How Does Sound Travel
Have you ever thrown pebbles in a pool of water? The impact of pebbles in water creates ripples of waves that spread in the pool. In a similar fashion, vibrations cause waves in the air. We hear the sound when these waves reach our ears. To understand how this happens, let us take the example of a loudspeaker.
When a loudspeaker is switched on, a membrane in the loudspeaker moves backwards and forwards, i.e., it vibrates. This causes the air molecules surrounding the loudspeaker to vibrate. If we imagine the air molecules to be like small balls, a sound wave travelling through air alternatively pushes these balls close together and then pulls them away from each other. The areas where they lie together are called compressions, and the areas where they lie away from one another are called rarefactions. A diagrammatic representation of the propagation of sound waves produced when a drummer beats the drum is shown in figure.
As the sound wave propagates, the molecules themselves do not move from one point to another, they only vibrate about a mean position. It is the effect that propagates and reaches our ears.
Sound Needs a Medium to Travel
Sound waves require a material like a solid, liquid, or gas to travel through. (See activity on making a toy telephone at the end of the chapter. There the string attached to the cups is the , medium.) They cannot travel through vacuum. This is because sound travels by producing a vibration in the molecules of the medium surrounding it and there are virtually no molecules – in vacuum.