What wave are longitudinal?

Answer 1

Here is a description of longitudinal waves and transverse waves and how they are the same and different. Examples are provided.

In one sentence, a longitudinal wave is any wave where the quantity that is oscillating is along the direction of propagation of the wave. There is no variation which is associated with any particular direction perpendicular to the direction of propagation.

With even greater brevity, a transverse wave is one which exhibits the oscillations to be occurring in a direction perpendicular to the direction of motion.

We speak of these two types of waves, longitudinal and transverse, because almost all waves that we encounter can be categorized as one or the other. (Not all, but almost all waves are one or the other.)

Both types of waves have all the usual characteristics of amplitude, frequency, wavelength and velocity, all related in the usual ways. The difference lies in the direction associated with the direction associated with the varying amplitude of oscillation.

The usual examples of transverse waves, the vibrations of a string or the waves on water, are easy to visualize as one can see the "up and down" motion of the vibrating medium. Light is also a transverse wave and that can not simple be seen.

The sound wave generated by a vibrating string is a longitudinal wave, as are all sound waves. The quantity that is oscillating in a sound wave is the pressure. For a pure tone, the sound wave is a series of peaks and troughs of pressure extending along the line that is the direction the wave is traveling. The pressure does not have a direction and the increases and decreases in pressure are associated with no direction perpendicular to propagation. That is enough to make it a longitudinal wave, i.e. it is enough that it is not a transverse wave.

The usual argument that a sound wave is a longitudinal wave utilizes the direction of motion of the air caused by the wave. At any particular point in space, as the wave move past, there is motion of each region of air as it moves sinusoidal forward and backward along the direction of motion. Oscillating movement along the direction of motion certainly meets the criteria necessary to be longitudinal.

In another example, a wave is traveling along a spring where the wave pattern is produced by the coils of the spring moving closer together and further apart and those motions traveling along the spring in a regular sinusoidal pattern. This is directly analogous to the properties of a sound wave and it is, therefore, an example of a longitudinal wave.

One could imagine a rope of lights where the colors at each point varied continuously and sinusoidal as the oscillating pattern move along a wire. This color wave also has no direction associated with the oscillating character, so it too would be longitudinal.

End note 1. Technically, not all waves propagate and some waves are standing waves. Standing waves are, however, a special case which occurs when two waves are combined, in which case the two waves will separately have a well defined direction and therefore allow the classification of them as longitudinal or transverse. In what follows, it will be assumed we are discussing propagating waves.)

End note 2. A wave is an oscillating process that moves through space. Normally a wave travels through a medium, though light is a wave that does not require a medium. While waves can have complex shapes, we have found that nearly all wave processes can be represented as being built up of plane waves, i.e. waves which vary in a sinusoidal manner and move only in one direction.

End note 3. There are the kinds of waves, torsional waves is one example. Also, in solids, elastic waves do not have to be compression wave, they can also be shear waves.