Everyone thinks they know what sound is, but it is important for your GCSE that you can give a precise definition. Start by humming a high note, and touch under your chin. What can you feel?
Sound is always produced by something vibrating. The vibrations will make matter - either solid, liquid or gas - near it vibrate. In this way, energy is taken away from the source of the vibrations.
We call these vibrations sound. Our ears pick them up and tell our brain what we can hear. The vibrations are longitudinal. Make sure you know what this means.