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Have you ever wondered how a string instrument can make different sounds when plucking different strings? Or how a singer could shatter a glass cup just by using their voice? Or how the Tacoma Narrows Bridge in Washington collapsed in 1940 despite being designed to hold winds whose speed is 193 kilometres per hour (Lamb, Morrissey and Kiger, 2021)? These all work on the principle of resonance.
But before that…
Before we get to understanding what resonance is, we need to first understand what goes on at a molecular level of an object: the particles of a solid object will vibrate at a certain number of times per second (what is known as frequency) at all times. This is what is known as the natural frequency of the object. As the particles vibrate, they will move about its position. As it moves from its original position to a certain distance away from its original position, this distance is known as amplitude.
Now, what exactly is resonance?
With the above understanding, let us go into the definition of resonance: a phenomenon that objects experience when the matching vibrations of another object increase the amplitude of an object’s oscillations (Byjus, n.d.). Basically, when there exists an external source that produces waves that has the same frequency as the natural frequency of some object and the waves hit the object, the particles in said object will move the largest distance away from its original position back to its original position, and this is what we call resonance.
Let us use the example of the glass shattering by a singer’s voice to illustrate this. The object in this case would be the glass. The singer would be the external source, where their voice would produce sound waves that will hit the glass. When these sound waves reach a certain frequency that is approximately equal to the natural frequency of the particles of the glass, these particles will oscillate with the highest amplitude. As this continues for a period of time, the internal structure of the glass will not be able to withstand the particles’ vibrations, which causes the glass shattering. This can also be known as destructive resonance.
Resonance can also be constructive, such as when someone is playing an instrument such as a guitar. When they pluck the string, the string is set into oscillation or vibration at the natural frequency of vibration of the instrument (Byjus, n.d.). This causes the string to oscillate and thus a sound is produced.
Graphically…
Below shows how resonance can be graphically represented on an amplitude-frequency graph (resonance is represented by the line labelled resonant frequency):
Note: the green dotted line represents the natural frequency of the object. The curves refer to what is happening in the object that is being affected by the external source of waves. The frequency axis represents the frequency of the waves from the external source, hence notice how as the frequency increases to the natural frequency, the amplitude of the oscillations of the particles increases to the maximum.
The graph also contains other curves with labels containing the word ‘damping’. Damping refers to the restraining of the vibratory motion (Britannica, 2024). If you are interested to find out more, you can watch this video (VROOK Learning, 2022)!
Works Cited
Britannica, The Editors of Encyclopaedia. (2024) damping. [online] Encyclopedia Britannica. Available at: https://www.britannica.com/science/damping. [Accessed 1 Nov. 2024.]
Byjus (n.d.). Resonance. [online] BYJUS. Available at: https://byjus.com/physics/resonance/. [Accessed 1 Nov. 2024]
Lamb, R., Morrissey, M. and Kiger, P.J. (2021). How Bridges Work. [online] HowStuffWorks. Available at: https://science.howstuffworks.com/engineering/civil/bridge10.htm [Accessed 1 Nov. 2024].
Sophie (2019). Damping and Resonance. [online] www.scienceandmathsrevision.co.uk. Available at: https://www.scienceandmathsrevision.co.uk/topic/damping-and-resonance/ [Accessed 1 Nov. 2024].
VROOK Learning (2022). What is damping? | Theory of damped oscillations with door closer example. [online] www.youtube.com. Available at: https://www.youtube.com/watch?v=N6wFJUitkG8 [Accessed 1 Nov. 2024].
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