There are so many diverse and interesting sounds in our daily lives that we tend to focus on the sounds themselves, but never really stop and think how we are able to hear them. What exactly is a sound anyway? In a nutshell, sounds are how our brain perceives waves that enter our ears.
Believe it or not, there is nothing inherently noisy about sound waves! The waves are the result of vibrations. For example, when you pluck a guitar string, it vibrates at a certain frequency (frequency is basically how quickly something vibrates). This causes the air around the guitar to get compressed (pushed together) in some areas, while the other areas expand.
Why does this happen? First, keep in mind is that, even though we can't see them, there are billions of molecules (tiny particles) floating around in air. Now, if you carefully watch a guitar string, you can see it go up and down very quickly when you pluck it. So when the string is moving towards you, it bangs into the air molecules and pushes them forward (thus compressing them). When the string moves away from you, it allows the air to expand. This pattern continues as the string vibrates, so you get alternating areas of compressed, expanded, compressed, expanded, etc.
This is like waving your hand in a pool of water. You hand moves at a much slower frequency than a guitar string, but it is a vibration nonetheless. If you've ever done that, then you know how difficult it is to move because your body spends so much energy pushing the water molecules out of the way.
You may have also noticed that waves ripple outwards. This is because the water near your hand gets pushed forward when your hand moves toward it, and it is allowed to expand when your hand moves away from it. The water waves that occur when your hand vibrates in water are similar to sound waves rippling outwards when something vibrates in air.
So, now that we understand what sound waves are, we can see that sound itself is not a result of plucking a string, banging a drum, etc. Only the waves are what result from those vibrations. Sound is just how our brain interprets those waves. It categorizes them according to their frequency, with high frequency (fast waves) sounding high-pitched like a violin, and low frequency (slow waves) sounding low-pitched like a bass.
When sound waves enter our ears, they cause bones in the ear to vibrate. Then those bones cause the fluid that lies in our inner ear to vibrate (like when you wave your hand in water).
Our inner ear also contains many tiny hairs along its surface. Each hair is programmed to respond to a certain frequency of waves in the ear fluid. When specific hairs come in contact with their "favorite" frequency, they send messages to the brain so it will know that that particular frequency has just entered the ear. These signals are interpreted as pitch. How is this amazing sensitivity achieved?
Our hair cells are basically arranged in a line. Each frequency "peaks" (is most intense) at a certain hair cell location, so each hair cell can respond to the one and only one frequency: the one which peaks where that hair cell is!
High frequencies tend to fizzle out quickly, so they only reach the hair cells that are close to the outside world. So, it should be no surprise that these hair cells respond to high frequencies. Low frequencies travel much further and "peak" further inside of your head. So your innermost hair cells are programmed to respond to low frequencies.
So, the lower a sound, the further inwards it peaked in your inner ear. The higher a sound, the further outwards it peaked.
Little Lion Experiment:
Stretch a rubber band around your thumb and index finger. Pluck the rubber band and notice the pitch. Stretch the rubber band by moving your thumb away from your index finger. What happens to the pitch now? Can you guess if the frequency is higher or lower?
After your pluck it, the band will keep vibrating but it won't move as far up or down (the height of the movement is called amplitude, and it corresponds to loudness). The frequency, however, shouldn't change (be careful not to move your fingers though!). If the frequency does change, then the pitch will change since frequency is interpreted by your brain as pitch. So, you can observe the frequency just by listening to see if the pitch changes over time! That allows you to see that the frequency stays the same even though the amplitude decreases.