This excerpt is 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.
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.
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.
For proper hearing…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.
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.
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.See Evolution News for the complete article.