group velocity

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Group velocity

The velocity of propagation of a group of waves forming a wave packet; also, the velocity of energy flow in a traveling wave or wave packet. The pure sine waves used to define phase velocity vp do not ever really exist, for they would require infinite extent. What do exist are groups of waves, wave packets, which are combined disturbances of a group of sine waves having a range of frequencies and wavelengths. Good approximations to pure sine waves exist, provided the extent of the media is very large in comparison with the wavelength of the sine wave. In nondispersive media, pure sine waves of different frequencies all travel at the same speed vp, and any wave packet retains its shape as it propagates. In this case, the group velocity vg is the same as vp. But if there is dispersion, the wave packet changes shape as it moves, because each different frequency which makes up the packet moves with a different phase velocity. If vp is frequency-dependent, then vg is not equal to vp. See Phase velocity, Sine wave, Wave motion

group velocity

[¦grüp və′läs·əd·ē]
(physics)
The velocity of the envelope of a group of interfering waves having slightly different frequencies and phase velocities.
References in periodicals archive ?
As neutrinos propagate, coherence between different mass eigenstates is eventually lost due to their different group velocities.
The average group velocities obtained for the 104 source-station paths considered in this study are listed in Supplement 5 for several periods.
Material, Rod Geometry, Group Velocities and Pencil Lead Breaks
The phase and group velocities of (22) are smaller than the velocity constant c.
The phase and the group velocities of guided waves usually are different and depend essentially on frequency.
The left-right and up-down symmetries for negative as well as for positive group velocities in both cases are observed.
4(b), where two EFCs surrounding the center point r have the almost opposite curvatures and intersect the k-conservation line at two valid points of A and B with the inward and outward group velocities respectively.
This method is based on the averaging of group velocities for each period from all travel-time curves available.
Therefore given the elastic properties, group velocities of all three modes are determined for a given direction [[Theta].
Using standard ultrasonic systems, the phase velocities of megahertz-frequency elastic waves in low-loss composites can usually be determined using Fourier or Hilbert spectroscopic methods, while group velocities are typically found using pulse-echo-overlap and similar methods.