IB HL Biomedical Physics – the Ear and Hearing

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The ear consists of three basic parts – the outer ear, the middle ear, and the inner ear. Each part serves a specific purpose in the task of detecting and interpreting sound. The outer ear collects and channels sound -a  longitudinal pressure wave- to the middle ear. Because of the length of the ear canal, it behaves like a resonance pipe open at one end, with antinode at the open end and node at the eardrum. It is capable of amplifying sounds with frequencies of approximately 3000 Hz – you should be able to verify this using an ear canal length of a few cm = a quarter wavelength, and wave speed about 340m/s. The eardrum is a flexible membrane like a drum skin, oscillating at the same frequency as the incoming sound. The 3 middle ear bones or ossicles are inside a fluid filled cavity and are levers, amplifying the pressure wave. The ossicles mechanically convert the vibrations of the eardrum into amplified pressure waves in the fluid of the  cochlea or inner ear with a lever arm factor of 1.3. Since the area of the eardrum is about 17 times larger than that of the exit point, the oval window, the sound pressure is concentrated, leading to a pressure gain of at least 22. This enhances the ability to hear very faint sounds where the incoming force on the eardrum is very small. Study the worked example on p692.

The pressure wave at the oval window is then transformed into a compression wave through the fluid in the inner ear which converts this energy into nerve impulses transmitted to the brain.

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