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Longitudinal Waves - Examples, Diagram, FAQs

Longitudinal Waves - Examples, Diagram, FAQs

Team Careers360Updated on 02 Jul 2025, 04:45 PM IST

Longitudinal waves belong to a class of mechanical waves, wherein particle displacement takes place in the same direction as the propagating wave. These waves move through compression and expansion as seen in the case of air, liquids, and even solids. These types of waves include sound waves which therefore are important in several natural and technological applications.

Define Longitudinal Wave

Waves can be classified into two types based on the direction of particles relative to the direction of the wave travel. They are transverse wave and longitudinal wave

The wave illustration of the two types of waves can be observed in the figure below.

Wave illustration of longitudinal and transverse waves

Figure 1 Wave illustration of longitudinal and transverse waves

A wave consisting of a periodic vibration travelling in the same direction as the direction that the wave travels is called a longitudinal wave. From the above figure, the longitudinal wave is a wave in which particle displacement is parallel to the direction of wave propagation.

A wave in which the particle displacement is perpendicular to the direction of wave propagation is called a transverse wave.
Also, read

What is Longitudinal Vibration?

If the direction of the particles present in the shaft moves parallel to the axis of the shaft then it is called longitudinal vibration.

Explain Longitudinal Waves With Diagrams

In Figure 2, a longitudinal wave in the air is represented graphically. The longitudinal wave image below is a graph of distance from the source versus the density of air. Particles are denser at a particular region on the curve. It is called the compression region and it is present on the top of the curve. The region where particles are less crowded is called rarefaction. It is present at the bottom of the curve. In the compression region, particles are close to each other while they part away in rarefaction. Peaks represent maximum compression and rarefaction.

A longitudinal wave in the air

Figure 2 A longitudinal wave in the air

The Velocity of Longitudinal Wave

Longitudinal waves transmit through media with some velocities. The velocity of the wave depends on the elasticity and density of the substance.

In solids, the velocity of a longitudinal wave is given by,

$v=\sqrt{\frac{E}{\rho}}$

Here, E is Young’s modulus and ρ is the density of the substance.

In liquids, the velocity of a longitudinal wave is given by,

$v=\sqrt{\frac{B}{\rho}}$

Here, B is the bulk modulus and ρ is the density of the fluid.

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Examples of Longitudinal Waves

The longitudinal wave includes examples such as sound waves, seismic P-waves, ultrasound waves, etc.

The Wavelength of Longitudinal Wave

The distance between two successive compressions or between two successive rarefactions is called the wavelength of a longitudinal wave. It estimates the size of waves.

Sound wave

Figure 3 Sound wave

The above figure represents a sound wave which is also a longitudinal wave. The wavelength, amplitude, and velocity of the longitudinal wave are shown in the diagram.

Amplitude: When the wave compresses particles in the medium, the distance between those particles is called the amplitude of the wave. It can also be stated as the distance between the equilibrium position of the medium and the compression or rarefaction. Waves produced by greater energy of disturbance produce greater amplitude.

Frequency: Frequency is the number of wavelengths per second. Frequency remains constant regardless of the change in speed.

Sound Wave

If the particles are denser at a particular region in the sound wave, then it is compression in the sound wave. If those particles are separated then it is called rarefaction in the sound wave.

The motion of a sound component is measured from its equilibrium position as it produces sound waves. The distance measured is called displacement in a sound wave. The displacement and pressure in a sound wave are interrelated. Pressure in a sound wave is either maximum or minimum when its displacement is zero. If the neighbouring particles are nearer to the point then maximum pressure is observed. So, minimum pressure is observed when neighbouring particles are away from the point.

Difference Between Longitudinal Wave and Transverse Wave

Longitudinal and transverse waves are differed by the following:

Longitudinal waves
Transverse waves
A longitudinal wave is a wave in which particle displacement is parallel to the direction of wave propagation.
A transverse wave is a wave in which the particle displacement is perpendicular to the direction of wave propagation.
The graph of distance versus density represents a longitudinal wave.The Displacement-distance graph represents the transverse wave.
Longitudinal waves have compression and rarefaction.The transverse wave has a crest and trough.
Longitudinal waves travel through any media.Only through solid and liquid surface transverse wave travel.
Sound waves, ultrasound waves, tsunami waves, and seismic P waves are some of the examples of longitudinal waves.Examples of transverse waves are the vibration of the guitar string, seismic S-waves, electromagnetic waves, etc.


Frequently Asked Questions (FAQs)

Q: Can longitudinal waves be used in quantum technologies?
A:
Yes, longitudinal waves, particularly in the form of phonons (quantized sound waves), play a role in quantum technologies. For example, acoustic waves are being explored for quantum information processing and as a means to control and manipulate quantum states in solid-state systems.
Q: Can longitudinal waves be used to study the Earth's core?
A:
Yes, longitudinal waves (P-waves) are crucial in studying the Earth's core. These waves can travel through both solid and liquid materials, allowing seismologists to probe the structure of the Earth's interior, including the liquid outer core and solid inner core.
Q: How do longitudinal waves contribute to the functioning of musical wind instruments?
A:
In wind instruments, longitudinal waves are created by vibrating air columns. The player's breath sets up standing longitudinal waves within the instrument's tube. The frequency of these waves, determined by the tube's length and any open holes, produces the desired musical notes.
Q: What is the role of longitudinal waves in noise cancellation technology?
A:
Noise cancellation technology uses the principle of destructive interference of longitudinal sound waves. It generates sound waves that are out of phase with unwanted noise, effectively canceling it out. This requires precise analysis of the incoming sound waves and rapid generation of counteracting waves.
Q: How do longitudinal waves behave in metamaterials?
A:
In acoustic metamaterials, the propagation of longitudinal waves can be manipulated in ways not possible with natural materials. This can lead to phenomena like negative refraction, superlensing, and even acoustic cloaking, where sound waves are guided around an object making it "invisible" to sonar.
Q: Can longitudinal waves be used to study phase transitions in materials?
A:
Yes, longitudinal waves, particularly ultrasonic waves, can be used to study phase transitions in materials. Changes in the speed and attenuation of these waves can indicate transitions between different phases, providing insights into material properties and behavior under various conditions.
Q: How do longitudinal waves contribute to the phenomenon of cavitation?
A:
Cavitation occurs when rapid changes in pressure caused by intense longitudinal waves create vapor-filled cavities in a liquid. These cavities then collapse violently, potentially causing damage to nearby surfaces. This phenomenon is important in fields ranging from marine engineering to medical ultrasound.
Q: What is the relationship between longitudinal waves and infrasound?
A:
Infrasound refers to longitudinal sound waves with frequencies below the lower limit of human hearing (typically 20 Hz). These low-frequency waves can travel long distances and are used to detect phenomena like earthquakes, volcanic eruptions, and even nuclear tests.
Q: How do longitudinal waves behave in granular materials?
A:
In granular materials, the propagation of longitudinal waves is complex due to the discrete nature of the medium. Wave behavior depends on factors like particle size, shape, and packing density. Studying these waves can provide insights into soil mechanics, powder processing, and even planetary science.
Q: Can longitudinal waves be used to measure the properties of materials?
A:
Yes, longitudinal waves, particularly ultrasonic waves, are used to measure various properties of materials. Techniques like ultrasonic testing can determine a material's thickness, density, elastic modulus, and detect internal flaws or defects non-destructively.
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