Mechanical Waves

A mechanical wave is a wave that carries energy through a medium — a material such as water, air, or a rope — by making the particles of that medium vibrate. The particles pass the energy along without traveling with it, so a mechanical wave transfers energy, not matter. Because it needs particles to move, a mechanical wave cannot travel through empty space. If you are new to waves, start with the introduction to waves.

In a transverse wave, the particles of the medium vibrate perpendicular (at a right angle) to the direction the wave travels. The high points are crests and the low points are troughs. The wavelength is the distance between two crests, and the amplitude is the height from the rest position to a crest.

A wave on a rope or ripples on a water surface are transverse.

In a longitudinal wave, the particles vibrate parallel to the direction the wave travels — back and forth along the same line. Where the particles bunch together is a compression; where they spread apart is a rarefaction.

Sound waves are the most common longitudinal waves.

The one difference to remember: in a transverse wave the particles move across the wave's path, while in a longitudinal wave they move along it. Both still carry energy through a medium, and both can add together where they meet, which is called superposition of waves.

• Sound — a longitudinal wave through air, water, or solids.
• Water waves and waves on a string — transverse.
• Seismic waves from earthquakes — both types (P-waves are longitudinal, S-waves are transverse).

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• Physics