What is a Mechanical Wave?

A mechanical wave is a disturbance that pulsates through some form of medium: a solid, a liquid, or a gas, carrying the original, initiating energy from one place to another. There are three types of mechanical waves: transverse, longitudinal, and boundary. Simple examples of waves and the media through which they move are sound in air, seismic waves in rock, and the vibrations of a guitar string.

One example of mechanical waves are ripples in a pond that move outward from where a stone is dropped. The wave moves through the medium even though the atoms and molecules of the medium itself generally return to their original position after they have been squeezed together, stretched apart or rotated by the wave passing through them. The mechanical wave may move in straight-line motion, rotational motion, or a combination of the two.

Waves possess the characteristics of frequency, period, wavelength, and amplitude. The distance between one wave pulse and the next is its wavelength and the time between pulses is the period. For a transverse waves, the amplitude is the distance that the wave displaces the medium. The number of waves passing through the medium over time is the wave’s frequency. The higher the frequency of a mechanical wave, the greater the energy it transmits.

Transverse waves transmit their energy in an up and down or a side-to-side fashion, forcing the medium to take the form of a crest and trough — as in a sine wave. A good example of this kind of wave is a seismic S-wave in which the Earth’s crust moves up and down, or side to side, perpendicular to the direction of wave propagation. The mechanical wave model of a transverse wave is given by the wave equation, an energy equation, the solution of which is a sine wave possessing the aforementioned characteristics of frequency, period, wavelength, and amplitude.

Longitudinal waves are density waves that move through the expansion and compression of particles of the medium. One expansion and compression is one wavelength. Good examples of these waves are sound waves in air and seismic P-waves in the ground. The greater the density of the material, the faster the waves move through it.

Boundary waves are mechanical waves that resonate between two dissimilar media. Air cavities and constrained strings in musical instruments, the vibration of a tuning fork, and the crown-like shape of a water droplet splash are good examples of boundary mechanical waves. The resonant sound of the tuning fork is limited by the shape and size of its boundary and the elasticity of the material composing it.