What’s happening in your ear? The outer ear, the pinna, helps you collect sound and holds your piercings, but the more interesting stuff, from a hearing perspective, is down the canal.
Sound waves hit our tympanic membrane, which is to say, our ear drum. Three tiny bones, the smallest in our bodies, the malleus, the incus and the stapes, transmit the movements of the ear drum to an oval window. The oval window is attached to the inner ear, which is a rigid structure. The stapes pushes on the window, moving fluid inside the inner ear. Below the oval window is a round window. As the oval window is pushed in, the round window bulges out. This allows the fluid to move, as liquids are much less compressible than air.
This whole structure of the ear drum, the little bones and the window, take a movement in air and turn it into a movement in liquid.
This liquid moves inside the cochlea, a snail shaped part of our ear. Inside that is the basilar membrane. This part of your ear is covered with tiny cells, called hair cells that wiggle in response to sound. The part of the membrane closest to the oval window responds to high frequency sounds, where the part furthest away responds to low frequency sounds.
When you hear a high frequency sound, the hair cells close to the oval window wiggle and send nerve signals to your brain. When you hear a low frequency sound, the hair cells further from the window are wiggling. These hair cells are fragile and can break in response to loud sounds. When they do break, they do not grow back. If you listen to ear buds too loudly, you can break these cells and your hearing will not come back. The hair cells closest to the ear drum are high frequencies and people tend to loose high frequency hearing first. When babies are born, their hearing is so sensitive that the lower threshold is just above individual air molecules hitting the tympanic membrane. People’s hearing gets worse from environmental factors including infections and exposure to loud sounds, but not from age. Wear protection to loud concerts and turn down your headphones to preserve your hearing.
When you are listening to two different frequencies, two different parts of your basilar membrane will wiggle. If those frequencies are far apart, you can hear each one individually. However, if the frequencies are close together, the parts of the basilar membrane that are wiggling can start to overlap. In this case, sometimes one sound can mask another, so we can’t hear both.
Image Sources:
Medical gallery of Blausen Medical 2014 (2014). WikiJournal of Medicine [Online] 1. Available at: https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014 [Accessed: 10 April 2024].
Further reading / watching:
Auditory Transduction (2002) (2009). [Online]. 6:43. Available at: https://www.youtube.com/watch?v=PeTriGTENoc [Accessed: 10 April 2024].
