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A Sound Question


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Imagine a pendulum. You gently strike it so that it begins to swing. Very gently, so that you can say that it's vibrating.


Now, it will 'vibrate' at a particular fixed rate. It makes no difference how much the pendulum is displaced from the 'rest' position, it shall vibrate only with a fixed rate. This frequency (rate) by which it is vibrating can be said to be it's natural frequency. The time period of oscillation will be the pendulum's 'favorite' time period.


As time proceeds, it eventually stops vibrating, because of various factors. Air resistance, for example. You want it to vibrate continuously. So you bilndfold yourself (don't ask why, just do as I say) and begin giving the pendulum the gentle pushes after regular time intervals.


Obviously the pendulum will vibrate continuously. Soon you'll find that the pendulum begins to oscillate at somewhat the same rate as the force you give. If you give the force at regular intervals of 0.6 seconds, the pendulum will be reaching one particular exterme after intervals of 0.6 seconds.


Okay, remove your blindfold and look at the pendulum vibrating.


Abruptly, change the time period of your gentle force application.


What happens? For a few moments, the vibration is completely disrupted. For a moment, the pendulum will try to maintain it's 0.6 second oscillation time period, but the force you're applying after say 0.3 seconds is changing the oscillation rate wierdly. Soon, the pendulum will reach your 0.3 second oscillation time.


Now, try and apply the natural frequency in your force. Suppose you had seen the pendulum's oscillation time as 1 second, before you were applying the forces. Begin applying the force with that regular interval. What do you see? You've obviously reached the pendulum's favorite frequency. You're pushing it when it reaches an extreme, so that it's goes further away, and the 'extreme' becomes further away from the center position.


The pendulum's oscillation 'amplitude', is going to keep rising. It's going to rise quite a lot. In fact, no other rate of force application is going to get it so far away from the mean position.


You have reached a situation... known as resonance.


Now, when you're considering sound, The sound can be considered like the force you were applying on whatever object.

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To understand resonace you must first have a good idea of how waves can intefere and superimpose with each other. Basically that means when two waves overlap if the peaks and troughs are in phase with each other they constructivly interfere, making the wave amplitude larger. With resonace this happens repeditivly, so that the waves amplitude becomes much larger.

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