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. Transverse waves, for instance, describe some bulk sound waves in solid materials (but not in fluids); these are also called "shear waves" to differentiate them from the (longitudinal) pressure waves that these materials also support.
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. A wave along the length of a stretched Slinky toy, where the distance between coils increases and decreases, is a good visualization, and contrasts with the standing wave along an oscillating guitar string which is transverse.
"Longitudinal waves" and "transverse waves" have been abbreviated by some authors as "L-waves" and "T-waves," respectively, for their own convenience. While these two abbreviations have specific meanings in seismology (L-wave for Love wave or long wave) 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.
- 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;
- y0 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.
The equations for sound in a fluid given above also apply to acoustic waves in an elastic solid. Although solids also support transverse waves (known as S-waves in seismology), longitudinal sound waves in the solid exist with a velocity and wave impedance dependent on the material's density and its rigidity, the latter of which is described (as with sound in a gas) by the material's bulk modulus.
Maxwell's equations lead to the prediction of electromagnetic waves in a vacuum, which are strictly transverse waves, that is, the electric and magnetic fields of which the wave consists are perpendicular to the direction of the wave's propagation. However plasma waves are longitudinal inasmuch as these are not electromagnetic waves but density waves of charged particles, but which can couple to the electromagnetic field.
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. Maxwell's equations, as we now understand them, retain that conclusion: in free-space or other uniform isotropic dielectrics, electro-magnetic waves are strictly transverse. However electromagnetic waves can display a longitudinal component in the electric and/or magnetic fields when traversing birefringent materials, or inhomogeneous materials especially at interfaces (surface waves for instance) such as Zenneck waves.
In the development of modern physics, Alexandru Proca (1897-1955) was known for developing relativistic quantum field equations bearing his name (Proca's equations) which apply to the massive vector spin-1 mesons. In recent decades some other 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  as a longitudinal electromagnetic component of Maxwell's equations, suggesting that longitudinal electromagnetic waves could exist in a Dirac polarized vacuum. However photon rest mass is strongly doubted by most physicists.
- 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.
- 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
- 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.
- Lakes, Roderic (1998). "Experimental Limits on the Photon Mass and Cosmic Magnetic Vector Potential". Physical Review Letters. 80 (9): 1826–1829. Bibcode:1998PhRvL..80.1826L. doi:10.1103/PhysRevLett.80.1826.
- Varadan, V. K., and Vasundara V. Varadan, "Elastic wave scattering and propagation". Attenuation due to scattering of ultrasonic compressional waves in granular media - A.J. Devaney, H. Levine, and T. Plona. Ann Arbor, Mich., Ann Arbor Science, 1982.
- Schaaf, John van der, Jaap C. Schouten, and Cor M. van den Bleek, "Experimental Observation of Pressure Waves in Gas-Solids Fluidized Beds". American Institute of Chemical Engineers. New York, N.Y., 1997.
- Krishan, S.; Selim, A. A. (1968). "Generation of transverse waves by non-linear wave-wave interaction". Plasma Physics. 10 (10): 931–937. Bibcode:1968PlPh...10..931K. doi:10.1088/0032-1028/10/10/305.
- Barrow, W.L. (1936). "Transmission of Electromagnetic Waves in Hollow Tubes of Metal". Proceedings of the IRE. 24 (10): 1298–1328. doi:10.1109/JRPROC.1936.227357.
- Russell, Dan, "Longitudinal and Transverse Wave Motion". Acoustics Animations, Pennsylvania State University, Graduate Program in Acoustics.
- Longitudinal Waves, with animations "The Physics Classroom"