The Ear and High Fidelity
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Just 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 magazine 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. In the era, home-base hi-fi equipment was a big deal, as was building out listening areas for optimal performance. Room floor, wall and ceiling materials and finishes, dimensions, and placement of furniture, window coverings, wall decorations, etc., were all taken into account for serious audiophiles. Hi-fi's "ultimate consumer," the ear itself works like a miniature hi-fi system
In 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 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 series-connected 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 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. 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 high-fidelity 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. Each speaker, in order to produce stereo-phonic 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. 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.
Posted December 8, 2022 |
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By Morris M. Rubin