**Longitudinal waves** are waves in which the displacement of the medium is in the same direction as, or the opposite direction to, the direction of propagation of the wave. Mechanical longitudinal waves are also called *compressional* or *compression waves*, because they produce compression and "rarefaction when traveling through a medium, and *pressure waves*, because they produce increases and decreases in pressure.

The other main type of wave is the "transverse wave, in which the displacements of the medium are at right angles to the direction of propagation. Some transverse waves are mechanical, meaning that the wave needs a medium to travel through. Transverse mechanical waves are also called "shear waves".

By "acronym, "longitudinal waves" and "transverse waves" were occasionally abbreviated by some authors as "L-waves" and "T-waves" respectively for their own convenience.^{[1]} While these two acronyms have specific meanings in "seismology (L-wave for "Love wave^{[2]} or long wave^{[3]}) and "electrocardiography (see "T wave), some authors chose to use "*l-waves*" (lowercase 'L') and "*t-waves*" instead, although they are not commonly found in physics writings except for some popular science books.^{[4]}

Longitudinal waves include sound waves ("vibrations in pressure, particle of displacement, and particle velocity propagated in an "elastic medium) and seismic "P-waves (created by earthquakes and explosions).

In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave, and waves can be either straight or round. A wave along the length of a stretched "Slinky toy, where the distance between coils increases and decreases, is a good visualization.

In the case of longitudinal harmonic sound waves, the "frequency and "wavelength can be described by the formula

where:

*y*is the displacement of the point on the traveling sound wave;*x*is the distance the point has traveled from the wave's source;*t*is the time elapsed;*y*_{0}is the "amplitude of the oscillations,*c*is the speed of the wave; and*ω*is the "angular frequency of the wave.

The quantity *x*/*c* is the time that the wave takes to travel the distance *x*.

The ordinary frequency (*f*) of the wave is given by

The wavelength can be calculated as the relation between a wave's speed and ordinary frequency.

For sound waves, the amplitude of the wave is the difference between the pressure of the undisturbed air and the maximum pressure caused by the wave.

Sound's "propagation speed depends on the type, temperature, and composition of the medium through which it propagates.

In an elastic medium with "rigidity, a harmonic pressure wave oscillation has the form,

where:

*y*_{0}is the amplitude of displacement,*k*is the angular "wavenumber,*x*is the distance along the axis of propagation,*ω*is the angular frequency,*t*is the time, and*φ*is the phase difference.

The restoring force, which acts to return the medium to its original position, is provided by the medium's "bulk modulus.^{[5]}

"Maxwell's equations lead to the prediction of electromagnetic waves in a vacuum, which are transverse (in that the electric fields and magnetic fields vary perpendicularly to the direction of propagation).^{[6]} However, waves can exist in plasmas or confined spaces, called "plasma waves, which can be longitudinal, transverse, or a mixture of both.^{[6]}^{[7]} Plasma waves can also occur in force-free magnetic fields. ^{[8]}

In the early development of electromagnetism, there were some like "Alexandru Proca (1897-1955) known for developing relativistic quantum field equations bearing his name (Proca's equations) for the massive, vector spin-1 mesons. In recent decades, some extended electromagnetic theorists, such as "Jean-Pierre Vigier and Bo Lehnert of the Swedish Royal Society, have used the Proca equation in an attempt to demonstrate photon mass ^{[9]} as a longitudinal electromagnetic component of Maxwell's equations, suggesting that longitudinal electromagnetic waves could exist in a Dirac polarized vacuum.

After "Heaviside's attempts to generalize "Maxwell's equations, Heaviside came to the conclusion that electromagnetic waves were not to be found as longitudinal waves in "*"free space*" or homogeneous media.^{[10]} But Maxwell's equations do lead to the appearance of longitudinal waves under some circumstances, for example, in "plasma waves or "guided waves. Basically distinct from the "free-space" waves, such as those studied by Hertz in his UHF experiments, are "Zenneck waves.^{[11]} The longitudinal modes of a "resonant cavity are the particular standing wave patterns formed by waves confined in a cavity. The longitudinal modes correspond to those wavelengths of the wave which are reinforced by constructive interference after many reflections from the cavity's reflecting surfaces. Recently, Haifeng Wang et al. proposed a method that can generate a longitudinal electromagnetic (light) wave in free space, and this wave can propagate without divergence for a few wavelengths.^{[12]}

**^**Erhard Winkler (1997),*Stone in Architecture: Properties, Durability*, p.55 and p.57, Springer Science & Business Media**^**"Michael Allaby (2008),*A Dictionary of Earth Sciences (3 ed.)*, Oxford University Press**^**Dean A. Stahl, Karen Landen (2001),*Abbreviations Dictionary, Tenth Edition*, p.618, "CRC Press**^**Francine Milford (2016),*The Tuning Fork*, pp.43-4**^**Weisstein, Eric W., "*P-Wave*". Eric Weisstein's World of Science.- ^
^{a}^{b}"David J. Griffiths, Introduction to Electrodynamics, "ISBN "0-13-805326-X **^**John D. Jackson, Classical Electrodynamics, "ISBN "0-471-30932-X.**^**Gerald E. Marsh (1996), Force-free Magnetic Fields, World Scientific, "ISBN "981-02-2497-4**^**Lakes, R. (1998). Experimental limits on the photon mass and cosmic magnetic vector potential. Physical review letters, 80(9), 1826-1829**^**Heaviside, Oliver, "*Electromagnetic theory*".*Appendices: D. On compressional electric or magnetic waves*. Chelsea Pub Co; 3rd edition (1971) 082840237X**^**Corum, K. L., and J. F. Corum, "*The Zenneck surface wave*",*Nikola Tesla, Lightning observations, and stationary waves, Appendix II*. 1994.**^**Haifeng Wang, Luping Shi, Boris Luk'yanchuk, "Colin Sheppard and Chong Tow Chong, "Creation of a needle of longitudinally polarized light in vacuum using binary optics," Nature Photonics, Vol.2, pp 501-505, 2008, doi:10.1038/nphoton.2008.127

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*Experimental Observation of Pressure Waves in Gas-Solids Fluidized Beds*". American Institute of Chemical Engineers. New York, N.Y., 1997. - Krishan, S, and A A Selim, "
*Generation of transverse waves by non-linear wave-wave interaction*". Department of Physics, University of Alberta, Edmonton, Canada. - Barrow, W. L., "
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