What Is the Nature of Sound Waves?

Sound waves are mechanical waves produced by vibrations and transmitted through a material medium. In gases and liquids, they are longitudinal waves: particles oscillate parallel to the direction in which energy travels.

This is part of C.2 Wave model and applies to both SL and HL students.

How Sound Travels

A vibrating source, such as a loudspeaker cone, pushes nearby air particles together, creating a region of increased pressure called a compression. When the source moves back, it produces a region of reduced pressure called a rarefaction.

These alternating pressure variations move through the medium. Individual particles only oscillate about their equilibrium positions; they do not travel from the source to the listener. Energy is transferred through the medium, but there is no overall transfer of matter.

These quantities are related by

$$v=f\lambda.$$

The speed of sound depends on the medium's physical properties, particularly its elasticity and density. Sound generally travels faster in solids than in liquids and faster in liquids than in gases. It cannot travel through a vacuum because there are no particles to transmit the vibration.

A common misconception is that air particles travel from the speaker to the listener. They do not: the particles oscillate locally while the wave transfers energy.

Exam Technique

For an IB Explain question, link the vibrating source to compressions and rarefactions, state that particle oscillations are parallel to energy transfer, and identify sound as mechanical. If calculating, use $v=f\lambda$ with consistent SI units: $v$ in $\text{m s}^{-1}$, $f$ in , and in .