Your Designed Body: Hearing Is a Symphony of Parts
Howard Glicksman and Steve Laufmann
Editor’s note: We are delighted to present this excerpt from Your Designed Body, the new book by engineer Steve Laufmann and physician Howard Glicksman.
To hear, your body must collect acoustic signals from the environment (pressure waves in the air), channel them to the right locations, convert them into nerve impulses, send them to the brain, and correctly interpret them into experiences like speech and music. And, just as with vision, if any one of those parts works incorrectly, or even just a bit less efficiently, hearing is either severely degraded or impossible.
The human ear can detect sound when the eardrum is displaced by as little as one-tenth the diameter of a single hydrogen atom. Yet it can also hear and correctly interpret sounds with acoustic pressure levels approaching the loudest sounds produced in nature (~1 kilopascal (kPa)).
And you can do more than register sounds of varying pitch and volume. From an early age you could tell from the sound of your mom’s voice just how much trouble you were in, and which direction she was calling from (so you knew which way to run). These and other features of human hearing require — and by now this should come as no surprise to readers — not just one or two clever engineering solutions, but a suite of ingenious solutions upon ingenious solutions.
The figure below illustrates the main parts of the body’s auditory system. Its many parts work together to gather sound waves from the environment and transmit them accurately and efficiently to the cochlea, where a subsystem called the organ of Corti converts them into nerve impulses and sends them to the brain.
The ear is divided into three regions: the outer (external) ear, the middle ear, and the inner (internal) ear. We’ll walk through these parts in order — that is, following a sound wave as it moves from outside the body to the inside where it’s converted into information and eventually into an experience.
The Outer Ear
The outer ear is made up of the pinna (ear flap), the ear canal, and the tympanic membrane (eardrum).
The pinna acts like a satellite dish, collecting sound waves and funneling them down the ear canal. But it does more than just collect. The pinna’s ridges and folds reflect and absorb certain frequency components of incoming sound waves. Since the pinna is not circularly symmetric, sounds coming from different directions have slightly different acoustic characteristics. This means certain frequencies in a sound will be slightly softer or louder depending on the direction they enter the ear. This allows you to tell the direction a sound comes from. This is why we instinctively look up when we hear a sound coming from above us.
To further help with this, we have two ears for stereo sound. We can detect differences as small as ten microseconds in the time of arrival of the same sound in each ear. We can also detect subtle differences in loudness between our two ears. Coupled with the fine-grained sound-shaping done by the outer ear, this allows us to tell the direction of a noise and hear in three dimensions. That is, our minds can generate a three-dimensional understanding of what’s going on around us based solely on sounds.
Close your eyes and listen carefully to the sounds you hear. Where are they, both in direction (left or right, front to back, up or down) and distance away from you? If you have good ears and are used to exercising this skill, your hearing should prove informative on this score.
The ear canal is a hollow tube about two centimeters long. It forms an acoustic channel between the pinna and the eardrum. The ear canal may not seem interesting at first glance, but its length plays a crucial role in hearing.
Much like a pipe in a pipe organ, the outer ear consists of a rigid tube open at one end and sealed at the other. Incoming waves bounce off the closed end and create standing waves in the tube (ear canal). This amplifies sounds at or near the tube’s resonant frequencies (constructive interference) and dampens sounds at other frequencies (destructive interference). This increases sensitivity to particular frequencies while diminishing the amplitude of others. Basically, it’s a passive amplifier!
For the human ear, this amplification is strongest at around 3,000 Hz. While this is higher than the central frequencies of human speech, it’s exactly the range where the percussive elements of the consonants in human speech are most prominent, and the consonants are essential for distinguishing the nuances of human speech.
The net effect is that the outer ear preprocesses incoming sound waves to maximize sensitivity to the natural frequencies of human speech. That is, our ears are fine tuned to hear best at the same frequencies we naturally speak.
The human ear can hear sounds from 20 Hz to around 20,000 Hz. Normal human speech ranges from 80–2,500 Hz. The lowest note on a tuba is 16 Hz, middle C on a piano is 262 Hz, and the highest note on a flute is 2,093 Hz.
The eardrum (tympanum) is a small membrane, about one centimeter in diameter, at the inner end of the ear canal. It’s a durable piece of skin tightly stretched across an opening in the bony skull. The eardrum vibrates at the same frequency as an incoming sound wave, enabling it to accurately and efficiently transmit sounds from outside the body to the inside. All the while, it maintains a barrier that seals the delicate inner workings of the ear from foreign matter and bacteria.
