as a whole is responsible for our ability to hear because it receives acoustic sound waves and converts them into neural impulses, which are then analysed by the brain. The organ of hearing consists of the outer ear, the middle ear and the inner ear.
The hearing process works as follows:
- The sound is received by the visible part of the ear (the auricle), where it is concentrated, and then travels through the ear canal to the eardrum.
- The sound causes the eardrum to vibrate.
- As a result, three tiny bones, the malleus, incus and stapes – which are connected and are collectively called the ossicles – also start to vibrate.
The last bone of the ossicular chain – the incus – puts pressure on the fluid-filled inner ear, in this way transmitting the sound wave to the inner ear, which is also called the cochlea and is shaped like a snail.
The cochlea contains small cells whose surface is covered by delicate hairs (thus called hair cells). The movement of the fluid is transformed by the sensory auditory cells into a complex neural signal. This electrical signal travels via the auditory nerve to the auditory centre of the brain. The neural impulses are analysed and interpreted on their way to the brain. This is how a hearing sensation is formed.
Hearing is followed by perception
This is why hearing is actually quite a simple process. Far more complex and far less explored, however, is how further processing of the impulses is carried out in our brain. The neural impulses must undergo a multi-stage journey before they penetrate the auditory cortex. On their way, everything we hear is either amplified or decreased in intensity, and everything is interpreted – as negative, positive or neutral – and can even be completely filtered out. Only the signals which arrive at the auditory cortex are actually perceived.