Compressions and rarefactions

Watch a slinky. Push and pull one end. Bunches of coils squeezed together and stretches of coils spread apart travel along it. But mark one coil with a ribbon: it only moves back and forth around its own place. It never travels along.

The stadium wave. At a cricket match, the crowd does a "Mexican wave". Each person only stands up and sits down, yet a wave seems to run all round the stadium. In sound too, the disturbance travels; the particles stay where they are and jiggle.

Inside a tube of air. Imagine a piston (like a pichkari plunger) moving back and forth in a tube:

• moving forward, it squeezes the air: a region of higher density (more air crowded together), called a compression (C);
• moving back, it leaves a region of lower density, a rarefaction (R).

If normal air had 10 particles in a box, a compression might have 15 and a rarefaction 5.

A sound wave is a series of compressions and rarefactions travelling through a medium. The air does not flow from the source to you; each particle only vibrates around its resting place, passing the push on, like a shove passing down a school queue.

Sound from a small source spreads out in all directions, as spherical waves. Air that is suddenly heated and expands, as in firecrackers or thunder, sends out a loud pulse of sound. An aircraft flying faster than sound makes a loud sonic boom.

Two kinds of wave:

• Longitudinal: particles vibrate along the direction the wave travels. Sound is longitudinal, like the slinky.
• Transverse: particles vibrate across the direction of travel, like a flicked rope or the stadium wave.

Waves that need a medium are mechanical waves; sound is one. Earthquake (seismic) waves come in both kinds; the longitudinal ones travel faster and are detected first. Light is a transverse wave that can cross a vacuum, which is how sunlight reaches us through space.