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What is sound?

  1. Feb 26, 2015 #1
    I know there is a previous post here about this, but it didn't seem to cover the angle I am getting at...

    I understand sound to be vibrations that travel through a medium such as air or water. However, what actually generates these vibrations? For example, hitting a piece of steel with a rod. Vibrations (sound waves) will be generated that travel through the air to my ear. If I do this in space, no sound waves will travel to my ear, however the initial act remains the same.

    Put another way, is a sound produced in a vacuum, but just not propogated?

    I assume without any basis for doing so that sound waves must have some sort of energy component. Where does that go if there is no medium to dissipate it??

    And what is it that actually generates the sound waves? What happens physically as the rod meets the iron?
  2. jcsd
  3. Feb 26, 2015 #2


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    Sound is produced in your ear.

    A vibrating source may or may not be adjacent to a medium through which the vibrations are transferred.

    It is propagated through any matter in contact with the steel. If you went ten yards down to the end of the steel beam and put your head against the steel you would hear sound.

    It simply stays in the steel and heats it up. This is why, in some cases, overheating is actially a problem in space. There is no convection or conduction to remove heat - only radiation.
    Last edited: Feb 26, 2015
  4. Feb 26, 2015 #3
    Sound in the air consists of a train of alternating closely spaced zones of high and low air pressure that travel from the source to your ear. These alternating high and low pressures cause your ear drum to vibrate back and forth. The signal from your ear drum is transmitted by nerves to your brain, which interprets them as sound. When you strike a bar, the bar starts to vibrate, and causes the air immediately adjacent to the bar to move back and forth, generating the alternating zones of high and low air pressure. In a vacuum, without the air to alternately compress and expand to form the train of alternating zones of high and low pressure, there is no such thing as sound.

  5. Feb 26, 2015 #4
    Thanks for these comments. The ear part I understand - that is the mechanism for us to hear a sound. But a sound exists regardless of whether we hear it. What I am more getting at is what Chestermiller said about the pressure waves. I'm not sure I am quite describing my question well enough!

    Take a sudden sharp sound, say the crack from a clap of the hands or the collision of two solid objects. From what you say, even though I hear it as a single sharp sound, it is still composed of a series of pressure waves just of very high frequency travelling through a medium.

    What causes the pressure waves though? Is there a physical description of the mechanics of generating those waves? If hands are clapped in space, the mechanism for creating the pressure waves must still be present, mustn't it? But no pressure waves are created. So if the mechanism is independent of the medium, is there a physical description of that mechanism? Or is this just a dumb or poorly framed question?

    Dave notes that the energy otherwise lost in producing sound remains in the object in space. Does that mean that a sound system speaker on the moon would heat up just from reproducing sound that can't be heard?
  6. Feb 26, 2015 #5
    As I said in my original post, the pressure waves are generated mechanically in the adjacent air by small scale vibrations of the body that causes the sound. It is also generated mechanically by the sudden large compression caused by your hands compressing air. This short burst of pressure variations travels through the air at the speed of sound to your ear. In the case of the collision of two large objects, the deforming objects mechanically generate both small scale and large scale expansions and compressions of the adjacent air. If you have a loud speaker, the speaker membrane vibrates back and forth and mechanically disturbs the adjacent air in the form of pressure waves.
  7. Feb 26, 2015 #6


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    Chestermiller covered it in his 2 posts ... but just in case its a mechanical vibration

  8. Feb 26, 2015 #7


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    The 'mechanism' is your hands disturbing the air, so no, it doesn't exist in space. You can't have pressure waves in a medium of air if you aren't in that medium.
  9. Feb 27, 2015 #8


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    If you supply an audio signal to the voice coil then it will vibrate more than it would in air. As there is less work done on the air, you could find that the amplifier is actually supplying less power. This sort of thing will also happen when you block the input to a fan and it races because it is not doing work on shifting the air. It ain't a simple question to answer.
    The coil may well get hotter without any air around to transfer the heat away.
  10. Feb 27, 2015 #9
    Thanks for these further clarifications. So sound is pressure waves propagated through a medium. I'm not sure I can quite visualise what 'pressure waves' are but that's OK, the idea is clear enough. If I clap my hands, the impact of hand on hand generates these pressure waves. If I strike a bar of steel in space, the pressure waves will travel through the steel but will not travel beyond that because there is no air.

