How Do Longitudinal Waves Move?

Longitudinal Wave Motion

• Particles in a longitudinal wave are displaced in a direction parallel to the direction the energy is being transported. Individual particles do not travel the entire distance of the wave. Rather, they oscillate back and forth. This creates a pressure, or compression, wave. The motion of a longitudinal wave is similar to that of a Slinky.

Creation of a Longitudinal Wave

• Longitudinal waves are typically created by a vibrating object. The object sets particles in a surrounding medium in motion, where they vibrate in the same direction. These particles then collide with neighboring particles, repeating the process. As this particle interaction continues, the longitudinal wave transports the energy of the initial object in the direction of the wave propagation.

Compressions and Rarefactions

• A vibrating object compresses the medium, increasing its density. This means more mass is packed into a smaller volume, creating an area of high pressure. Next, the object vibrates in the opposite direction, allowing the medium to expand. This decreases its density, creating an area of low pressure. The expansion phase is called a rarefaction. Thus, a longitudinal wave is a cycle, or series, of compressions and rarefactions, or regions of high and low pressure.

Longitudinal Wavelength

• A wavelength represents the distance that a disturbance in a medium travels in one cycle. For a transverse wave, this means the distance from crest to crest, or trough to trough. However, a longitudinal wave does not have crests or troughs. Instead, a longitudinal wavelength is measured from compression to compression, or rarefaction to rarefaction. This can also be described as the distance between high-pressure or low-pressure regions.

Longitudinal Frequency

• Frequency represents how often a cycle, or wavelength, occurs. A wave's frequency is measured in the number of cycles per unit of time. The typical unit for this measurement is the Hertz, where one Hertz equals one cycle per second. Every particle in a longitudinal wave vibrates at the same frequency, since its vibrations are due to the motions of adjacent particles. The period of the wave represents the time between compressions or rarefactions. For example, if a wave has a frequency of 60 Hertz, then the wave's period would be equal to one-sixtieth of a second. In terms of sound waves, frequency is also known as pitch.

Longitudinal Instensity

• The intensity of a wave is called amplitude. For a transverse wave, this represents the height of a crest. However, longitudinal waves do not have crests. Instead, the amplitude of a longitudinal wave is a measure of the intensity of the wave's energy. It measures how intense the compressions and rarefactions are. For sound waves, intensity is measured in decibels. The decibel scale is logarithmic, meaning a doubling in intensity, or loudness, is equal to an increase of 10 decibels.