The Human Ear
Understanding how humans hear is a complex subject involving
the fields of physiology, psychology and acoustics. In this part of Lesson 2,
we will focus on the acoustics (the branch of physics pertaining to sound) of
hearing. We will attempt to understand how the human ear serves as an
astounding transducer, converting sound energy to mechanical energy to a nerve
impulse that is transmitted to the brain. The ear's ability to do this allows
us to perceive the pitch of sounds by detection of the wave's frequencies, the
loudness of sound by detection of the wave's amplitude and the timbre of the
sound by the detection of the various frequencies that make up a complex sound
wave.
The ear consists of three basic parts - the outer ear, the
middle ear, and the inner ear. Each part of the ear serves a specific purpose
in the task of detecting and interpreting sound. The outer ear serves to
collect and channel sound to the middle ear. The middle ear serves to transform
the energy of a sound wave into the internal vibrations of the bone structure
of the middle ear and ultimately transform these vibrations into a
compressional wave in the inner ear. The inner ear serves to transform the
energy of a compressional wave within the inner ear fluid into nerve impulses
that can be transmitted to the brain. The three parts of the ear are shown
below.
The
Outer Ear
The outer ear consists of an earflap and an approximately
2-cm long ear canal. The earflap provides protection for the middle ear in
order to prevent damage to the eardrum. The outer ear also channels sound waves
that reach the ear through the ear canal to the eardrum of the middle ear.
Because of the length of the ear canal, it is capable of amplifying sounds with
frequencies of approximately 3000 Hz. As sound travels through the outer ear,
the sound is still in the form of a pressure wave, with an alternating pattern of high and low
pressure regions. It is not until the sound reaches the eardrum at the
interface of the outer and the middle ear that the energy of the mechanical wave becomes
converted into vibrations of the inner bone structure of the ear.
The
Middle Ear
The middle ear is an air-filled cavity that consists of an
eardrum and three tiny, interconnected bones - the hammer, anvil, and stirrup.
The eardrum is a very durable and tightly stretched membrane that vibrates as
the incoming pressure waves reach it. As shown below, a compression forces the
eardrum inward and a rarefaction forces the eardrum outward, thus vibrating the
eardrum at the same frequency of the sound wave.
Being connected to the hammer, the movements of the eardrum
will set the hammer, anvil, and stirrup into motion at the same frequency of
the sound wave. The stirrup is connected to the inner ear; and thus the
vibrations of the stirrup are transmitted to the fluid of the inner ear and
create a compression wave within the fluid. The three tiny bones of the middle
ear act as levers to amplify the vibrations of the sound wave. Due to a
mechanical advantage, the displacements of the stirrup are greater than that of
the hammer. Furthermore, since the pressure wave striking the large area of the
eardrum is concentrated into the smaller area of the stirrup, the force of the
vibrating stirrup is nearly 15 times larger than that of the eardrum. This
feature enhances our ability of hear the faintest of sounds. The middle ear is
an air-filled cavity that is connected by the Eustachian tube to the mouth.
This connection allows for the equalization of pressure within the air-filled
cavities of the ear. When this tube becomes clogged during a cold, the ear
cavity is unable to equalize its pressure; this will often lead to earaches and
other pains.
The
Inner Ear
The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve. The
cochlea and the semicircular canals are
filled with a water-like fluid. The fluid and nerve cells of the semicircular canals provide no role in the task of
hearing; they merely serve as accelerometers for detecting accelerated movements
and assisting in the task of maintaining balance. The cochlea is a snail-shaped
organ that would stretch to approximately 3 cm. In addition to being filled
with fluid, the inner surface of the cochlea is lined with over 20 000
hair-like nerve cells that perform one of the most critical roles in our ability
to hear. These nerve cells differ in length by minuscule amounts; they also
have different degrees of resiliency to the fluid that passes over them. As a
compressional wave moves from the interface between the hammer of the middle
ear and the oval window of the inner ear through the cochlea,
the small hair-like nerve cells will be set in motion. Each hair cell has a
natural sensitivity to a particular frequency of vibration. When the frequency
of the compressional wave matches the natural frequency of the nerve cell, that
nerve cell will resonate with a larger amplitude of vibration. This increased
vibrational amplitude induces the cell to release an electrical impulse that
passes along the auditory nerve towards the brain. In a process that is not clearly
understood, the brain is capable of interpreting the qualities of the sound
upon reception of these electric nerve impulses.