Download Auditory System The Human auditory system is a very minute, yet

Bruce Gray
Section 15
Term Paper
Auditory System
The Human auditory system is a very minute, yet incredibly sophisticated system. We
can say that the function of the auditory system is to turn sound waves into nerve impulses which
are then interpreted in the brain. This system is made up of several small parts are working in
conjunction with each other to carry sound waves to the brain which are then interpreted into
what we call sound. There are three main bones in this system: The Hammer or Malleus, the
Anvil or Incus, and the Stirrup or Stapes bones, collectively these three bones are known as
Ossicles. These Ossicles are located in what is called the middle ear just past the ear canal and
the eardrum in the outer ear system.
As sound enters the ear the vibrations cause the ear drum to vibrate. When the ear drum
vibrates it causes the Malleus, which attaches to the eardrum at one end, to vibrate as well at the
same frequency. At the other end of the Malleus a hinge is formed with the Anvil and as
vibrations move in to the inner ear the Anvil carries the vibration on to the Stapes. As we pass
the Stapes we move into the inner ear. Inside the Inner ear we find one of the most important
parts of the auditory system, the Cochlea. The footplate, found at the innermost point of the
Stapes, connects to the Cochlea at an opening called the Oval Window. As the Stapes vibrates
the footplate moves in and out of the Oval Window causing a ripple effect in the fluids of the
inner ear. This motion transfers the vibrations through the inner ear and to the brain where it is
then interpreted as sound.
If it were not for the Malleus, Incus, and Stapes only about 0.1 percent of any sound
would make it through the ear into the brain where it could then be interpreted. Without these
bones it would be like trying to listen to a conversation with your head under water, the vast
majority of any sound would simply reflect off the surfaces in the ear and what does make it
through the brain would have difficulty interpreting with any coherence.
However, there are limits to what can be done by the various parts of the inner ear. The
Human ear is capable of hearing only a certain range of sounds. Sound can be measured in two
ways, through Decibels (dB) which measure the volume of a sound and through frequency, that
is, how fast the sound wave is moving.
Decibels increase exponentially in power. If we start at a whisper that is barely audible
we can say that this whisper measures about 30 decibels and from this point we can move up the
scale until we reach roughly 130-140 decibels, such as the sound of a gun going off near the ear
or a jet during takeoff. It is at this latter level that damage occurs to the ear almost
instantaneously. However, damage can also occur after prolonged exposure as well, for
example, forty hours of exposure to 90 dB produces roughly the same amount of damage to the
ear as a single gunshot. The ear has no protection against these high decibel sounds. While the
ear does produce wax which can buffer the sound this is extremely limited as the wax produced
in the ear is meant to block the entry of harmful bacteria, not loud noises. Once damage has
occurred in the ear it stays with the individual, the body does not or cannot repair damage done
to the bones or organs in the auditory system.
The ear also picks up sound based on vibration. As stated earlier it is the vibrations of
sound waves which cause the inner ear to vibrate and carry sound to the brain. These vibrations
are measured in frequencies of Hertz (Hz). The frequencies between 100 Hz and 1000Hz
increase on a linear level, for example, the difference in pitch between sounds of 300 and 450 Hz
will be the same between 450 and 600 Hz and 750 and 900 Hz etc… For frequencies between
1000 Hz and 10,000 Hz the difference in pitch is related to the ratio of the two frequencies. For
example, 1,500 Hz and 3,000 Hz have a ratio of 1:2 because 3,000 Hz is twice as much as 1,500
Hz. Because of this ratio the difference in pitch of 3,000 Hz to 6,000 Hz or 4,500 Hz to 9,000
Hz are all equal.
Using the frequencies we can measure the power of a sound to determine how “loud” it
is. This power is determined by the square of the amplitude of a sound. If we have a sound at a
certain amplitude and we double it then the power is increased by a factor of two squared, or
four. If we triple the amplitude then power is increased by nine. This adds up very quickly.
Thus, if we take the smallest sound we can hear, say a tiny whisper at 30 dB, then the loudest
sound we can hear without crossing the ear’s pain threshold will be 1,000,000,000,000 times
greater than the whisper.
When the ear combines both frequency and volume we get sound. In order to distinguish
between sounds the ear picks up differing frequencies. Let’s say we take two sounds one at 60
dB and one at 40 dB and both are vibrating at 1,000 Hz, since both are vibrating on the same
frequency the sound with the higher volume takes precedence and we do not hear the 40 dB
sound at all. If all sound vibrated at the same frequency then only the loudest sounds would be
heard. It is because all sound vibrates at different frequencies that we are able to hear more than
one person speaking at a time or why we are able to distinguish the different instruments in a
band or symphony. Truly the ear and the auditory system are one of the greatest marvels
contained in the human body.
Works Cited
Ladefoged, Peter. Elements of Acoustic Phonetics. 2nd ed. Chicago: The University of Chicago
Press, 1996. 75-91. Print.
Vetter, Douglas E. "How do Three Tiny Bones Amplify Sound into the Inner Ear?." Scientific
American. 298.4 (2008): 114. Print.