Longitudinal Waves

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Created by: Team Physics - Examples.com, Last Updated: July 9, 2024

Longitudinal Waves

Longitudinal waves are a type of mechanical wave in which the displacement of the medium is parallel to the direction of the wave’s propagation. These waves consist of alternating compressions and rarefactions, where particles of the medium move back and forth in the same direction as the wave travels. Common examples of longitudinal waves include sound waves in air, where the vibrations of air molecules create regions of higher and lower pressure that move through the medium. This type of wave is essential in various fields, including acoustics, seismology, and communication technologies.

What is Longitudinal Wave?

A longitudinal wave is a type of wave where the particles of the medium move parallel to the direction of the wave’s travel. Examples include sound waves in air and seismic P-waves. These waves create regions of compression and rarefaction as they propagate through a medium.


Longitudinal Wave Formula

The fundamental formula for a longitudinal wave is given by:

y( x, t) = y₀​ cos ( kx−ωt+ϕ )


  • y( x, t) is the displacement of the medium at position xxx and time ttt
  • y₀​ is the amplitude of the wave
  • k is the wave number, defined as k=2π​/λ with λ being the wavelength
  • ω is the angular frequency, defined as ω=2πf with f being the frequency
  • ϕ is the phase constant

Examples of Longitudinal Waves

  1. Sound Waves:
    • These waves travel through air or other media by compressing and rarefying particles in the same direction as the wave moves.
    • Example: A person’s voice travels as sound waves when they speak.
  2. Seismic P-Waves:
    • Primary waves are the fastest type of seismic waves produced by earthquakes and travel through the Earth’s interior.
    • Example: These waves are the first to be detected by seismographs during an earthquake.
  3. Ultrasound Waves:
    • High-frequency sound waves used in medical imaging to create images of the inside of the body.
    • Example: Ultrasound is used to monitor the health of a fetus during pregnancy.
  4. Pressure Waves in Fluids:
    • Waves that propagate through fluids (liquids and gases) due to changes in pressure and density.
    • Example: Shock waves from an explosion traveling through the air or water.
  5. Sound Waves in Solids:
    • Similar to sound waves in air, but these waves travel through solid materials.
    • Example: Vibrations from a tuning fork moving through a metal rod.
  6. Slinky Toy Waves:
    • Demonstrates longitudinal waves by compressing and releasing coils in a slinky.
    • Example: When one end of a slinky is pushed and pulled along its length, compressions and rarefactions move through the slinky.
  7. Compression Waves in Springs:
    • Waves in a compressed spring where coils move parallel to the wave direction.
    • Example: A wave traveling through a compressed and released spring toy.
  8. Acoustic Waves in Liquids:
    • Sound waves that travel through liquids by compressing and decompressing the medium.
    • Example: Underwater sound waves used in sonar technology.
  9. Pulses in Air Columns:
    • Sound waves traveling through air columns, such as in musical instruments.
    • Example: The sound produced in a flute or a pipe organ.
  10. Shock Waves in Gases:
    • Intense pressure waves that travel through gases at high speeds.
    • Example: The sonic boom created by an aircraft breaking the sound barrier.

