The inner ear contains the cochlea, an organ that plays one of the most important roles in human hearing. This intriguing little coiled up mass of flesh is a complex apparatus consisting of three adjacent tubes – the Scala Vestibuli, Scala Media, and Scala Tympani. The microscopic membrane that separates these components allows sound waves to travel through the cochlea as if it is one unified tube. Even when fully unravelled the entire cochlea only measures 3.5 cm in length. Now that you have a basic understanding of the inner, it’s time to take a look at how it process sound waves:
Vibrations Enter the Inner Ear Through The Stapes
In order to make it to the inner ear, vibrations first have to encounter the stapes bone in the middle ear. This is the smallest bone in the human body and it is responsible for transmitting sound vibrations into the fenestra ovalis (oval window). The stapes is extremely sensitive, so when sound makes contact with it pressure waves are easily generated and then transferred to the perilymph fluid inside the cochlea. The exact manner in which the stapes oscillates will depend on the frequency and volume of the sound, allowing for the transmission of notes and other reproducible/recognizable noises.
Sound Waves Form on The Basilar Membrane
When perilymph fluid is moved by the stapes it causes a standing sound wave to form on the surface of the rigid basilar membrane that covers the entire cochlea. The wave travels along the thousands of reed-like fibres on the basilar membrane like ripples on a pond. Near the stapes the basilar membrane’s fibres are short and stiff, but they get progressively longer and limber as you move towards the base of the cochlea. The textural difference between these fibres is what ultimately allows for sound recognition because each section of fibre has different resonant frequencies. In other words, when a sound wave of a certain frequency encounters the right spot on the basilar membrane it causes that particular area of the membrane to resonate rapidly. Since the membrane has different resonant properties along its surface, each spot is essentially designated to its own specific frequency that can cause it to reach its peak resonance.
Hair Cells Convert Vibrations Into Electrical Signals
The cochlea is covered in millions of microscopic hairs called stereocilia. These hairs detect which part of the basilar membrane is resonating most and then transfer this information to the brain via nerve impulses to let us know which specific sound we’re hearing. Higher sound frequencies peak along the side of the basilar membrane that is closest to the oval window (where the short and stiff fibres are), whereas lower frequencies peak along the side of the membrane furthest from the oval window (where the fibres are longer and more limber).