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12.3 Hearing and the Ear -- Part 1
Waves spread through a medium.
When a wave encounters an obstacle, it spreads into the region behind it.
The spread of the wave depends on the wavelength.
If the size of the obstacle is smaller than the wavelength, there will be significant diffraction into the region behind it.
A person sitting behind a pillar in an auditorium can hear the performer because of the long wavelength sound waves behind the pillar.
Because the wavelength of light is smaller than the pillar, the light cannot diffract into the region behind the pillar.
There is no significant reflection when objects are smaller than the wavelength.
Diffusion is to blame for this too.
The wave diffracts around the small obstacle, like the water spreads around a stick.
Light and sound waves can be focused with curved reflectors.
There is a limit to the size of the focused spot.
Waves have consequences in the process of hearing and seeing.
The sensation of hearing is caused by the response of the nerves in the ear.
Most of the skin is pressure-sensitive, and the nerves in the ear are not the only ones that respond to pressure.
The ear is more sensitive to pressure variations than any other part of the body.
There is a drawing of the ear.
The outer ear, middle ear, and inner ear are the main parts of the ear.
The inner ear contains sensory cells that convert sound to nerve impulses.
The purpose of the outer and middle ears is to amplify the sound.
The pinna helps the animal locate the source of sound by rotating it toward the source.
The pinna is small and fixed in humans, so it doesn't seem to contribute much to the hearing process.
A drawing of the ear with various structures cut away and simplified to show the basic relationships more clearly is a semidiagrammatic drawing.
The middle ear muscles are not present.
The ear canal of an average adult is about 0.75 cm in diameter and 2.5 cm long, a configuration that is good for sound waves.
The ear is sensitive to sound waves in this range.
The sound has to be coupled from air to the sensory cells in the inner ear in order for an animal to hear it.
Most of the sound energy is reflected at the interface, so it's inefficient to put sound waves into a fluid.
The middle ear has an efficient path for sound waves to enter the inner ear.
The hammer is attached to the eardrum and the stirrup is connected to the window in the inner ear.
When sound waves hit the eardrum, the ossicles send the sound waves to the window, which in turn causes the fluid of the inner ear to change.
The muscles that control the volume of the middle ear are connected to the ossicles.
The transmission of sound to the inner ear is reduced if the sound is loud.
The middle ear is used for more than one purpose.
The person's own voice, chewing, and movements of the head can cause noise in the inner ear.
The sound of the vocal cords is transmitted through the bones into the inner ear, but it is not as strong as it could be.
We hear ourselves talking mostly from the outside.
Talking with ears plugged can show this.
Air enters through this tube to keep the middle ear moist.
The movement of air through the Eustachian tube is aided by swallowing.
A sudden change in the air pressure can cause a pressure discrepancy on the two sides of the ear.
The force on the eardrum causes a painful sensation until the pressure in the middle ear is adjusted.
If the Eustachian tube is blocked, the pain is even worse.
The cochlea is shaped like a snail shell.
The wide end of the cochlea has an area of about 4mm2.
The cochlea is formed into a spiral.
There are three ducts.
The two canals contain the cochlear duct.
The sound wave came from the fluid in the canal.
The sound wave, which travels along the tympanic canal and through the helicotrema into the vestibular canal, stimulates the auditory nerves to send electrical signals to the brain.
The round window at the end of the tympanic canal is where the excess energy in the sound wave is dissipated.
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