The ear consists of three parts. The outermost of these, the floppy shell on the side of our heads that we call “the ear,” is formally the pinna (from the Latin for “fin” or “feather,” a bit oddly). On the face of it, the pinna would seem ill-designed to do its job. Any engineer, starting from scratch, would design something larger and more rigid—more like a satellite dish, say—and certainly wouldn’t allow hair to cascade over it. In fact, however, the fleshy whorls of our outer ears do a surprisingly good job of capturing passing sounds—and, more than that, of stereoscopically working out where they come from and whether they demand attention. That is why you can not only hear someone across the room speak your name at a cocktail party but turn your head and identify the speaker with uncanny accuracy. Your forebears spent eons as prey to endow you with this benefit.
Although all outer ears function in the same way, each set, it appears, is uniquely built and as distinctive as the owner’s fingerprints. According to the British scientist and author Desmond Morris, two-thirds of Europeans have free-hanging earlobes and one-third have attached lobes. Whether tethered or flapping, the earlobes make no difference to your hearing or indeed anything else.
The passage beyond the pinna, the ear canal, ends in a taut and sturdy piece of tissue known to science as the tympanic membrane and to the rest of us as the eardrum, which marks the boundary between the outer ear and the middle ear. The tiny quiverings of the eardrum are passed on to the three smallest bones in the body, collectively known as ossicles and individually known as the malleus, incus, and stapes (or hammer, anvil, and stirrup, because of their very vague resemblances to those objects). The ossicles are perfect demonstrations of how evolution is so often a matter of make-do. They were jawbones in our ancient ancestors and only gradually migrated to new positions in our inner ear. For much of their history, those three bones had nothing to do with hearing.
The ossicles exist to amplify sounds and pass them on to the inner ear via the cochlea, a snail-shaped structure (cochlea means “snail”) that is filled with twenty-seven hundred delicate hairlike filaments called stereocilia, which wave like ocean grasses as sound waves pass across them. The brain then puts all the signals together and works out what it has just heard. All this is done on a sublimely modest scale—the cochlea is no bigger than a sunflower seed, the three bones of the ossicles would fit on a shirt button—yet it works incredibly well. A pressure wave that moves the eardrum by less than the width of an atom will activate the ossicles and reach the brain as sound. You genuinely cannot improve upon that. As the acoustics scientist Mike Goldsmith has put it, “If we could hear quieter sounds still, we would live in a world of continuous noise, because the omnipresent random motion of air molecules would be audible. Our hearing really could not get any better.” From the quietest detectable sound to the loudest is a range of about a million million times of amplitude.
To help protect us from the damage of really loud noises, we have something called an acoustic reflex, in which a muscle jerks the stapes away from the cochlea, essentially breaking the circuit, whenever a brutally intense sound is perceived, and it maintains that posture for some seconds afterward, which is why we are often deafened after an explosion. Unfortunately, the process is not perfect. Like any reflex, it is quick but not instantaneous, and it takes about a third of a second for the muscle to contract, by which point a lot of damage can be done.
Our ears are built for a quiet world. Evolution did not foresee that one day humans would insert plastic buds in their ears and subject their eardrums to a hundred decibels of melodic roar across a span of millimeters. The stereocilia tend to wear out anyway as we age, and they do not, alas, regenerate. Once you disable a stereocilium, it remains lost to you forever. There isn’t any particular reason for this. Stereocilia grow back perfectly well in birds. They just don’t do it in us. The high-frequency ones are at the front and the low-frequency ones farther in. This means that all sound waves, high and low, pass over the high-frequency cilia, and this heavier traffic means they wear out more quickly.
In order to gauge the power, intensity, and loudness of different sounds, acoustic scientists in the 1920s came up with the concept of the decibel. The term was coined by Colonel Sir Thomas Fortune Purves, chief engineer of the British Post Office (which in those days was in charge of the British telephone system, hence the interest in sound amplification). The decibel is logarithmic, which means that its units of increment are not mathematical in the everyday sense of the term but increase by orders of magnitude. So the sum of two 10-decibel sounds is not 20 decibels but 13 decibels. Volume doubles about every 6 decibels, which means that a 96-decibel noise is not just a bit louder than a 90-decibel noise but twice as loud. The pain threshold for noise is about 120 decibels, and noises above 150 decibels can burst the eardrum. For purposes of comparison, a quiet place like a library or the countryside is about 30 decibels, snoring is 60 to 80 decibels, a really loud nearby thunderclap is 120 decibels, and standing in the wash of a jet engine at takeoff would be 150 decibels.
The ear is also responsible for keeping you balanced thanks to a tiny but ingenious collection of semicircular ducts and two tiny associated sacs called otolith organs, which together are called the vestibular system. The vestibular system does everything that a gyroscope does on an airplane, but in an extremely miniaturized form. Inside the vestibular channels is a gel that acts a little like the bubbles in a carpenter’s level, in that the gel’s movements from side to side or up and down tell the brain in which direction we are traveling (which is how you can sense whether you are going up or down in an elevator even in the absence of visual clues). The reason we feel dizzy when we jump from a merry-go-round is that the gel keeps moving even though the head has stopped, so the body is temporarily disoriented. That gel thickens as we age and doesn’t slosh around as well, which is one reason why the elderly are often not so steady on their feet (and why they especially shouldn’t jump from moving objects). When loss of balance is prolonged or severe, the brain doesn’t know quite what to make of it and interprets it as poisoning. That is why loss of balance so generally results in nausea.
Another part of the ear that intrudes upon our consciousness from time to time is the Eustachian tube, which forms a kind of escape tunnel for air between the middle ear and the nasal cavity. Everyone knows that uncomfortable feeling you get in your ears when you change heights rapidly, as when coming in to land in an airplane. It is known as the Valsalva effect, and it arises because the air pressure inside your head fails to keep up with the changing air pressure outside it. Making your ears pop by blowing out while keeping your mouth and nose closed is known as the Valsalva maneuver. Both are named for a seventeenth-century Italian anatomist, Antonio Maria Valsalva—who also, not incidentally, named the Eustachian tube, after his fellow anatomist Bartolomeo Eustachi. As your mother doubtless told you, you shouldn’t blow too hard. People have ruptured eardrums from doing so.
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