|In order for us to
hear, the ear must first direct sound waves into the hearing
By Ernie Fisher
The inner ear has the body’s smallest bones and they are fully
developed at birth. They are the malleus (the hammer), the incus,
(the anvil) and the stapes (the stirrup).
In order for us to hear, the ear must:
a) Direct sound waves into the hearing part of the ear, the pinna
or outer ear.
b) Sense the variations in air pressure.
c) Convert these variations into an electrical stimulus for the brain
The pinna faces forward and allows us to learn the direction of sound,
but also enables us to hear sounds better in front than from behind
(home theatre buffs, take note).
Our ability to hear and pinpoint sound from sides—the horizontal
plane—depends on the comparative intensity and relative phase
of sound reaching each ear. It’s the phase/intensity difference,
which is analyzed and processed by the brain stem (our brain tells
us the result, which is what we know as imaging).
When you sit in front of the loudspeakers at a rock concert, you’ll
expose your ears to about 120 dB sound pressure levels that will
begin damage to your hearing in about seven minutes, which explains
why more than one-third of the North American population has significant
hearing impairment by the age of 65. The many rock concerts in the
good old 70’s are likely responsible for a lot of this problem.
The eardrum or tympanic membrane has at least two layers—the
outer layer, which is continual with the skin of the outer ear, and
the central layer, the active vibrating area in response to sound.
At the eardrum, sound or air pressure variations are converted into
mechanical energy—the eardrum’s reactive movements. Another
transformation takes place when the mechanical energy is converted
into wave patterns of the basilar membrane. The basilar membrane
forms a partition between the cochlear canal and the tympanic canal
and houses the organ of Corti. Anchored in the Corti structure are
some 20,000 hair cells, with filaments varying in length in a manner
somewhat analogous to harp strings. These are the sensory hearing
cells, connected at their base with the auditory nerve.
It’s not known how the brain distinguishes high-pitched from
low-pitched sounds. One theory proposes that the sensation of pitch
is dependent on which area of the basilar membrane is made to vibrate.
It’s also not known how the brain distinguishes between loud
and soft sounds, though some scientists believe that loudness is
determined by the intensity of vibration of the basilar membrane.
In a small portion of normal hearing, sound waves are transmitted
directly to the inner ear by causing the bones of the skull to vibrate;
i.e., the auditory canal and the middle ear are bypassed. This kind
of hearing, called bone conduction, is utilized in compensating for
certain kinds of deafness and plays a role in the hearing of extremely
loud sounds. Now it may make sense when I have referred to a listening
experience that includes the ears and the body.
There is 135dB difference between the threshold of hearing (0 dB)
and pain—the power doubles 45 times. By the way, at the threshold
of 0 dB it is estimated that the eardrum still moves, if only to
the extent smaller than the diameter of a hydrogen molecule.
When we perceive sound, the most sensitive range is from about 2
kHz to 5 KHz—the range of voices, inner detail, special understanding
Finally and perhaps the most important to understand is how we process
what we hear is the shape of the sound scope at the ear drum, the
difference in intensity between left and right ears and the difference
in time arrival between reflections from the ear itself. This, of
course, relates to imaging and spatial elements and may well be the
reason why some folks simply can not appreciate good sound and are
satisfied with a portable radio, ipods and background, elevator music.
So much for the inner ear, but, of course there also is the middle
ear. It can be regarded as an impedance-matching transformer, which
matches the impedance of air (in the ear canal) to the inner ear.
Obviously, there is much more to our brain/hearing system than is
known to most audio folks. More important to the serious listener
is what is perceived as tonal connection to our senses, pleasing
musicality and authenticity of a performance — agree? Well,
that’s the way it is.
For more in-depth, science-based knowledge, I recommend reading “This
Is Your Brain On Music” written by Daniel J. Levitin. He runs
the laboratory for Musical Perception, Cognition and Expertise at
McGill University in Montreal. It’s a must read for all those
who love audio and music.