A mechanical wave is a propagating disturbance in matter. It requires a medium whose parts can be displaced and that provides both inertia and a restoring interaction.

Local motion, travelling pattern

As a mechanical wave passes, particles of the medium move around their equilibrium positions. The disturbance and energy propagate through the material even though the material itself does not travel with the wave over the same distance.

What sets the wave speed

Mechanical-wave speed depends on the competition between restoring stiffness and inertia.

A taut string, for example, carries transverse waves with speed

$$v=\sqrt{\frac{T}{\mu}},$$

where $T$ is the tension and $\mu$ the mass per unit length.

Greater tension increases the restoring effect and raises the speed; greater linear density increases inertia and lowers it.

Examples

Waves on strings, seismic waves, water-surface waves and sound are all mechanical, although their geometry and restoring mechanisms differ.

Because the medium matters, mechanical waves can change speed, wavelength, reflection and transmission behavior when they encounter a different material or boundary.