IB HL The Functioning of the Ear (3)

Because I’m old, I can only hear up to 10kHz. Normal people hear from 20Hz-20kHz and the upper frequencies decrease with age because of decreased flexibility in the aural mechanisms, thickening eardrum, ossicle degradation and so on. A few minutes spent with p697 on ‘hearing defects’ might be valuable here followed by q9, p699.

The ear’s sensitivity is frequency-dependent and our previous value for 1pW per square metre is only true for a frequency of 1kHz – the most sensitive region being as we can see from the graph below is at about 3kHz. If the ear canal is about 3cm in length or a quarter of a wavelength, what frequency does this correspond to? (2.83kHz) Many other mammals have different ranges dependent on the configuration of the outer ear, the way the pinna collects the sound and their electrical responsiveness.

Threshold of Hearing – what would this look like for an older person with conductive hearing loss?

Pitch is what we usually mean by perceived frequency, but a better way to think about it is it’s related to both pitch and intensity. A soft sound at a particular frequency is perceived as being of lower pitch than the same frequency but louder or more intense.

The sound is converted to an electrical signal by this (grossly simplified) mechanism. The sound wave interacts with hairs of different lengths like a Barton’s pendulum with a hair cell somewhere along the Organ of Corti and receptors on the Basilar Membrane which decreases in stiffness along its length. A particular frequency causes one of the hairs to oscillate with large amplitude or resonate and hence send its electrical signal to correspond to this frequency. The longest hairs are furthest away from the oval window hence respond to lowest frequencies.


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