March 1959 Popular Electronics
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
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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
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
. Starting 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 do it?
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
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. Various parts
of the human ear perform
many functions analogous to those performed by musical instruments and
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.
Sound waves 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
This 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.
The 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 monophonic one.
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.
Both ears 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 following example.
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 3D
. 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
The ear 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.