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Resonant frequencies

  1. Nov 29, 2005 #1
    Hi. I'm having some trouble understanding resonance. I was reading about how ears work on howstuffworks.com, and it noted that the ear picks up different frequencies basically because little hairs inside the ear resonate at different freqencies. It said that this is also the concept behind tuning forks.

    I don't understand why though. Why does a tuning fork repeat a sound at a certain frequency?
     
  2. jcsd
  3. Nov 29, 2005 #2

    Chi Meson

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    Like any simple harmonic oscillator, there is a natural frequency. Think of pushing a kid on a swing; depending on the length of the swing, there is only one frequency that the swing will have. You have to push at exactly the same frequency in order to increase the amplitude of the swing.

    If you set up a large number of tuning forks, each of a different frequency, and then make a noise nearby, the frequency of the noise will act like pushes against the tuning forks. But only the tuning fork that matches the frequency of the noise will increase in amplitude and start ringing itself. For the others, its like pushing the swing at the wrong moment, or "out of phase."
     
  4. Nov 29, 2005 #3
    I know there is a natural frequency, I just don't understand why.

    If we are relating a swing to a tuning fork, how do you push at the wrong moment?

    And why does the fork with the right frequency start ringing? Where does it get this energy from? Why don't any of the other forks ring (other than they aren't the right frequency)? Is it simply that the sound wave only interferes constructively with the right tuning fork, (and possibly both constructively and destructively with the others)?
     
  5. Nov 30, 2005 #4
    You do this by hitting the tuning fork really hard, or not very hard at all, there will still be a sound but it won't be resonating (just like if you push someone on a swing really weakly or really hard, they wil lstill move by they won't move in the "resonating" fashion).

    Hmm, just re-read you question again, and although the above is correct it isn't exactly what you asked. If a tuning fork is already resonanting (like a swing is swinging nicely), you ask what is the analogy for pushing the swing at the wrong moment? Well you have to remember that a tuning fork generally resonates faster than a swing (think about ho the two prongs on the fork move back and forth when they are vibrating). This would make it difficult for you to say when it is a "bad" time to strike the fork again (its obvious in the swing case because its going slow enough for us to see whats going on).

    Indeed, I think if you start a tuning fork resonanting and then hit again you may destroy the resonance quickly, or it may only disturb it. You have some chance of actually re-enforcing the resonance of the fork also, but I'd imagine you'd have to hit it just right and be really lucky!
     
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