Let’s look at the Apollo 11 moon landing to understand how sound and hearing work.
Imagine it, July 2, 1969, Neil Armstrong’s lunar module has just landed on the moon. He utters the now famous words “The Eagle has landed,” and some guy in Houston is listening to him roughly 240,000 miles away. What an achievement! But as humans we take it for granted. How did we hear him from the moon? It’s a journey, just like traveling to the moon in a rocket. It’s what we do every day to speak, hear, and communicate. All the conditions have to be just right, and it’s miraculous!
Remember, in space no one can hear you scream.
Why? Because there are no air molecules to push around. But Neil happened to be inside a spacecraft that also contained oxygen and other air molecules. He started by taking a breath in, and slowly releasing it. With his vocal cords contracted, he sent air up into his mouth, to the tip of his tongue behind his teeth. With vocal cords buzzing, Neil opened his mouth, removed his tongue from the back of his teeth, and spoke the word “The”. The magical thing is, Neil has a large vocabulary. He can describe anything to anyone, as long as they share the same language. But how could he be heard on Earth? After all he was on the moon thousands of miles away in a tin can!
Neil Armstrong utters the now-famous words on that day in 1969, “The Eagle has landed.”
Those words hit a microphone in the spacecraft. The microphone houses a thin sheet of metal and a diaphragm that can feel the air molecules bouncing against it. Neil’s speech has concentrated energies, frequencies, starts, and stops. As the sound waves hit the microphone, it vibrates to the pressure created by the sound waves. The amplifier then transforms the impulses into electrical energy, which becomes the audio signal.
Hearing aids also have tiny microphones, no bigger than the head of a pin but must be able to capture all these dynamic sounds.
Once sound reaches one’s ear, the magic of hearing occurs.
Let’s go back to Neil right before he takes the first step on the moon. Houston hears, “The eagle has landed.” over a headset which reassembles the sound waves from the radio waves. The sound waves enter the ear canal of the person wearing the headset in Command Center. Even before they hit that person’s eardrum, the shape of the ear canal (tube with a closed end and an open end) will amplify a certain part of the signal on its own, mainly the softer, harder to hear consonant sounds. The sound waves travel to the eardrum, vibrating it. The bones behind the eardrum move in frequency and amplitude. The bones then convert the sound energy into mechanical energy. The bones end in the middle ear and connect to the oval window, a membrane that separates the middle ear from the inner ear. The inner ear is like a pressurized cavity that has two sections separated like a battery.
The sound wave that enters here from the vibration of the bones, travels through the turns of the inner ear, just like inside a seashell, two and a half turns with high frequency receptors at the beginning and low frequency receptors at the end. There’s a sensitive and fragile part of the ear at this point that gets excited and brings the two battery connections together creating an electrical charge. The electrical charge travels out of the ear and into the brain. As the charge travels along the path to the brain there are stops just like a railroad line where the signal is boosted. Ultimately, the charge reaches the auditory section of the brain where it integrates with the rest of the higher functioning parts of the brain. These parts of the listener’s brain understand language and the idea that was in Neil’s head. Wow, what a feat!
This short video illustrates how sound waves enter the ear canal and travel through the various components of the middle and inner ear.
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