Defined Words: sound, tympanic membrane, ear
To understand how your hearing works, we’ll have to look back at how it developed through our evolutionary line. Let’s start with our evolutionary grandparents, the fish. Just like your real grandparents, fish lead a very different life than you in a very different environment.
If you were ever to visit your fishy ancestors, you’ll notice that they are pretty jumpy and understandably so. Underwater ecosystems were the first battle grounds of predation and unlike on land, fish have three dimensions in which to move at all times. When looking around, we use the phrase “keep your head on a swivel” but fish can’t turn their heads nearly as much as us. Instead, they have developed sensory organs to detect changes in the pressure of water around them.
Have you ever dragged your hand through a pool of water? You’ll notice that the water in front of your hand is being pushed forward and away in a ripple effect. Your hand is pushing on the water molecules which are in turn pushing on each other. Those ripples in water can be detected by fish, who can then determine where the movement came from and how big the moving thing is. A huge wave coming from a fish’s left means something big is coming at it from the left side.
The fish that were able to detect pressure changes and anticipate possible danger were better able to survive and pass down that trait. The reason people tell you not to tap on fishbowls is because the resulting vibrations can confuse and terrify the fish. Tapping can even damage their sensors if the amplitude of the wave is great enough.
Over time, the oceans became too crowded to find sufficient food causing some organisms to begin exploring the land. While those pressure sensors could detect movement in water, they weren’t great at detecting movement through air. If a predator is running at you, you aren’t going to feel the movement of the air that it’s pushing out of its way; there’s too much space between the air molecules. However, you will detect slight vibrations in the air that the predator might make by crunching some dead leaves or rustling some bushes.
We call the movement of air sound. Depending on the frequency and amplitude of the vibrations, we can hear sounds at different pitches and volumes. You cannot feel the air that the predator pushes out of the way while running at you, but you sure can hear all of the noise its making.
Since those first creatures found their way onto land, the basic mechanism for detecting vibrations in air has been relatively the same. Vibrating air enters a canal in the organism, usually on the side of the head. The air then meets a thin tympanic membrane (sometimes called your “ear drum”) and the membrane begins to vibrate at the same frequency and amplitude as the air that entered the canal.
The membrane will then pass those vibrations on to very small bones which also resonate at the same frequency and amplitude. Bones are much denser than a membrane, so the vibrations are much easier to detect for the communication cells.
As you can imagine, a small hole for air to enter doesn’t give the best results for picking up vibrating air. Remember, gaseous molecules are zipping around with lots of space in between them. Over time, individuals developed structures to bounce the surrounding air into the canal. That outer structure is called an ear.
Those that could hear had a better chance of survival and could pass down their traits. This is why you’ll notice our ear has a bunch of grooves that can bounce air into our canals.
Rabbits and deer are examples of organisms that grew much larger structures to bounce the sound in; you’ll notice these animals will always angle their ears toward you when they’re startled.
Gases are molecules in a state of constant motion with no attachments and will therefore push outward on any container that holds it. If there is more movement or more molecules, the gas will push on the container with greater pressure. The air behind your tympanic membrane follows the same rules, which means that too much or too little air compared to the surroundings can stretch the membrane.
Luckily, our ancestors adapted a means to equalize that pressure and it comes in a simple form: a tube leading from behind your tympanic membrane to the back of your mouth. If you’ve ever been on an airplane, you know the feeling of unbalanced pressure on your ears. People will usually tell you to yawn or chew gum to remove the unpleasant feeling. Those solutions just open up the tube and give the air a chance to flow to equilibrium.
Your hearing can alter the sounds you hear after some habituation. For example, listening to whispers is much easier after spending time in silence. On the other side, the large amplitude of loud music doesn’t hurt your ears after you listen for a little while. Your brain can even create interfering sounds to cancel some of the excess noise. The reason you hear ringing in your ears after a loud concert is because your brain is still ready to muffle loud sounds.
Your sense of hearing is nothing more than your ability to detect vibrations of air from your surroundings. But you already know this, especially if you’ve ever been to a loud concert. Sometimes a show is so loud that you can feel the music in your throat or chest, or even in the floor! The vibrations here have enough energy to vibrate not only the air but the liquid water in your body or the solid of your bones and the floor.
From detecting potential predators to creating wonderful music, our sense of hearing has played an integral role in our ascension to what we are today. Remember not to take for granted what we use every day.