How do musical instruments make sound?

It all starts with a wobble

Rest your fingers gently on the front of your throat and hum. Can you feel a buzz? That buzz is a vibration: something moving back and forth very quickly. Every sound you have ever heard started with a vibration.

When something vibrates, it pushes and pulls on the air around it. This sends out waves of slightly squashed and stretched air, called sound waves. When they reach your ear, they make your eardrum vibrate too, and your brain turns that into sound.

A musical instrument is a clever way of making vibrations on purpose. Every instrument needs something that can vibrate and a way to get it started, by plucking, bowing, blowing or hitting.

High notes and low notes

How high or low a note sounds is called its pitch. Pitch depends on the frequency: how many times something vibrates each second. Frequency is measured in hertz (Hz). One hertz means one vibration per second.

Fast vibrations make high notes and slow vibrations make low notes. Musicians often tune the note A to 440 hertz. The next A up, one octave higher, vibrates exactly twice as fast: 880 times a second. Most young people can hear sounds from about 20 hertz up to about 20,000 hertz, and we lose some of the highest sounds as we get older.

Strings

Guitars, violins, cellos and harps all make sound with vibrating strings. A string's pitch depends on three things. Shorter strings play higher notes, which is why pressing a guitar string against the neck makes the note go up. Tighter strings play higher notes, so players turn the tuning pegs to tighten or loosen them. Heavier, thicker strings play lower notes.

A string on its own is quiet, because it is too thin to push much air. So it passes its vibrations to the instrument's wooden body. The body has a big surface that moves lots of air, so the sound becomes much louder.

Wind and percussion

In a wind instrument, the thing that vibrates is the air inside a tube. A flute player blows across an edge, a clarinet player makes a thin reed vibrate, and trumpet players buzz their lips. A longer tube of air plays a lower note. When you uncover holes on a recorder, the vibrating column of air gets shorter, so the note goes higher.

Percussion instruments are hit, shaken or scraped. Drums, xylophones, triangles and clapsticks all belong to this family. On a drum, the stretched skin vibrates. Tighten the skin and the note goes up.

The yidaki: a tree that sings

A very special wind instrument comes from northern Australia. The Yolŋu people of north-east Arnhem Land call it the yidaki. It is also known as the didgeridoo, and many Aboriginal language groups have their own names for it.

Termites make the hard part. They eat out the inside of a eucalyptus trunk or branch, leaving a hollow tube. Makers choose a good hollow log, strip off the bark, clean out the inside and add a mouthpiece of beeswax. A yidaki is usually about 1.2 to 1.5 metres long, and different lengths make different sounds.

The player buzzes their lips into the tube, a bit like a trumpet or tuba player, making a deep, rumbling sound called a drone. Skilled players use circular breathing: they fill their cheeks with air and squeeze it out while quickly breathing in through the nose, so the sound never stops.

The yidaki is very old. Rock paintings of people playing it have been dated to between 1,000 and 1,500 years ago. It also comes with cultural rules. The yidaki belongs to the Yolŋu people, and in many communities it is traditionally played only by men, so the best way to learn about it is from its custodians.

📚 Words to know

Vibration

A fast back-and-forth movement. All sounds start with something vibrating.

Pitch

How high or low a sound is.

Frequency

How many times something vibrates each second. Higher frequency means higher pitch.

Hertz

The unit for frequency. One hertz (Hz) is one vibration per second.

Circular breathing

A way of playing without stopping, by pushing air out from the cheeks while breathing in through the nose.

🧪 Try this: a band from the recycling bin

1. Stretch three or four rubber bands of different thicknesses around an open plastic container or shoebox. Pluck each one. Which sounds lowest? (Keep your face back in case a band snaps.)
2. Slide a pencil under the bands near one end. Press a band down at different points with your finger and pluck the part between your finger and the pencil. Does a shorter part sound higher or lower?
3. Line up three plastic bottles with different amounts of water in them. Blow gently across the top of each until it hums. The bottle with the most water has the shortest column of air, so it plays the highest note.
4. Now you have strings and a wind section. Tap an upside-down container with a wooden spoon for percussion, and play a song!