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Sound and Vibrations

Primary schoolPhysicsVibrating string, drum skin and ruler; pitch from their size, tightness and thickness

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About this experiment

Every sound starts with something vibrating. Pluck a string, tap a drum or twang a ruler on the edge of a table and listen. Then change the instrument: make the string shorter, tighter or thinner, make the drum smaller or the ruler shorter, and hear the pitch go up. Hit harder and the sound gets louder but the pitch stays the same. Watch the air particles pass the vibration along to your ear.

Learning goals

  • Explain that sounds are made by vibrating objects
  • Describe how sound travels through the air to the ear
  • Find out how length, tightness and thickness change the pitch of a sound
  • Tell the difference between pitch (high or low) and loudness (loud or quiet)

How to use it

  1. 1Choose String and press Pluck. Watch the string and listen. What is moving?
  2. 2Make the string shorter, then longer. Pluck it each time. When is the sound higher?
  3. 3Make the string tighter, then thinner. Which changes make the pitch go up?
  4. 4Choose Drum. Compare a small drum with a big drum, and a tight skin with a loose one.
  5. 5Choose Ruler. Start with a long piece over the edge and make it shorter step by step. What happens to the sound?
  6. 6Change How hard and play again. Does the pitch change, or only the loudness? Look at the sound wave to check.

The model

A stretched string: shorter (L), tighter (T) or thinner (μ, mass per metre) gives a higher frequency f (for older pupils)
A smaller drum or a shorter ruler vibrates faster and sounds higher

Assumptions

  • The string is steel, like a guitar string, fixed at both ends. The drum skin is clamped all around the rim, and the ruler is steel, clamped at one end.
  • The sound you hear is made from the vibration frequencies of each object, so the string, drum and ruler each sound different even on the same note.
  • The picture and the moving air are slowed down 40 times. The sound you hear and the numbers are at real speed.
  • A very long ruler vibrates fewer than 20 times a second. You can see it wobble, but the sound is too low for people to hear well.

Try this in class

  1. Q1Why does a guitar have strings of different thickness?
  2. Q2How does a guitarist play a higher note on the same string?
  3. Q3Why does a big drum sound lower than a small drum?
  4. Q4If you shout in space, can anyone hear you? Why not?
  5. Q5Put your hand on your throat and hum. What can you feel?

Frequently asked questions

What is the difference between pitch and loudness?

Pitch is how high or low a sound is. It depends on how many times per second something vibrates. Loudness depends on how big the vibrations are. A hard pluck makes bigger vibrations, so the sound is louder, but the pitch stays the same.

How does the sound get to my ear?

The vibrating object pushes on the air next to it. Those air particles push on the next ones, and the squeeze travels through the air as a sound wave until it reaches your ear and makes your eardrum vibrate too.

Why do the string, drum and ruler sound different?

Each one vibrates in several ways at once, at different speeds. The mix of these extra vibrations, called overtones, is different for each object, which is why a guitar and a drum never sound the same.