How to Define Mechanical Waves?

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Originally published Jun 26, 2017, updated Feb 20, 2021
How to Define Mechanical Waves?

Mechanical waves are the type of waves that involve the oscillation of matter. They require a material medium for propagation and thus, transfer most of their energy through this medium. While these waves cover very large distances, the movement of the medium, on the other hand, is very limited and is sometimes almost immobile. Thus, the medium of propagation hardly seems to move from its initial position of equilibrium. Mechanical waves carry bundles of energy as they move. The direction of this energy is in the direction of wave propagation. Mechanical waves can only be produced and propagated along media that have inertia and elasticity. In contrast to this, electromagnetic waves do not need any medium for propagation and can even travel in a vacuum.

Properties of Mechanical Waves

Mechanical waves make use of initial input energy to begin their propagation and once this energy starts to operate, the waves travel through the entire medium until all of its energy gets exhausted. Another important property of this wave is the unusual way its amplitude is measured, that is, displacement has to be divided by its reduced wavelength. When this result approaches unity, disturbances due to non-linear effects may be significantly visible. A few examples of mechanical waves include sound waves, water waves and seismic waves.

While talking about waves, it is important that we know a few important terms related to it. For example, the frequency of a wave motion is the number of full cycles the wave completes in a given time. Its unit of measurement is Hertz written as Hz and it is the inverse relation of time. Similarly, the wavelength of a wave motion is the maximum distance between one point on the wave and another similar point on the next cycle of the wave. Its unit of measurement is called metres. Another very common term used to describe waves is its amplitude. The amplitude of the wave is the maximum displacement it is capable of undergoing when displaced from its initial fixed position and is also measured in metres. The frequency and wavelength of a wave thus, follow an inverse relation which means that increasing the frequency of the wave will lead to a decrease in its wavelength and vice versa. The formula to calculate the speed of a wave is as follows:

V= velocity of the wave;

W= wavelength;

F= frequency;

So, V= W X F;

Types of Mechanical Waves

Mechanical waves are basically of three types, that is, longitudinal, transverse and surface waves. Let us describe each of them in details.

Longitudinal waves are those types of waves in which the vibrations in the medium always move parallel to the direction in which the wave propagation goes. It is composed of compressions and rarefactions. While a compression can be defined as the closest distance in the wave, a rarefaction, on the other hand, is the farthest distance covered in a longitudinal wave. Sound waves are an example of longitudinal waves as they vibrate parallel to the direction of motion of the wave. As the index of refraction in a longitudinal waves increases, so does the speed of a wave, due to a much closer proximity of all the atoms that get compressed in the medium.

Transverse waves are the type of mechanical waves in which the vibrations are perpendicular to the direction of wave travel. Just like longitudinal waves consist of rarefactions and compressions, similarly transverse waves also comprise of crest and trough. A crest is considered to be the highest point to which the wave moves while a trough is the lowest point in a wave motion. The distance concerning two successive crests or two successive troughs is the measure of the wavelength which also means that the distance between a crest and a trough will be half its wavelength. To see a transverse wave, move an end of a string and notice a wave being generated at ninety degrees to the direction of wave motion.

Another type of mechanical waves are the surface waves. These ways travel along a surface that has two different media in between. They are further subdivided as Love waves and the Rayleigh waves. Rayleigh waves are basically a type of ground roll that travels in the form of ripples on the surface of the medium and are much slower in speed compared to any other type of wave. In fact, Rayleigh waves have as much as 80 percent of the velocity of other waves for a continuous elastic motion. On the other hand, Love waves are a type of surface waves that are transverse in nature. They are the waves that have the largest amplitude and also travel at a speed slightly greater than those of Rayleigh waves. To see surface waves, we can simply look at the ripples in the pond or a swimming pool.

Mechanical Waves vs. Electromagnetic Waves

The most common way to categorize waves is by their ability to transfer energy through empty spaces or vacuum. Based on this, the waves are classified as electromagnetic waves and mechanical waves. Electromagnetic waves are those types of waves that get produced by the motion of charged particles and can transfer their energy even through empty spaces called vacuum. Electromagnetic waves get produced when charged particles near the Sun cause vibrations and subsequently begin their journey towards the Earth by using vacuum as their medium of propagation. Had electromagnetic waves not travelled through vacuum and reached the Earth, there would have been no possibility of life on earth. Examples of electromagnetic waves include light waves which make life possible on this planet. On the other side, mechanical waves are the opposite of electromagnetic waves. They cannot transmit their energy through a vacuum and need a definite medium for their propagation. They use the medium to transfer their energy from one place to the other. The most common type of mechanical wave is sound waves. Thus, sound waves cannot travel in a vacuum and this is the reason an astronaut is unable to communicate in space as the vacuum-filled space does not allow sound waves to travel. Therefore to sum up, sound waves belong to the category of mechanical waves while light waves belong to the category of electromagnetic waves.

Sound Waves as a Type of Mechanical Waves

Sound waves are everywhere around us. The music that we hear and the guitar that we play or the noises that we make are all a part of sound. They can cover long distances based on the material they are travelling through and the intensity of vibrations produced by them. The duration and amplitude of the waves also depend on the amount of energy they are capable of transferring in their respective medium. The medium of propagation for these waves can either be air or water. In fact, for any wave to generate, the first necessity is a medium that is capable of producing disturbances at different places. A medium is nothing, but a series or continuation of interconnected particles that interact with one another. The second most important thing for wave generation is a vibrating object that can disturb the very first particle in the medium. These disturbances can be easily produced by using a vibrating string or a tuning fork. Finally, the wave gets generated when the sound waves push a particle in the air and disturb it from their equilibrium position, which thereby exerts another force on its neighbor as the result of a push or a pull and allows them to get displaced from their initial fixed position of equilibrium.

Rayleigh Waves as a Type of Mechanical Waves

Rayleigh surface waves are the type of mechanical waves that consist of both longitudinal and transverse motions. It is most commonly found in solids and move in elliptical orbits wherein the major axis of the ellipse formed is at right angles to the surface of the solid. This means that the greater the depth of the solid surface more is the width of the elliptical path covered by the wave. They are different from water waves in the way that the water waves have all its particles moving in a clockwise direction while the Rayleigh waves trace the path in an anticlockwise manner. However, particles with a depth greater than one-fifth of its wavelength trace a clockwise direction of motion. Rayleigh waves are the ones that are responsible for causing earthquakes as they travel at slower velocities, but with a very large amplitude. They cause side-to-side and up-down motions during earthquakes which cause great destruction in the particular area leading to a severe loss of life and property.

Max Malak
Academic and Business Consultant

You can find me reading Hemingway in the far south of Spain. Fueled by caffeine.