Sounds are produced by vibrations. You can feel these when you put your fingers against your throat and talk. A violin bow causes a string to vibrate, and lips vibrating cause the sound produced by brass instruments. By rubbing the handles of a bowl it can be made to vibrate and when it does the water contained within it vibrates. Polystyrene balls on the surface of a drum illustrate how a small sound is caused by a small vibration. The size of a vibration is called the amplitude. The bigger the amplitude the louder the sound.