A longitudinal wave is an oscillation or vibration that travels within a medium in parallel to the direction of motion. When one vibration particle is disturbed, it passes the disturbance onto the next particle, transporting the wave energy. As energy is being transported, the particles of the medium get displaced by a left and right motion. For example, if a longitudinal wave is traveling east through a medium, the disturbance will vibrate in parallel at an alternating left-to-right direction instead of the up-and-down motion of a transverse wave.
Longitudinal waves can be broken down into two categories, which are non-electromagnetic and electromagnetic. The main difference between the two is that electromagnetic waves can transmit energy through a vacuum, while non-electromagnetic waves cannot. Non-electromagnetic waves include pressure and sound waves. Plasma waves are considered to be an electromagnetic longitudinal wave.
P-waves, or pressure waves, are a type of compressional or elastic wave that can travel through various mediums such as gases, solids and liquids. During a seismic event, pressure waves are generated as a result of alternating compression and decompression. For example, the oscillation produced by an earthquake causes vibrations that travel through land and water. Out of all the seismic wave types, pressure waves are considered to be the fastest, allowing them to travel great distances. Pressure waves are recorded by an instrument called a seismometer that measures ground motions.
Earthquakes can also produce sound waves. Like pressure waves, sound waves are compressional in nature, meaning the waves expand and compress matter while moving through it. Sound waves, much like p-waves, require a medium to transport energy from one area to another. The speed at which sound travels is dependent upon the medium it passes through. Generally sound waves travel faster in solid media versus non-solid media, such as air.
Using a tuning fork is a good example of how sound is produced and transmitted via an air medium. The fork’s movements produce vibrations that disturb the air particles, producing a series of compression and decompression actions. These actions generate a pure musical note as a result.
Unlike pressure and sound waves, plasma waves cannot be produced directly from an earthquake. Instead, plasma waves must be generated from an electrical or magnetic source. A laser is an example of a device which produces plasma waves when it interacts with ionizing gases. This type of longitudinal wave can exist in an ionized or charged state. An electromagnetic wave can also take the form of a transverse wave, while a pressure wave will always be a longitudinal wave.