10. Nature of Sound

10.1 Sound as a Mechanical, Longitudinal Wave

Sound is produced whenever an object vibrates — that is, moves rapidly back and forth about a fixed mean position. These vibrations disturb the particles of the surrounding medium (air, water, or a solid), causing them to vibrate in turn about their own mean positions and pass the disturbance on to neighbouring particles, without the particles themselves undergoing any net, permanent displacement from their original locations. This transfer of a vibratory disturbance from particle to particle constitutes a sound wave. Sound is classified as a mechanical wave because it requires a material medium (solid, liquid, or gas) to travel — unlike electromagnetic waves such as light, sound cannot travel through a vacuum, since there are no particles in a vacuum to vibrate and pass on the disturbance. This was famously demonstrated by placing a ringing electric bell inside a bell jar and progressively pumping out the air with a vacuum pump: as the air is removed, the sound of the bell becomes progressively fainter and eventually inaudible, even though the bell can still be seen vibrating, proving that sound needs a medium while light does not.

Sound is also classified specifically as a longitudinal wave, meaning that the particles of the medium vibrate back and forth along the same direction in which the wave itself travels (as opposed to a transverse wave, such as a light wave or a wave on a plucked string, where particles vibrate perpendicular to the direction of wave travel). As a sound wave passes through air, it creates alternating regions of compression (where air molecules are pushed closer together, resulting in locally higher pressure and density) and rarefaction (where air molecules are spread further apart, resulting in locally lower pressure and density), and it is this alternating pattern of compressions and rarefactions travelling through the medium that constitutes the propagating sound wave.

10.2 Speed of Sound and the Factors Affecting It

The speed of sound depends critically on the nature and physical state of the medium through which it travels, and, for a given medium, on the temperature of that medium. In general, sound travels fastest through solids, slower through liquids, and slowest through gases. This ordering arises because the speed of sound depends on how quickly a disturbance in one particle can be transmitted to the next, which in turn depends on the elasticity (stiffness, or resistance to being deformed) and the density of the medium — specifically, speed increases with greater elasticity and (for a given elasticity) decreases with greater density. Solids, having their particles held rigidly and closely together by strong intermolecular forces, transmit vibrations most efficiently and quickly; gases, with their particles far apart and only weakly interacting, transmit vibrations comparatively slowly.

For sound travelling through a gas such as air, speed increases with increasing temperature, because at a higher temperature the gas molecules move faster and collide with each other more frequently, transmitting the disturbance more rapidly. As a rough rule, the speed of sound in air increases by about 0.6 m/s for every 1°C rise in temperature. Speed of sound in air is essentially unaffected by ordinary changes in atmospheric pressure at constant temperature (a subtle but frequently-tested point), and it increases slightly with increasing humidity, since moist air is less dense than dry air at the same temperature and pressure (water vapour molecules being lighter than the nitrogen and oxygen molecules they displace).

10.3 Speed of Sound in Various Media

Exam tip: The general order to remember is: speed in solids > speed in liquids > speed in gases, and within a gas, speed increases with rising temperature. 'Sound travels fastest in solids and does not travel at all through vacuum' is one of the most frequently repeated one-line facts across SSC/RRB papers.