Sound waves transfer energy by causing successive compressions and rarefactions in the particles of the medium without transporting the medium particles themselves. Sound in solids can also manifest as transverse waves, causing crests and troughs in the propagation medium.
Waves can be characterized by two basic parameters: amplitude and frequency. Focusing on a single point vibrating in a medium excited by a sound wave, amplitude is the maximum distance traveled by the vibrating point relative to its rest position. Taking a snapshot of a propagating sound wave at any instance shows consecutive compressions and rarefactions, regions in which the medium material is compressed together and spread widely apart, respectively. The distance between the centers of two successive compressions or rarefactions is the wavelength of the wave.
The frequency is the inverse of the wavelength, so the larger the frequency, the smaller the wavelength. Having a higher frequency makes the wave have a larger number of more closely-grouped compressions and rarefactions as it propagates. As the amplitude and frequency of the wave increase, the energy transferred by the propagation of the wave also increases. Extremely short but low-amplitude ultrasonic waves can be used to heat polymer materials and weld them together.