What is sound?

Make it stop, I’m trying to study! Ugh, can’t you just put headphones on? They’re not helping, you’re making so much noise the whole house is shaking! How can music make the walls and floors of a building shake? Let’s take a closer look at the instruments Jenny, Aki and Esi are playing.

Jenny is playing the electric guitar. Whenever she plucks a string, the string moves back and forth very quickly. It vibrates. Aki is playing the bass. Like Jenny’s electric guitar, the bass strings also vibrate when Aki plucks them.

Esi is playing the drums. When she bangs a drum with her drumstick, the membrane vibrates. If you touched the guitar string or drum membrane lightly, you would feel the vibrations! When a part of an instrument vibrates, it produces sound. But what is sound?

The air is made up of tiny particles. When an object such as a guitar string or a drum membrane vibrates, the vibrations make the air particles around them move back and forth as well. These particles then bump into the air particles next to them, those particles hit the air particles next to them, and so on. So in the air around the vibrating object, this bumping of particles creates regions of higher pressure — compressions, and lower pressure — rarefactions. Think of it like the pattern of waves created when you drop a stone into water.

Once the vibrations reach the eardrums, you can hear sound! We call the journey of vibrations from an object, through the air, a sound wave. While all sound is a wave, not all sound waves are the same. One way in which sound waves differ, is how far apart the rarefactions and compressions are. The distance between the two closest compressions, is related to the sound wave’s frequency.

The closer together the compressions are, the higher the frequency of the sound, and therefore the higher the pitch. Like the highest tone Jenny can play on her guitar! Aki, however, can play very low sounding tones on her bass. These sound waves have compressions further apart. They are at a lower frequency.

To measure frequency, we count how many compressions pass through a certain point, every second. The unit of this measurement is hertz. The human ear can only detect certain frequencies — those between 20 and 20 000 hertz! There is a whole world of sound that humans can’t actually hear! We’ve just learned that sound is particles vibrating and those vibrations moving through space.

So for sound to travel, there need to be particles — that’s why there’s no sound in a vacuum! The sound we hear, normally travels through air. But think, if you’re underwater and try to listen out for sounds around you, you still hear them! Sound does travel through liquids too, because liquids, like air, are made up of particles! As for solids — sound waves can travel through these, too!

Again, because solids are made up of particles. That’s why Michael feels the walls and floor shaking when Jenny’s band is practising! Alright, alright, we’re done for today anyway. Or nearly... let’s take it one more time from the top?