March 1959 Popular Electronics
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as optimizing the transmission path between an RF transmitter and
receiver helps guarantee the best possible fidelity in receiving
an exact copy of the transmitted signal, so too, does optimizing
the signal path for an audio signal help guarantee a faithful replicate
of the original sound. This article from the March 1959 edition
of Popular Electronics is a primer on the topic of understanding
how the human ear perceives sound, and how to best facilitate a
good match between the speaker and the ear drum.
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The Ear and High Fidelity
Hi-fi's "ultimate consumer," the ear itself works like a
miniature hi-fi system
By Morris M. Rubin
the world of hi-fi, with its tweeters, woofers, tuners, amplifiers
and so on, it is easy to forget that all of these are servants of
one master, the Human Ear. One can almost visualize the great and
noble Ear sitting in the midst of this host of hi-fi components,
receiving their ;services like a feudal baron receiving the produce
of his serfs. Hearing Is Believing
at the dawn of life as an humble part of a fish's respiratory organ,
the ear has developed into a most remarkable instrument. Stop for
a moment and think of the widely differing sounds that it is called
on to recognize ; the breathing of a sleeping baby, the roar of
a jet plane and the magnificence of a symphony orchestra.
When the hi-fi fan talks of highs and lows, of distortion and
peaks, of recording and playback, he is speaking of attempts to
feed his ear a select sample of the multitude of different sounds
it can recognize.
Let us imagine someone sitting in a comfortable
chair in his living room, about to listen to a Tchaikowsky piano
concerto on his hi-fi rig. The opening chords are played. He immediately
recognizes them as having been produced by a piano. How does he
To answer this question, we must know something about
how the ear works. The Ear in Three Parts
The ear is made up of three main sections, the outer, the middle,
and the inner ear.
The outer ear is what we see sitting
on the sides of our head. Anatomists call it the pinna. It is probable
that in days gone by we could move the pinna to judge sound direction.
But now it remains motionless and just collects the sound. From
the pinna the sound proceeds down a passage called the auditory
canal (a distance a little less than an inch) to the eardrum.
The eardrum marks the beginning of the middle ear. It is
shaped like the cone of a loudspeaker, and works roughly the same
way, but in reverse. (The loudspeaker cone couples mechanical vibrations
to the air; the eardrum couples air vibrations to the mechanical
parts of the ear.) Attached to it is a bone called the hammer which
is connected to another bone called the anvil which in turn is connected
to the stirrup. These three bones form the ossicular chain and work
in a Rube Goldberg fashion, with one bone activating the next. The
base of the stirrup, the last element in this seriesconnected mechanical
circuit, fits into the oval window, the entrance to the inner ear.
of the human ear perform
many functions analogous to those performed by musical instruments
and electronic devices.
In the inner ear we find the
cochlea, where the real work of separating the lows from the highs
is carried on. This snail-shaped, tapering coil narrows down from
its widest part at the oval window to an apex.
travel into the outer ear and strike the eardrum. The eardrum responds
to the pattern of sound waves in very much the same way that a voice
coil and speaker cone respond to a pattern of electrical impulses.
Submicroscopic vibrations of the eardrum are transmitted to the
ossicular chain. This chain acts like a mechanical step-up transformer,
matching the impedance of the eardrum to the higher impedance of
the liquid in the cochlea. The gain of this system is about 20.
The stirrup moves in the oval window and sets up a vibration
of the liquid in the cochlea canals. This in turn shakes the membrane
holding the Organ of Corti which, through its nerve cells, analyzes
the movements of this liquid. The pattern of vibrations transmitted
by the liquid to the Organ of Corti almost exactly matches the original
sound wave pattern. Organ of Corti
is the "heart" of the hearing system. The Organ of Corti floats
on the flexible membrane separating the lower canal from the cochlea
canal. It is to this structure, which contains about 25,000 specialized
sensory nerve cells, that the designers of communication and highfidelity
equipment direct themselves. This is where the auditory nerve connects
the ear to the brain.
As even the largest and most complicated
computer cannot duplicate the complexity of human thought, not even
the finest and most expensive microphone can match the ear's ability
to discriminate between a variety of sounds. The function of the
Organ of Corti can be easily understood when it is compared to the
action of a piano. The long heavy piano strings make low-frequency
sounds when they are struck and the thin shorter strings produce
the higher notes,
Similarly, the cochlea is wide at one
end and narrow at the other. Since the Organ of Corti responds to
the vibrations of the liquid in the canals, it is easy to see that
it will pick up low-frequency vibrations at its widest end where
there is the most fluid, and the high frequencies at its narrow
end where there is little fluid.
Organ of Corti works in precisely the same way as does a microphone.
It converts the mechanical energy of sound vibrations into electrical
impulses. Thus, sound is analyzed in the cochlea, the report is
sent via the auditory nerve to the brain, and there it is interpreted.
The brain thumbs through its files, calling upon its vast store
of memories and associations and says, "This is the sound of a piano
- no question about it!" Music for Two Ears
Within the past few years, the ear has acquired a new but worthy
servant - stereophonic reproduction of sound. No matter how hi the
fi of a record or a playback-instrument, the ear cannot be fooled
into thinking that a sound is "real" if its source is a conventional
A monophonic system will serve the ear many
delicacies of loudness, frequency, and so on, but the meal falls
flat without the spice of spacial perception. Stereophonic reproduction
adds this last, but almost indispensable, spice.
receive the same sound stimulus only if the sound is produced from
a source directly in front of the listener. Any deviation to one
side will cause the sound wave patterns reaching each ear to be
slightly different. This can be visualized with the help of the
Think of two small boats rocked in the
wake of a passing ship. They are both responding to the same wave
pattern, but one may be at the crest of one wave while the other
is at the trough of another. Sound waves also have what might be
called troughs and crests. Because of the difference in distance
from the sound source caused by ears being on the opposite sides
of the head, each will receive the sound wave at a slightly different
point. One ear will get a stimulus that is a tiny bit closer to
the crest than that received by the other. Sound
. In order to satisfy the ear's demands for more "realistic"
sound reproduction, engineers have developed a sound system that
instead of having only one sound source has two. But just adding
an extra loudspeaker to a monophonic hi-fi system will not give
the ear the sensation of space perception.
in order to produce stereophonic sound (that is, sound with the
dimension of space perception) must send out a message that varies
slightly from the message sent out by the other speaker. Each ear
then receives a different stimulus and the reproduced sounds will
become "three-dimensional." The brain combines the two differing
sounds into a composite three-dimensional image.
will, no doubt, demand further attention and more varied entertainment
as time goes on. But let us not forget that even this ruler of the
world of sound is in the service of a greater master - the incredibly
complex and wonderful human mind.