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Hearing
If a tree falls and no one is there, does the tree make a sound?
A psychologist would answer, “There are sound waves, but there is not sound or noise.
Sounds and noise are psychological experiences created by the brain in response to
stimulation.
Sound
Sound waves – changes in pressure caused when molecules of air or fluid collide with
one another and then move apart again, transmitting energy at every collision. Sine
wave – what we hear as a pure tone molecule of air first contract then expand
Frequency of the waves measure in cycles per second, expressed in a unit called hertz
Frequency determines the pitch of the sound – how high or low it is
Human ear response to frequencies from about 20 Hz to 20,000 Hz
The height of the wave represented its amplitude – which with frequency, determines the
perceived loudness of a sound
Loudness is measures in decibels.
As we grow older we lose some of our ability to hear low-intensity sounds.
There are seldom pure tones produced most of the time instruments produce overtones –
The complex pattern of overtones is called timbre or texture of the sound
Hearing undergoes adaptation too.
The Ear
Hearing begins when sound waves strike the eardrum and cause it to vibrate.
Three tiny bones
Hammer, anvil and stirrup hit each other in sequence and carry the vibrations to the
inner ear.
Stirrup is attached to a membrane called the oval window. Below oval window is the
round window. This equalizes the pressure in the inner each when the stirrup strikes the
oval window.
The vibrations are transmitted to the fluid inside the cochlea. It is divided lengthwise by
the basilar membrane. Stiffer near the oval and round windows and gets more flexible
toward its other end. The basilar membrane is pushed up and down rippling in response
to the movement of the cochlear fluid.
On top of the basilar membrane and moving in sync with it is the organ of Corti. Here
messages from the sound waves finally reach the receptor cells for the sense of hearing.
Thousands of tiny hair cells that are embedded in the organ of Corti. Signals are
transmitted through the auditory nerve to the brain.
Hearing is truly bilateral. Each ear sends messages to both cerebral hemispheres. The
switching station where the nerve fibers from the ears cross over is in the medulla. The
primary destinations for the auditory messages are the auditory areas in the temporal
lobes of the two hemispheres.
Place theory – the brain determines pitch by noting the place on the basilar membrane at
which the message is strongest.
Frequency theory – pitch is determined by the frequency with which hair cells in the
cochlea fire
Volley principle – refines frequency theory; suggests that receptors in the ear fire in
sequence, with one groups responding, then the next and the third etc.
Hearing disorders
Damage of eardrum
Small bones may not work properly
Damage to basilar membrane
Disease, infections and long-term exposure to loud noi8se can harm the ear
Hearing aids simply amplify incoming sound
Conductive hearing loss – surgery is recommended
Conductive is due to a stiffening of the connections between the bones
Cochlear implants can offer hope to those whose deafness is due to cochlear
damage.
If you have auditory nerve damage none of these things will work.
Tinnitus – high pitched hum all the time.
Other Senses:
Amell: smell is 10,000 times as sensitive as taste
Smell undergoes adaptation like other senses
Two different sensory systems:
Detect and discriminate among common odors
Communicating sexual aggressive or territorial scent signals