Sound and Wave

Sources of sound such as a drum, a guitar and the human voice have some part which vibrates. The sound travels through the air to our ears and we hear it. It becomes clear that air is an essential factor once we, for example, pump out a glass containing a ringing electric bell. The sound disappears though the striker can be seen hitting the gong. Evidently sound cannot travel in a vacuum like light can.

Sound also gives interference and diffraction effects. Because of this and its other properties, we believe it is a form of energy which travels as a wave, but of a type called progressive longitudinal.


In a longitudinal wave the particles of the transmitting medium vibrate to and fro along the same line as that in which the wave is traveling and not at right angles to it as in a transverse wave.

A sound wave produced for example by a loudspeaker consists of a train of compressions and rarefactions in the air. The speaker has a cone which is made to vibrate in and out by an electric current. When the cone moves out the air in front is compressed: when it moves in the air is rarefied (‘thinner’). The wave progresses through the air but the air as a whole does not move. The air particles vibrate backwards and forwards a little as the wave passes, and this causes the sound you hear when the wave reaches your ears.