Resonance - Its Causes and Importance. What is Resonance?

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Let me begin with some examples of resonances which we come across in our day-to-day life. This will help us in understanding why it happens.

  1. We all travel in buses, cars and many other means of transport, preferably think of an old one. You might have felt that when the bus or car starts to accelerate from stand still condition, you feel that at a particular range speed of the bus it seems as if the whole bus is rattling and jolting a lot and you would feel that the whole bus would come down on you. But after the bus has passed through the acceleration phase for a few seconds it seems as if nothing has happened to the bus. This sudden rattling and jolting of the bus during a particular range of speed of the bus indicates “Resonance”.
  2. While most of us would have visited an ENT specialist at least once in a life time and you might have never missed the opportunity of the tuning fork being used on you. This too works on the same principle where in you induce the resonance in the structure of the component by hitting it against a table or wall, making it to vibrate continuously and produce an audible humming noise.

I hope these two examples would have really indicated to you how “Resonance” feels like. Now why does this phenomenon happen?

Eery object in this world tends to have a natural frequency. When I say “Natural frequency”, even if you are novice to the field of vibration, just think of it as a number with a unit of Hertz(Hz) e.g. say 50 Hz.

Just a small formula to say what Hz is for a rotating body, Hertz = speed in rpm/60

Each object tends to have a natural frequency like this which could be anything from 0.1 to infinity. Further each object tends to have a number of natural frequencies like first natural frequency, second natural frequency and so on. This natural frequency is largely dependant on the following characteristics of the object:

  1. Shape of the Object
  2. Material property of the object
  3. Degrees of freedom in the object basically this relates to the supports to the object
  4. Medium in which the object is in like air, water, etc.

Based on the above criterions, we can mathematically calculate the natural frequency of the object.

You may wonder what does the natural frequency of an object have to do with the resonance in the object?

When the object is made to approach or work in its natural frequency, say for example if the motor casing has a first natural frequency of 25 Hz and the motor is to run at 1500 rpm (i.e. 25 Hz) the casing will start to vibrate in a very excited manner, with no control on the amplitude of vibration. This is just like excitement what we get in the stomach when we reach the topmost point in the giant wheel. Feel as if this excitement will never stop. This uncontrollable “excitement” in the motor is “Resonance”.

Well just think about what this resonance can cause to the motor. The motor will start to jump, rattle, jolt and will do whatever thing possible leading to the motor’s breakage and ultimate failure. This is the reason why resonance is a very important term in the field of vibration and in the day to day articles we use.

Every component we use has been designed in such a way that, they operate a lot away from their natural frequency or “natural excitement” range.