<P> 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". </P> <P> By acronym, "longitudinal waves" and "transverse waves" were occasionally abbreviated by some authors as "L - waves" and "T - waves" respectively for their own convenience . While these two acronyms 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 . </P> <P> 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). </P> <P> 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 . </P>

Why the sound waves are called longitudinal waves