Sound is made when something vibrates (shakes). The molecules in the air (or a liquid or solid) are rapidly pushed closer together and pulled further apart. Musical instruments make pleasing sounds. Noise is any sound that is unwanted by the hearer.

Pitch

Slow vibrations make low sounds. Fast vibrations make high sounds. The low-ness or high-ness of a sound is called its pitch. An example of a low-pitched sound is the rumble of distant thunder or the beat of a bass drum. Birdsong is an example of a high-pitched sound.

Some sounds are so high-pitched we cannot hear them. Some animals, such as dogs, are able to hear these sounds, called ultrasounds. Bats produce squeaking ultrasounds, then use their large ears to listen to the echoes that bounce back. This helps them to avoid obstacles and catch insects in flight, even in darkness.

Making waves

Sound travels in waves. Sound waves spread out from the source just like the ripples on a pond when a pebble is thrown into it. If sound waves hit a hard surface, they bounce back off it. We may hear the returning sound as an echo.