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Hows sound generated?

  1. Jun 2, 2013 #1
    When a metal ball rolls on a metal surface, we can hear some noise. But when the metal ball rolls on a cloth, we cannot hear anything. Why is that?

    This question has bothered me a lot...
  2. jcsd
  3. Jun 2, 2013 #2

    Simon Bridge

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    The sound comes from the ball vibrating over different surfaces - the vibration will be different for different textures ... pebbly surfaces make a different noise to a steel grate for example. So it stands to reason that it should be different over something soft too, like felt.
    So what is the problem?

    When you walk on carpet, your steps sound different from when you walk on wood, or concrete, or a metal catwalk ... does that bother you the same?

    If you shout in a bare room, you get an echoey effect (very noticeable if you record it) but, if the room is carpeted and furnished, there is almost no echo, and if you line the walls and ceiling with soft material or foam rubber shapes then the echo can be eliminated completely. Why is this?

    Have a think about these things - what sort of things will affect how you hear sound waves - and you should be able to figure it out.
  4. Jun 2, 2013 #3
    The mass of atoms, their size, and how they are arranged determine the density of a substance. So the metal surface has a more compact molecular structure and density than the cloth. The structure affects density. The density affects has well vibrations (sound waves) radiate outward. The atoms in the metal are more densely packed and thus vibrate at a higher pitch (frequency) because they're banging into each other more often when struck, and also create a greater amplitude (low-high pressure oscillation) which is perceived by the human ear as louder. Rolling over cloth does the exact same thing, but cloth has a less dense atomic structure.
  5. Jun 3, 2013 #4

    Simon Bridge

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    There is also the role that "damping" plays isn't there?
    The atoms in metal are "banging into each other ... when struck"?
  6. Jun 3, 2013 #5
    Was referring to the natural frequency (pitch, note) of metals

    frequency = speed/wavelength

    At the particle level, a stiff or rigid material is characterized by atoms and/or molecules with strong attractions for each other. When a force is applied in an attempt to stretch or deform the material, its strong particle interactions prevent this deformation and help the material maintain its shape. In general, rigid materials such as steel (metal) have the strongest interactions between particles.

    If I'm correct, the force applied to the metal surface by the ball causes a vibration (wave) that propagates through the metal, passed from particle to particle SPEED-ily. Anytime we increase SPEED in our above equation we get a higher frequency (pitch or note).

    So, hopefully I got this concluded properly; Metal has a stronger partial attraction than cloth, which allows the sound wave to SPEED through faster, and in accordance with our frequency equation we get a higher note, usually more easily discerned by the human ear up to a certain point.
    Last edited: Jun 3, 2013
  7. Jun 3, 2013 #6
    In a way it is really counter intuitive that a ball doesn't make more noise on a piece of cloth than on a piece of metal. After all the friction on the cloth is much higher so there is more energy lost from the the rolling ball. The answer to this problem lies in the fact that the vibrations in the metal are weakly damped therefore any sound that is produced keeps bouncing around in the metal. This doesn't happen with the cloth, where the sound gets turned into heat after moving a small number of fibres. Then there is more involved stuff, because the rolling friction that causes the sound is due to deformations of the rolling ball and the surface, which look different for combinations of materials where one is hard and the other one soft or if both are hard, but the main reason is due to the microscopic damping which is much larger in the cloth.
  8. Jun 3, 2013 #7

    Simon Bridge

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    I was sort of hoping to get OP to think about it a bit first though :(
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