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Question: Where do sound waves go after they are heard?
This is a very thoughtful question, but the answer is a bit complicated, as we first need to understand the nature of sound. Sound is the vibration of objects (e.g., a guitar string) that travel as a vibration of air molecules. If you pluck the guitar string, you can feel the vibrations by touching the guitar. The rate of vibration is called frequency and how far the string (and air molecules) moves in each direction is called amplitude (intensity) - we hear these physical properties primarily as pitch and loudness.
Air molecules try to stay a certain distance away from each other. Thus, air molecules moved by the vibrating guitar string cause nearby air molecules to vibrate, pushing them together (compression), then pulling them apart (rarefaction). Sound is this movement of alternating compressions and rarefactions in all directions from the sound source (e.g., vibrating guitar string). Most sounds we hear are from air molecule vibration. However, when you speak, your skull bones vibrate at low frequencies, adding that to your voice traveling through air to your ears, making your voice sound deeper in pitch. At the same time, a recording of your voice lacks the low frequency skull vibrations, allowing you to hear what your voice may sound like to others.
As sound travels farther distances, the vibration rate (frequency) does not change, but the amplitude is spread out over increasing numbers of molecules. Suppose there are 100 molecules 1 inch away that are caused to vibrate. Those 100 molecules cause vibration in 10,000 molecules 2 inches away, 100,000,000 molecules 4 inches away, and 10,000,000,000,000,000 molecules 8 inches away. Just like the sides of an expanding balloon, the farther the sound spreads, the weaker it gets. At some distance, the sound will be too weak to cause the next molecules to vibrate.
Now, knowing the nature of sound, how do we hear it? The vibrating air molecules are collected by our ears and funneled to our ear drums. Each ear drum is connected to a series of three tiny bones (auditory ossicles, the smallest bones in our body) that provide a bridge across our middle ear. The middle ear is an air filled space connected to our nose. When we go up or down quickly, this connection allows us to balance pressure on the two sides of our ear drum. The innermost auditory ossicle connects to a membrane (oval window) in the cochlea. The outside of the cochlea is mostly bone and looks like a snail shell. It is filled with liquid and contains sensory cells that convert vibration (sound) into electrical activity in the auditory nerve. Our brain interprets this electrical activity as sound.
What happens to the vibrations in your cochlea? Think about drinking milk through a straw. Milk at the end of the straw in the glass moves in or out depending upon whether you suck or blow on the straw in your mouth. The cochlea is like the straw. It is filled with fluid held between two flexible membranes (oval and round windows). Like the straw, the vibrations (movement) at the oval window cause movement at the round window. Round window movement should vibrate the air molecules in your middle ear. Fortunately, the soft tissue lining your middle ear and nose probably absorbs these vibrations, so that the sound does not come out your nose - that would be gross.
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