    So 'sound' depends on a medium to carry the vibrations, and the pressure waves are energetic. So in a vacuum, it is as DaveC says above, the energy otherwise used to vibrate the air molecules has to be absorbed within the object and radiated off as heat? Any object generating sound in our atmosphere (eg a struck tuning fork) cannot bleed off energy as sound waves if in space, so it must heat up?
  11. Feb 27, 2015 #10


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    Have you ever played with a slinky spring? You can launch longitudinal waves along the spring and you can see the compressions travel along it. In the case of sound, the situation is very much the same.
    You seem to have the right idea about what happens to the energy. Most vibrators will vibrate for much longer in a vacuum because energy is being dissipated at a slower rate.
  12. Feb 27, 2015 #11


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    No, the effect of your hands on the surrounding air molecules generates the pressure waves. Crudely speaking, as your hands come together, they quickly squeeze the air between them. As the air rushes out of the space that that your hands are now filling, it disturbs the air surrounding your hand and triggers a brief burst of pressure waves.

    If you clap your hands in vacuum, there is no air to squeeze out.

    Your reasoning would apply better to something like a bell or a tuning fork. You strike it and set it vibrating. The vibrations continue in the object for some time after the striking, but you can hear them only if there is air or some other medium between the object and you.
  13. Feb 27, 2015 #12
    When the rods meet each other, the kinetic energies of the rods (due to its motion) have to become another form of energy due to the law conservation of energy.

    If this phenomenon takes place in a medium, this energy transforms into sounds waves and a little bit of heat due to the collision of the rods. As you know, sound waves are vibrations that need a medium to propagate. Since in this context there is a medium, we can hear the sound waves when it hits our eardrum.

    Now if this took place in vacuum, the kinetic energies of the rods will still be transformed into sound and heat. The only difference this time is, you'll not be able to hear it because vacuum is not a medium.

    Nevertheless, there is a conservation of energy :) Hope this cleared your doubt. If not, please do ask :)
  14. Feb 27, 2015 #13
    It's not sound in the sense that the OP understands sound. I think you meant to say "vibrations in the rods" rather than "sound."

  15. Feb 27, 2015 #14
    Oh yes, thankyou for the correction :)
  16. Feb 27, 2015 #15


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    I think of it as more than a semantic quibble to say, no, a VIBRATION exists regardless of whether or not a human is around to interpret it as sound. "Sound" is what we feel from our ear drums.

    I realize that one talks of "sound waves" as existing regardless of observation, so maybe I'm just back to the tree falling in the forest --- it just depends on your definition of sound.

    EDIT: this was just an expansion of Chester's post #13
  17. Feb 27, 2015 #16


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    I'm not sure the word "sound" has the same exclusively psychological connotations that "colour" and "music" have.
  18. Feb 28, 2015 #17
    Just as eyes are sensitive to light, the ear is sensitive to the "vibrations" in the medium. These vibrations are sensed by the eardrum, and we perceive it as sound. So, colour and sound are all our perception. On the other hand, vibrations and wavelengths are not.
  19. Feb 28, 2015 #18
    Materials are somewhat elastic. When you create a motion in some medium, there is a restoring force. This causes the material to bounce back and oscillate as a wave. The frequency of the wave depends on the restoring force and the mass density (more dense substances take more force to move, but often also have higher restoring forces). Since there is a frequency dependence to the restoring motion, the wave propagates in space. That is sound. It can't travel in a total vacuum, but it can travel through a solid that is surrounded by vacuum.
  20. Feb 28, 2015 #19
    Thanks, I think it is clear now. I did understand that sound only propagates in a medium. My question, poorly framed as it was, sought to firstly identify how sound waves are generated in the first place (explained as pressure waves) and secondly what happens in a vacuum to the energy that would otherwise be lost in generating the sound waves in an atmosphere (explained as heat and also through taking longer to stop vibrating).
  21. Feb 28, 2015 #20


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    We are having a diversion here, about semantics. Sound is vibration with a certain range of frequencies, detected by human ears. That's a fairly generally used definition and it is along the same lines as the definition of light. Otoh, colour is what our eyes make (subjectively) of various combinations of wavelengths of light. Music is, likewise, how our brains / ears assess various combinations of sequences of sound frequencies. To my mind, the two pairs of terms differ, significantly. We, or anyone who is bothered enough about it, should really be discussing this elsewhere, I think.
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