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Sound and Heat - Whats the difference

  1. Aug 30, 2009 #1
    Sound and Heat -- Whats the difference

    When we hear sound from a metal object ( like a tuning fork or a huge bell with gong) when they are striked by an external means we say that the atoms of the metal vibrate about their mean positions and so set the air around them also in compressive mode. This creates compressions and rarefactions in the air producing sound waves.

    Fine, this theory does make sense and we do hear sound due to the vibrations in the metal.( a bell, a tuning fork or any string instrument like guitar is a perfect example of this).


    Isnt it true that the atoms similarly vibrate around their mean positions when the metal is subjected to an external heat source. When we say that we supply heat to a metal the atoms absorb the heat energy and start to vibrate around their mean positions. And the amplitude and freqeuncy of the vibration are the indication of its thermal energy and tempreature.

    The point is that in both the cases ( that of heating and striking the metal with some external means to produce sound) both involve the same action ie the vibrations of atoms around their mean positions.

    1. Then what is the exact difference between these vibrations that we hear sound in one case and feel the heat in the other when in both cases the atoms do the same thing ie they vibrate.

    2. So why dont we hear sound from a iron bar which is being heated to red hot but still we dont hear any sound from it, though the atoms are vigorously vibrating.

    3. Similarly why dont we feel heat when we strike a tuning fork but hear sound alone, again the atoms only being vibrating.

    What is the differnce in the vibrations that differentiates between the production of Sound and heat despite the basic behaviour of the atoms being the same in both cases.

    What produces sound and what produces heat.
  2. jcsd
  3. Aug 30, 2009 #2


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    Re: Sound and Heat -- Whats the difference

    Hi Rohan. Because thermal vibrations are random with the atoms all more or less "doing there own thing" the amplitude is very small (approx 10^-11 m) and the frequency very large (10^12 Hz is a typical figure at room temperature though obviously it depends on the atomic mass).

    The bottom line is that human hearing extends to about 10^4 Hz so there is no chance of "hearing" 10^12 Hz.
    Last edited: Aug 30, 2009
  4. Aug 30, 2009 #3
    Re: Sound and Heat -- Whats the difference

    Does this imply that if we built a detector capable of 'hearing' 10^12 Hz it would detect hot objects? So there is another way of detecting something's heat other than detecting the IR photons it emits?

    Anyway thanks to the OP, this question was actually pretty interesting, PF is awesome :)
  5. Aug 30, 2009 #4
    Re: Sound and Heat -- Whats the difference

    Thanks Uart. Your answer is convincing as far as the frequency range audible to humans and the requency of the heated metal atoms is concerned.

    But then what exactly is the reason that influences the difference in the amplitude and the frequency of the vibrations of atoms in both the cases. Ie what is the reason that make the amplituude go high and frequency low in the case of creating sound.
    And what makes the frequency high and amplitude low in case of heating the metal.

    What is the physical reason or fundamental that brings about this change in each case.

    And also there is an interesting incidence where there is heat produced as well as sound created when there is a plastic collision ( impact) between two bodies. This can be the typical example of conservation of momentum problem where tthe two bodies moving collide with each other and the impact is inelastic ie the second body plastically deforms the first one on impact and they both stick together as one unit and move forward.
    Here the plastic defromation produces a sound ( when they collide hard) and also heat is given out at deformation.

    Then how does sound and heat both are produced at the same time and why. Can somebody explain how these both things occur the same time and Also what would be the condition of the vibrating atoms regarding frequency and amplitude of the atoms .

    How do both low and high frequency ( for sound and heat ) are produced at the same time and how does the energy of impact distribute itself to produce both types of frequencies.

    Another example would be when a heavy object falls on ground from a height and we hear a sound (dh..a a..pp) and also experience some heat at the spot of impact.

    How does this occur.
  6. Aug 30, 2009 #5


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    Re: Sound and Heat -- Whats the difference

    When you hit something to make sound (like a spoon hitting a tin-can or whatever) then you're setting a huge bunch of molecules moving all in the same direction, that's the big difference.