Examples of Longitudinal Waves in Real Life

Examples of Longitudinal Waves in Real Life
  1. Musical Instruments: In wind instruments like flutes and trumpets, sound is produced by vibrating air columns. These vibrations create longitudinal waves that travel through the instrument and produce musical notes.
  2. Railway Tracks: When a train moves on railway tracks, vibrations travel along the tracks. These vibrations are longitudinal waves that help signal the approach of the train.
  3. Heartbeats Detected by Stethoscopes: A doctor uses a stethoscope to listen to the heartbeat, which is a longitudinal wave traveling through the chest cavity as the heart pumps blood.
  4. Shock Waves from Thunder: The sound of thunder is a longitudinal wave created by the rapid expansion of air following a lightning strike, causing compressions and rarefactions in the atmosphere.
  5. Speaker Vibrations: When a speaker emits sound, its diaphragm moves back and forth, creating longitudinal waves in the air that we perceive as music or voices.
  6. Airplane Sonic Booms: When an airplane travels faster than the speed of sound, it creates a sonic boom, a powerful longitudinal wave that moves through the air.
  7. Earthquake Early Warning Systems: These systems detect the initial P-waves of an earthquake, providing early warnings before the more destructive S-waves and surface waves arrive.
  8. Echoes in Caves: When you shout in a cave, the sound waves travel through the air, reflect off the walls, and return as an echo. These reflected waves are longitudinal waves.
  9. Pressure Waves in Blood Vessels: As the heart pumps blood, pressure waves move through the arteries. These waves help propel the blood through the circulatory system.
  10. Vibrations in Engine Parts: In a car engine, the rapid movements of pistons create pressure waves that travel through the engine components, aiding in the combustion process.
  11. Acoustic Waves in Underwater Communication: Submarines and underwater robots use acoustic waves to communicate. These longitudinal waves travel through water, allowing messages to be transmitted over long distances.
  12. Vibrations in Skyscrapers: Tall buildings experience longitudinal vibrations due to wind and seismic activity. Engineers design these structures to withstand such forces.
  13. Communication in Drilling Operations: In oil drilling, longitudinal waves are used to transmit data from the drill bit to the surface. These waves carry information about the geological formations encountered.
  14. Vibration Alerts in Mobile Phones: When you receive a call or message, your phone vibrates due to a small motor inside. This motor creates longitudinal waves that propagate through the phone’s casing, alerting you through tactile feedback.
  15. Ultrasonic Cleaning: Ultrasonic cleaners use high-frequency longitudinal waves to clean delicate items such as jewelry, medical instruments, and electronic components. These waves create microscopic bubbles in a cleaning solution, which implode and dislodge dirt and contaminants from the surfaces of the items being cleaned.

Examples of Longitudinal Sound Waves

  1. Public Address Systems: When announcements are made over a public address system in places like schools, airports, or stadiums, the sound waves travel through the air as longitudinal waves, reaching large audiences.
  2. Radio and TV Broadcasting: Although the signals are transmitted as electromagnetic waves, when they are received by radios or televisions, they are converted back into sound waves that travel through the air as longitudinal waves.
  3. Whales’ Songs: Whales communicate over long distances in the ocean using powerful vocalizations. These sounds travel through water as longitudinal waves, allowing whales to communicate with each other even when they are miles apart.
  4. Speech Recognition Systems: Devices like smart speakers and voice-activated assistants use microphones to detect sound waves produced by human speech. These longitudinal waves are then processed to understand and respond to voice commands.
  5. Theater Surround Sound Systems: In movie theaters, surround sound systems create an immersive a experience by directing sound waves throughout the room. These sound waves are longitudinal and move through the air to reach the audience from different directions.
  6. Echolocation in Bats: Bats emit high-frequency sound waves that bounce off objects and return to them. This process, known as echolocation, involves longitudinal sound waves traveling through the air, helping bats navigate and hunt in the dark.
  7. Drums and Percussion Instruments: When a drum is struck, the membrane vibrates and creates sound waves that travel through the air as longitudinal waves, producing rhythmic sounds.
  8. Hearing in Humans and Animals: Sound waves enter the ear canal, causing the eardrum to vibrate. These vibrations, which are longitudinal waves, are then transmitted through the ear’s structures to be interpreted as sound by the brain.
  9. Home Audio Systems: Speakers in home a systems produce sound by creating longitudinal waves that travel through the air, allowing listeners to enjoy music, movies, and other a content.
  10. Birdsong: Birds produce songs by vibrating their syrinx, creating sound waves that travel through the air as longitudinal waves, which are used for communication, mating calls, and territory defense.

