What the heck is Resonance?

Let me start with the thought from a great man,

"When a mathematician engaged in investigating physical actions and results has arrived at his own conclusions, may they not be expressed in common language as fully, clearly, and definitely as in mathematical formulae? If so, would it not be a great boon to such as well to express them so, translating them out of their hieroglyphics that we might also work upon them by experiment?"
Michael Faraday

  

"Resonance" is an elusive concept to most Newbies and even many Pros too. They know the equations, they can count the effects, advantages, disadvantages etc with text book precision..., and yet...they dont "see" how it comes to exist! or How it manifests!

The Big fat Daddy Chicken is here, to finally shed some light on the topic for all of you unfortunate li'l chickies. Come follow me...Be enlightened!

One of the "Dictionary definitions" of the word "resonate" is

'To correspond closely or harmoniously'

. It might not make any sense to you right now, but keep it at the back of your mind. It will suddenly make sense, as I go about explaining the stuff.

Hmm....Lets forget about electronics for some time. Have you guys ever played on a swing? if you have not,...then stop right here and go get some life dude...OMG!

OK. Lets say you swing alone. So, basically you would SUSTAIN the swing action by pushing the ground forward when the swing reaches about the lowest point of motion (ie, closest to ground). Now, imagine this happening,...you are swinging really big, with short pushes at just the right moment. you should "feel" it to understand what I am about to say.

I said, you are pushing at the "right moment". what does that mean? Your periodic push should be at the same period as the swing (one push per swing action). More technically,

It should match the frequency of the swing. But, that is not enough!

If you have already started swinging somehow and now you are only bothered about MAINTAINING a certain swing movement (ie, you dont want to swing faster or slower), then you will need to push the ground at "specific intervals", with "just enough force", "at certain point of the swing motion". From your experience, you should know that the "specific interval" mentioned above corresponds to the period of the swing itself(one push per swing). "just enough force" is only to avoid the swing slowing down. "certain point in swing motion" is usually at the highest point while the swing is moving backwards, just when the swing slows to stop and starts to move forward again. This feels like a dance between you and the swing, both working "in resonance"...so to say.

Same is true in couple's ball room dance. The dance will be most effortless and "clean" (not messy), if both members move complementing each other. That is resonance between them.

Let me give you an even better example. Imagine that you have a flexible, springy stick,....perhaps something like an extended telescopic antenna or the top half of a professional fishing rod. Assume that you attached a small heavy object like a metal ball to the tip of the stick. We call the end with the ball as the "top" and the other end as the "bottom". if you were to grab the bottom with your palm, hold it upright and shake your fist in small twisting motion, then the ball would swing from side to side.

Now, lets say you start shaking your fist at low frequency, like may be 1Hz. That is a rate of 1 shake per second. Then as time goes by, you increase the frequency slowly. The amount of swing of the ball will keep changing with your shake frequency. As you increase the frequency, you will see that at some particular shake frequency, suddenly, shaking becomes very easy to you (less stress felt on your arms), with smallest shake, the ball makes the biggest swing! This is when we say that the whole system (your fist and the stick+ball) is in resonance with each other. if you increase the frequency of shake further, you will feel increased stress in your arms, the swing of the ball will be smaller and will look "messy". So, the system is not in resonance anymore.

if you understand this concept and "feel" what resonance means, you will start seeing resonance in everything. if you disturb a system, it will oscillate at it's resonant frequency. The frequency at which a guitar string vibrates when it is plucked is it's resonant frequency because it likes to vibrate in the easiest way (remember, the easiest frequency to vibrate is called it's resonant frequency or natural frequency). When you hit the wall with your head, you hear a "THUD", which is the wall vibrating at it's resonance frequency and you head vibrating in it's resonant frequency. Now, don't try that at home. When you blow into a blow-whistle, you are disturbing the air inside the whistle (and disturbances in air which fall between 20Hz and 20Khz is called audible sound), the air column vibrates at the resonant frequency decided by the shape and size of the whistle cavity. The disturbances bouncing back and forth within the cavity adds up into the loud whistling sound. When you change the fingering patterns while playing flute, you are actually changing the cavity shape(or size), thereby changing it's resonant frequency (that is the note you are playing). The story continues..., try to find more examples from your environment.

There is a story of a bridge which collapsed because an army troop marched through at the bridge's resonant frequency. When the army marches, all the members they stamp on the ground (bridge) at the same time, repeating periodically. At every stamp, the bridge vibrates(ie, moves up and down) and if the next stamp comes at the right moment (when the bridge is moving down), then the force will add to the vibration of the bridge. Over many cycles, it will add up over and over to make the bridge vibrate violently and fall apart eventually. This is supposed to be a myth, but interesting enough to understand and remember "resonance".

LETS MAKE A MECHANICAL FREQUENCY DETECTOR! using the flexible springy stick with ball, we could make a frequency detector. Think about it yourself and try to come up with a design of your own. This will help you understand the concept irrespective of wether you actually manage to make a design or not. The journey is the reward! I will explain how I would make one.

I would make a number of different sticks with balls, each of a different size (ie, different resonant frequency, because it depends on the length, springiness and weight of the ball). now if, you shake your hands at an unknown frequency, I would give you each stick, one by one and note the amount and ease of "swing". The stick, whose resonant frequency is closest to your shake can be easily identified this way. So, I could estimate your shake frequency to be very close to the resonant frequency of that particular stick! Does that sound dumb??..hehe..I was just trying to make it simple. A more elegant design is shown in picture below.

All sticks are attached at their bottom to one "roller stick". Then you are asked to twist the roller stick at unknown frequency. All the sticks will swing together now, and the one which resonates to your shake will stand out from others. It's movement will look very different.

If you got interested enough to read more, then grab a copy of "Fundamentals of Physics" by Resnick, Halliday and Walker and chew on it. Yet another of my recommendation for general Physics is "The Feynman Lectures on Physics" by Richard Feynman. This guy is a Nobel winner and he explains complex concepts in a "for Chicken-Brains" style....very readable, simple and enjoyable. You may find copies of these books at Amazon.com, barnesandnoble.com or may be even in 'books.google.com'.

 

OK. Enough blah blah. Lets come back to electronics. why is resonance interesting electronics? How does it occur? Why should you bother to understand it? Hmm...

Read "Resonance, Tanks and Circuits" to know the answers.

Don't forget to leave your comments and ratings before checking out the books because it would encourage me to continue writing. Tell me if it is dumb, good, silly, enlightening, whatever you think about it. if you have suggestions to improve the material, am open....Just tell me what.