    As for alexgmcm's previous question about detecting 10^12 Hz sound, firstly it doesn't transmit though air in quite the same way as a regular sound wave would. The vibrating atoms in the hot metal will impart random motion to the adjacent air molecules (which is really just regular conduction heating) and then we would need to "hear" that molecular velocity.

    Because this molecular velocity is so random and the molecule mass so small compared with macroscopic things (like a microphone diaphragm or the cochlea in our ears for example) you get very little net motion imparted to these macroscopic scale things. There are air molecules striking all the time - very randomly from all different directions - so overall their contribution to large scale motion is extremely small.

    It's interesting however that these molecular level vibrations do really manifest themselves on an (almost) macroscopic level in Brownian motion. Brownian motion is where extremely small particles (like very small grains of pollen or dust) get imparted with random thermal motion when suspended in an otherwise completely "still" fluid. It's caused by that fact that statistically the random motions of all the colliding molecules don't quite cancel completely - and the smaller the object the more pronounced will be the net "macroscopic" imbalance.

    (I'm no expert on hearing so the following is speculative) but I suppose that if some of the hair-cells in our Cochlea were small and fine enough to undergo significant Brownian motion then we really would be about to "hear" temperature. I'd guess it would just sound like a background noise that increased with increasing air temperature. I think it's even possible that this process already does actually occur within our ears but at a level below the threshold of human hearing. This would be a good thing too, as it would be a hell of a nuisance if we really could hear temperature as noise.
    Last edited: Aug 30, 2009
  7. Aug 30, 2009 #6
    Re: Sound and Heat -- Whats the difference

    The natural frequency of a solid, such as a tuning fork, is determined by the inertia of the atoms and the the bonds between them, not by the source of the disturbance. The disturbance determines the initial time and therefore the phase of the vibration. Therefore the sound of the tuning fork depends on the entire lattice having been hit at a particular time and in a particular direction by the hammer, causing it to vibrate as a whole body. However, heat involves many small collisions at random times and in random directions. After the superposition of the one big concerted disturbance and the many small random disturbances, it's only the orderly mode of vibration that will be able to produce the orderly compression waves in the surrounding air, which we hear. In addition to sound waves, the eardrum is also tapped lightly at random times due to thermal energy, but the sense of hearing isn't activated by that. Instead, the sense of touch responds to thermal energy, e.g. the air feels hot.
    Last edited: Aug 30, 2009
  8. Aug 30, 2009 #7


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  9. Aug 14, 2010 #8
    Re: Sound and Heat -- Whats the difference

    if large vibrations caused by heat can melt and boil objects
    can large sound vibrations do the same
  10. Aug 14, 2010 #9
    Re: Sound and Heat -- Whats the difference

    also, if heat and sound are both vibrations,
    how can heat pass through a vacuum but sound cant
  11. Aug 14, 2010 #10


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    Re: Sound and Heat -- Whats the difference

    Heat can be transferred in more ways than just conduction. In space it us transferred via radiation.
  12. Aug 14, 2010 #11
    Re: Sound and Heat -- Whats the difference

    so heat is not just a vibration?
  13. Aug 14, 2010 #12
    Re: Sound and Heat -- Whats the difference

    This is a very interesting thread. Does kinetic heat always travel in waves? I always just assumed that particles vibrated randomly on their own, transferring momentum to other particles that came in contact with them? If kinetic heat indeed always is organized into wave transfers, what is the maximum frequency such a wave can have and still be heat? Does a perfectly sound-absorbent material convert the sound into heat upon absorption then?
  14. Aug 14, 2010 #13
    Re: Sound and Heat -- Whats the difference

    i just remembered something i read about quantum theory
    it said that everything is both a wave and a particle
    this works for light so i assume it must have something to do with
    heat being a vibration (wave) and a particle (radiation from the sun)

    also i read something about developing a sound laser
    which might have something to do with this :)
  15. Aug 20, 2011 #14
    Re: Sound and Heat -- Whats the difference

    Heat will face the same limitations as sound in a vaccum - no matter no conductance.

    Great thread.
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