Longitudinal wave

"Longitudinal waves" and "transverse waves" have been abbreviated by some authors as "L-waves" and "T-waves", respectively, for their own convenience.[1] While these two abbreviations 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]

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

${\displaystyle y(x,t)=y_{0}\cos \!{\bigg (}\omega \!\left(t-{\frac {x}{c}}\right)\!{\bigg )}}$

where:

• y is the displacement of the point on the traveling sound wave;
  

  Representation of the propagation of an omnidirectional pulse wave on a 2d grid (empirical shape)
• x is the distance from the point to 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

${\displaystyle f={\frac {\omega }{2\pi }}.}$

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

${\displaystyle \lambda ={\frac {c}{f}}.}$

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.[5]

In May 2022, NASA reported the sonification (converting astronomical data associated with pressure waves into sound) of the black hole at the center of the Perseus galaxy cluster.[6][7]

Maxwell's equations lead to the prediction of electromagnetic waves in a vacuum, which is strictly transverse waves, due to the fact that they would need particles to vibrate upon, the electric and magnetic fields of which the wave consists are perpendicular to the direction of the wave's propagation.[8] However plasma waves are longitudinal since these are not electromagnetic waves but density waves of charged particles, but which can couple to the electromagnetic field.[8][9][10]

After Heaviside's attempts to generalize Maxwell's equations, Heaviside concluded that electromagnetic waves were not to be found as longitudinal waves in "free space" or homogeneous media.[11] 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.[12]

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[13] 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 almost all physicists and is incompatible with the Standard Model of physics.

• Transverse wave
• Sound
• Acoustic wave
• P-wave
• Plasma waves

• 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. S2CID 32056359.
• Russell, Dan, "Longitudinal and Transverse Wave Motion". Acoustics Animations, Pennsylvania State University, Graduate Program in Acoustics.
• Longitudinal Waves, with animations "The Physics Classroom"