# Longitudinal wave

A longitudinal wave that traverses a surface from left to right.
In the case of a longitudinal wave (a), the direction of oscillation coincides with the direction of propagation. In the case of a transverse wave (b), on the other hand, the direction of oscillation is perpendicular to the direction of propagation.

A longitudinal wave , also called a longitudinal wave , is a physical wave that oscillates in the direction of propagation . The standard example of longitudinal waves is sound in gases or liquids. Its counterpart is the transverse wave , the amplitude of which is perpendicular to the direction of propagation.

## Propagation principle

Longitudinal waves are pressure waves . This means that in a medium, zones with overpressure or compressive stress (or negative pressure or tensile stress) propagate or shift or spread in the direction of propagation.

The individual particles in the propagation medium, atoms or molecules , swing back and forth in the direction of propagation by the amount of the amplitude. After passing through the oscillation, the particles move back to their rest position, the equilibrium position. No energy is lost through the propagation of the oscillation, apart from friction losses between the particles.

• In a stretched body (spring, ribbon, rod, wire) the amount of the amplitude is the same at all points of the medium.
• When spreading from a concentrated source into space, the power density decreases with the square of the distance from the source, since the area enclosed by the solid angle grows quadratically with the distance.

The power of a longitudinal wave is proportional to the square of the amplitude of the displacement or the compressive stress; see also sound pressure and sound velocity .

## Examples

Mechanical longitudinal waves can propagate in any medium, whether solid , liquid or gaseous .

A typical example of a longitudinal wave is sound , which can only occur as a longitudinal wave in gases and liquids.

## Comparisons and characteristics

Longitudinal waves have a higher speed in the same solid medium than transverse waves of the same type with otherwise the same parameters.

Longitudinal seismic waves are called P waves. They always arrive first and have a lower potential for destruction than transverse waves during earthquakes . In contrast, the (transversal) water waves can cause “ tsunamis ”.

In plasmas and in other electrical conductors there are electromagnetic longitudinal waves in addition to electromagnetic transverse waves.