2. Your ears are extraordinary
organs. They pick up all the
sounds around you and then
translate this information into a
form your brain can understand.
One of the most remarkable things about this process is
that it is completely mechanical.
3. Your sense of smell, taste and vision all involve
chemical reactions, but your hearing system is based
solely on physical movement.
In this lesson, we'll look at the mechanical systems
that make hearing possible.
4. We'll trace the path of a sound, from its original
source all the way to your brain, to see how all the
parts of the ear work together.
When you understand everything they do, it's clear
that your ears are one of the most incredible parts of
your body!
5. To understand how your ears hear sound, you first
need to understand just what sound is.
An object produces sound when it vibrates in matter.
This could be a solid, such as earth; a liquid, such as
water; or a gas, such as air.
Most of the time, we hear sounds traveling through
the air in our atmosphere.
6. When something vibrates in the atmosphere, it
moves the air particles around it.
Those air particles in turn move the air particles
around them, carrying the pulse of the vibration
through the air.
7. To hear sound, your ear has to do three basic things:
Direct the sound waves into the hearing part of the ear
Sense the fluctuations in air pressure
Translate these fluctuations into an electrical signal
that your brain can understand
8. How Hearing Works
The ear is made up of three different sections:
the outer ear,
the middle ear, and
the inner ear.
These parts work together so you can hear and
process sounds.
9. The pinna, the outer part of the
ear, serves to "catch" the sound
waves.
Your outer ear is pointed forward
and it has a number of curves. This
structure helps you determine the
direction of a sound.
10. If a sound is coming from behind you or above you, it
will bounce off the pinna in a different way than if it is
coming from in front of you or below you.
This sound reflection alters the pattern of the sound
wave. Your brain recognizes distinctive patterns and
determines whether the sound is in front of you,
behind you, above you or below you.
11. When the sound waves hit
the eardrum in the middle
ear, the eardrum starts to
vibrate.
Hammer, avnil and stirrup
help sound move along on its
journey into the inner ear.
13. The vibrations then travel to the cochlea, which is
filled with liquid and lined with cells that have
thousands of tiny hairs on their surfaces.
There are two types of hair cells:
the outer and inner cells.
The sound vibrations make the tiny hairs move.
16. The outer hair cells take the sound information,
amplify it (make it louder), and tune it.
The inner hair cells send the sound information to
your hearing nerve, which then sends it to your brain,
allowing you to hear.
17. The Specificity of Receptors Depends on:
Each receptor has a low threshold for its particular
stimulus. The threshold is the minimal strength of
stimulus necessary to set off an impulse.
The receptor sends only one kind of message to the
central nervous system no matter what the nature of
the stimulus. This is known as Muller’s doctrine of
specific nerve energies.
18. Sound
Hearing is one of the several ways of picking up
vibrations.
Many receptors exist to detect vibrations, ranging
from simple sensory hairs to the ear.
Vibrations are picked up by the organ of Corti
within the cochlea.
Vibrations of the fluid in the cochlea causes the
basilar membrane to move, thereby bending the
hairs and creating a generator potential.
This triggers impulses in the sensory neurons; the
impulses then travel along the cochlear nerve to the
brain.
19.
20. Different pitches stimulate different parts of the
cochlea.
Normally, the human ear can detect sounds
ranging from 16 to 20,000 cycles per second.
Repeated or sustained exposure to loud noise
destroys the neurons of the Organ of Corti.
Once destroyed, the hair cells are not replaced,
and the sound frequencies interpreted by them
are no longer heard.
21. Hair cells that respond to high frequency
sound are very vulnerable to destruction,
and loss of these neurons typically produces
difficulty understanding human voices.
Much of this type of permanent hearing
loss is avoidable by reducing exposure to
loud noises in the environment, such as
industrial and machinery noise, gunfire,
and loud music
22.
23. Procedure
The sine wave generator is set up and switched on in
a room that is as quiet as possible.
The generator is set to 20 kHz and a value of 20 kHz
is set on the digital display by turning the frequency
knob.
The amplitude knob is fully turned on.
24. The headphones are connected to the headphone
output of the sine wave generator and are placed on
the head so that the ears are well covered.
The leader of the experiment gradually reduces the
frequency until the test subject just hears the sound.
The measurement is recorded.
The measurement should be repeated several times
with the same test subject.
25. The generator is then set to 200 Hz and a value of 10 Hz
is set on the digital display by turning the frequency
knob.
The amplitude knob is turned up halfway.
The leader of the experiment gradually increases the
frequency until, according to the test subject, the
individual sounds merge into a continuous tone.
This merging frequency is recorded.
The measurement is repeated several times with the
same test subject.
26. The upper acoustic threshold and merging frequency
should, for comparative purposes, also be determined
in younger and older test subjects according to the
procedure.