Characteristics of Longitudinal Waves

  1. Particle Motion Parallel to Wave Direction: In longitudinal waves, the particles of the medium move back and forth in the same direction as the wave travels. This motion creates areas of compression and rarefaction.
  2. Compressional and Rarefactional Phases: Longitudinal waves consist of alternating regions of compression, where particles are closer together, and rarefaction, where particles are spread apart. This is different from transverse waves, which have crests and troughs.
  3. Wavelength: The wavelength (λ\lambdaλ) of a longitudinal wave is the distance between successive compressions or rarefactions. It is a crucial parameter that determines the wave’s properties and behavior.
  4. Frequency: The frequency (fff) of a longitudinal wave is the number of compressions or rarefactions that pass a point in one second. It is measured in Hertz (Hz).
  5. Amplitude: The amplitude of a longitudinal wave is the maximum displacement of particles from their rest position. Higher amplitude means more energy is being transferred by the wave.
  6. Speed: The speed (vvv) of a longitudinal wave depends on the medium through which it is traveling. It is determined by the properties of the medium, such as its density and elasticity. The speed can be calculated using the formula v=fλv = f \lambdav=fλ.
  7. Medium Requirement: Longitudinal waves require a medium (solid, liquid, or gas) to propagate. They cannot travel through a vacuum because they rely on particle interaction to transmit energy.
  8. Energy Transfer: Longitudinal waves transfer energy through the medium by causing the particles to oscillate. The energy is transferred in the direction of the wave’s propagation.
  9. Pressure Variations: Longitudinal waves create pressure variations in the medium. Compressions correspond to regions of high pressure, while rarefactions correspond to regions of low pressure.

Difference Between Longitudinal And Transverse Wave

CharacteristicLongitudinal WavesTransverse Waves
Particle MotionParallel to wave directionPerpendicular to wave direction
Wave PropagationCompressions and rarefactionsCrests and troughs
Medium RequirementRequires a medium (solid, liquid, or gas)Can travel through a medium or a vacuum
ExamplesSound waves, seismic P-waves, ultrasoundLight waves, water waves, electromagnetic waves
Wave FeaturesCompressions (high pressure) and rarefactions (low pressure)Crests (high points) and troughs (low points)
PolarizationCannot be polarizedCan be polarized
Energy TransmissionEnergy travels in the same direction as the waveEnergy travels perpendicular to the wave direction
Equation of Motiony( x, t) = y₀​ cos ( kx−ωt+ϕ )y( x, t) = y₀​ sin ( kx−ωt+ϕ )
WavelengthDistance between successive compressions or rarefactionsDistance between successive crests or troughs
Practical ApplicationsMedical ultrasound, acoustic communicationPolarized sunglasses, radio waves, X-rays


How do longitudinal waves differ from transverse waves?

In longitudinal waves, particles move parallel to wave direction. In transverse waves, particles move perpendicular to wave direction.

What are compressions in longitudinal waves?

Compressions are regions where particles are closest together, resulting from the wave’s compressive force.

What are rarefactions in longitudinal waves?

Rarefactions are regions where particles are spread apart, resulting from the wave’s expansive force.

How are longitudinal waves produced?

Longitudinal waves are produced by vibrating objects that create compressions and rarefactions in the medium.

Can longitudinal waves travel through a vacuum?

No, longitudinal waves require a medium (solid, liquid, or gas) to propagate, unlike electromagnetic waves.

What is the wavelength of a longitudinal wave?

The wavelength is the distance between two consecutive compressions or rarefactions in the wave.

What is the frequency of a longitudinal wave?

Frequency is the number of compressions or rarefactions that pass a point per second, measured in Hertz (Hz).

What is the amplitude of a longitudinal wave?

Amplitude is the maximum displacement of particles from their equilibrium position, indicating the wave’s energy.

How does the speed of longitudinal waves vary in different media?

Longitudinal wave speed depends on the medium’s density and elasticity, traveling fastest in solids, slower in liquids, and slowest in gases.

Why are sound waves considered longitudinal waves?

An example of a liquid is water. It flows and takes the shape of its container.

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