The Middle Ear
The middle ear is an enclosed air-filled chamber, beginning at the inner surface of the eardrum and ending at the cochlea.
The middle ear contains the ossicles, the three smallest bones in the body. These are the malleus (hammer), incus (anvil), and stapes (stirrup). They were given these familiar names because they resemble those objects in shape. Together, they transmit the vibrations of the eardrum into the inner ear.
To do this, the malleus is attached to the eardrum and the incus, the incus is attached to the malleus and the stapes, and the stapes is attached to the incus and the oval window of the cochlea, as shown in the figure below.
Sound waves make the eardrum vibrate, which vibrates the malleus, which vibrates the incus, which vibrates the stapes, which vibrates the oval window of the cochlea. But the key to hearing is how these bones are precisely shaped and interconnected to modify incoming vibrations.
Interestingly, these bones are fully formed at birth and do not grow as the entire body around them grows from infancy to adulthood. These are the only bones in the body with this property.
How does the body grow all its other bones while keeping just these specific ones from growing? What mechanisms and control systems are needed? So far, neither medical science nor biology has answers, but engineers know that such things don’t happen by accident, so there seem to be many interesting discoveries yet to be made.
Less-than-Obvious Problems
As you’d expect by now, there are some less-than-obvious problems with hearing that the body needs to solve.
First, just like all the body’s cells, the cells in the tissue surrounding the middle ear need oxygen for respiration. Since the middle ear is filled with air, these cells have direct access to a ready supply. But they will gradually absorb all the available air, causing a vacuum effect, which would reduce eardrum movement and impair hearing.
Without a way to replenish its air supply, the ear would quickly lose hearing acuity. To solve this problem, it uses a small tube, called the eustachian (auditory) tube, that connects the middle ear to the back of the throat. When you swallow or yawn, this tube opens, allowing fresh air to enter the middle ear. This equalizes the middle ear’s air pressure with the pressure outside the body. This tube can get clogged, as during a head cold, preventing the middle ear from equalizing pressure, which, as we all know, degrades hearing and causes earaches.
As a second and more formidable problem, sounds entering the body come through the air, but the cochlea is filled with fluid. The cochlea’s fluid, as we’ll see, serves a vital purpose, but it presents a thorny acoustic problem for accurate hearing. Because air is much less dense than liquid, and far more compressible, without some skillful engineering most of the energy of the sound wave would simply be reflected back into the ear canal. A rough analogy would be throwing a rubber ball at the sidewalk. Most of the ball’s energy is reflected in the ball’s bounce back to the thrower. Very little is transmitted to the sidewalk.
For proper hearing, then, the body needs to amplify the signal between the eardrum and the cochlea. The best way to do this is with a lever system. Since the malleus is attached to the eardrum and the stapes to the cochlea, this leaves the middle bone, the incus, to serve as a lever. But not just any lever will do. Only a very specific configuration of that lever will properly translate the pressure waves in the air into corresponding pressure waves in the fluid.
Impedance Transformation
The middle ear must provide a mechanical advantage to accurately bridge the different densities of air and fluid, and do so with minimal loss of either loudness or tonality. Mechanical engineers call this impedance transformation, a tricky problem to overcome in even a simple system.
The ear’s solution involves the precise shapes and configurations of all three bones of the middle ear. The malleus has a larger surface area than the stapes. Also, the two arms of the incus’s lever have different lengths. Each provides mechanical advantage. Pressure waves hitting the large area of the eardrum are concentrated into the smaller area of the stirrup so that the force of the vibrating stirrup is nearly fifteen times greater than that of the eardrum. This makes it possible to hear even the faintest sounds.
These bones can only do their job effectively when surrounded by air. If they were immersed in fluid, the viscosity of the fluid would degrade their mechanical properties. This drives the need for an air supply to the middle ear.
The three bones of the middle ear, and the ways they’re held in place by various tendons, act as a four-bar mechanism. The specific configuration in the ear is called a double-crank rocker. Engineers use four-bar mechanisms to fine tune mechanical relationships in systems where exacting precision and sophistication are needed, as they most certainly are in the middle ear. To achieve the necessary mechanical advantage, the shapes of the parts and the positions of the several hinge points must be precisely tuned, with little room for error.
So, hearing hinges on the precise configuration of these three tiny bones, with their very specific shapes which are essential to their purposes. Nowhere do we see this more clearly than in the bones of the middle ear.
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