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• Sounds travel at different speeds through gases, solids and liquids.
• You hear an echo when a sound wave reflects off a surface and reaches your ear.
• You can work out the speed of sound using the equation:

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Sound waves are vibrations that are passed on between particles. The speed of sound is the distance a sound travels divided by the time it takes.

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Watch the video to see how you can measure the speed of sound in air:

Example

Remember: In air sound travels at 340 m/s (metres per second).

• If you put your ear on the desk and tap the edge of the desk, you are hearing the sound through a solid.
• If you are underwater in a swimming pool you are hearing sound through a liquid.
• Sound can travel through anything made of particles.
• How quickly sound travels depends on how close together the particles are.
• The particles in gases are further apart than liquids, and so sound travels slower in a gas than a liquid. The particles in a solid are closer still and so sound travels fastest in solids.
• If there are no particles, like in the vacuum of space, sound can’t travel at all.

• You hear an echo when a sound bounces off something and comes back to your ear.
• An echo is made by a sound wave reflecting off a surface.
• The sound has to travel there and back, therefore, the distance travelled by the sound for an echo is twice the distance of the surface that it is bouncing off of.

Example

Some rooms, like sports halls, produce lots of echoes, but other rooms, like bedrooms, produce less echoes.

That’s because hard, smooth surfaces are good at reflecting sound.

Soft surfaces are good at absorbing sound. Rooms with lots of soft surfaces, like carpets and curtains, produce less echoes.