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Resonant Frequency

  1. Mar 21, 2005 #1

    I have a question on resonant frequencies. Everyone knows the example of breaking a wine glass by singing an extended note at the resonant frequency of that glass. My question is:

    Can other objects be "broken" by this same process? For example, is it possible to produce the resonant frequency of a wall at such a high amplitude that it actually breaks the wall down, theoretically?
  2. jcsd
  3. Mar 21, 2005 #2
    When you send energy toward something like this at it's resonant frequency
    the amplitude of the vibration you set up in the wall is limited by the
    rate of energy absorption in the wall. That is, if the wall is "gooey" then
    instead of vibrating it hard, you warm it up instead. In order to break it,
    you have to make it vibrate hard enough that cracks form. That means directing enough energy at it to overcome whatever it absorbs and still
    make it vibrate hard enough to crack.

    This concept of losses and how much energy something will absorb is
    called Q for quality factor in engineering. A glass has a potentially very high
    Q because glass makes an excellent spring (low mechanical losses) so
    that a small power can be built into a large intensity in the glass over time.
    This trick doesn't work with plastic because plastics make bad springs.

    With raw intensity you can break down any wall. But a perfect crystaline
    wall could be broken with very small amounts of power like a human voice
    if enough energy can be transferred to it over time.
    Last edited: Mar 21, 2005
  4. Mar 21, 2005 #3
    What are typical Q values for walls? I would think that they would most likely not be "gooey". How much energy are we talking about here? Say I wanted to break down the walls of Jericho like in the Bible. Using what we know about resonant frequencies, would it be possible to break down a wall of that size?
  5. Mar 21, 2005 #4
    that is very much possible :

    The frequence of the waves in the air has to match the eigen frequence of the material out of which the bridge is built.

  6. Mar 23, 2005 #5
    It is indeed quite possible. The worst-hit buildings in earthquakes are very often ones which had the misfortune to match a resonant frequency matching the frequency of the quake, for instance.

    However, the resonant frequency of an object can be changed by non-elastic deformations - so if you're applying a steady frequency to a wall, you'll only keep producing ever-larger vibrations as long as you haven't done enough damage to make it resonate at a different frequency (or stop oscillating at all in the mode you're exciting).
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