What Are the Causes of Longitudinal Waves?

Types of Longitudinal Waves

Longitudinal waves are pressure, or compression, waves. The most common type of longitudinal waves is sound. For example, when a storm produces thunder, the sound wave rattles windows. When sound waves strike the human ear, they produce pressure variations that cause the parts of the inner ear to vibrate. The longitudinal wave from an explosion can compress internal tissues and organs, potentially leading to death. The most powerful longitudinal waves are seismic waves, known as P waves. These seismic pressure waves are caused by earthquakes.

Waves represent a periodic disturbance within a medium that transmits energy from the source to another point. In longitudinal waves, this disturbance displaces particles of the medium in a direction parallel to the direction of the wave, called the direction of propagation. This displacement produces a sequence of compressions, called condensations, and decompressions, called rarefactions. These pressure waves represent a change in volume. When they strike an object, they cause it to vibrate back in forth in the same direction as the wave is moving.

Longitudinal waves are described in terms of three primary properties: wavelength, frequency and amplitude. The wavelength of a longitudinal wave typically represents the distance from one compression to the next. However, the wavelength will be the same from any point along the wave to the corresponding point on the next wave. The frequency of a longitudinal wave represents the number of cycles, or compressions, within a given period of time. The typical period of time for measurements is one second. In this case, the number of cycles in one second is represented in Hertz. The amplitude of a longitudinal wave represents the change in pressure from the high-pressure condensations to the low-pressure rarefactions. Sound amplitude is measured in decibels.

Longitudinal seismic waves are called P waves, or primary waves. These are the fastest of the seismic waves, arriving first after an earthquake. These longitudinal waves are body waves, meaning they flow through the interior of the planet. As compression, or pressure, waves, longitudinal seismic waves can flow through both solids and fluids. Because these waves travel so quickly -- approximately 225 miles per hour through solid crust -- they typically pass rapidly and cause little damage. In an earthquake, it is the slower surface waves that cause the bulk of the damage.