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When two objects collide and undergo a partially inelastic collision (so every one we experience in every-day life), they rebound to a certain degree, but kinetic energy is not conserved. Thus, the energy of motion must be converted to another type of energy. Thus, when two object collide, the most common forms of energy that kinetic energy is converted to are sound energy and thermal energy.

Some collisions are louder than others just because they cause a bigger local variation of pressure (which then propagates through air until finally gets heard). So, why some objects, even if the velocity before and after the collision seems to be the same, are louder than others? I mean, how do the different material properties enter in the phenomenon?

How and why, by means of solid and fluid mechanics or with an heuristical microscopical reasoning, can this process be explained?

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  • $\begingroup$ There is no difference to how sound is produced in this situation and any other. See physics.stackexchange.com/questions/31469/… for general discussion. In fact, I think its just a duplicate of that question. $\endgroup$
    – DJBunk
    Commented Jul 14, 2012 at 3:57
  • $\begingroup$ So, some collisions are louder than others just because they cause a bigger local variation of pressure (which then propagates through air until finally gets heard). Right? So, why some objects, even if the velocity before and after the collision seems to be the same, are louder than others? I mean, how do the different material properties enter in the phenomenon? $\endgroup$
    – usumdelphini
    Commented Jul 14, 2012 at 4:03
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    $\begingroup$ Many comments deleted. I note that the most prolific participants both claimed to have an answer to the question, but that neither one wrote an answer. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jul 15, 2012 at 21:44

3 Answers 3

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Sound is longitudinal pressure waves in the air, It can be produced in a couple of ways in a collision.

  1. The impact can set up pressure waves and or ringing in the bodies themselves which then interact with the surrounding medium to produce audible sounds. This is the mechanism of a bell.

  2. If enough air is forced out of the space between the two bodies as they approach sound can be made either directly (the displacement is the initiating event) or a secondary effect of that out-rushing air interacting with the edges of the bodies. This mechanism is rare enough in everyday life that I can't think of an example off the top of my head.

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    $\begingroup$ A very interesting video showing physically based sound simulations: archive.org/details/Synthesi2001 The paper describing the simulation can be found at cs.berkeley.edu/~job/Papers/obrien-2001-SSP.pdf $\endgroup$
    – mmc
    Commented Jul 16, 2012 at 3:39
  • $\begingroup$ Thanks for the answer. So, when two stones hit each other and they are in my hands, each one make the other vibrate by means of the impulse given and this vibration is trasmitted to the surrounding air to produce an audible wave pressure? I mean, can I neglect the detailed "solid" effects and just consider every collision as a source of vibration of the objects (even if the vibration is partially suppressed as in the case of my hands on the stones) ? $\endgroup$
    – usumdelphini
    Commented Jul 16, 2012 at 3:56
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    $\begingroup$ Isn't the clapping of hands an example for no. 2? $\endgroup$
    – mart
    Commented Jul 16, 2012 at 7:59
  • $\begingroup$ @usumdelphini The details of what happens in each of the two bodies are exceedingly important to the nature of the sound (volume, spectrum, duration...). Consider the distinctive sound of a high quality bell or gong: it implies that there are modes in the body of the object with very high $Q$ values. Talk to a percussionist. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jul 16, 2012 at 13:35
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    $\begingroup$ @mart Er...I would have though that most of the sound of clapping came from flesh on flesh, but can provide nothing concrete to back up that assertion. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jul 16, 2012 at 13:39
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So, why some objects, even if the velocity before and after the collision seems to be the same, are louder than others? I mean, how do the different material properties enter in the phenomenon?

As dmckee stated

Sound is longitudinal pressure waves in the air...

  1. The impact can set up pressure waves and or ringing in the bodies themselves which then interact with the surrounding medium to produce audible sounds. This is the mechanism of a bell.

However, what was implied but not necessarily apparent is that size and shape will change how loud the sound is. I.e. Given a bell and a block of the same metal of the same mass, the bell will be louder due to the larger surface area (it will also sound for longer due to harmonic resonance, which stores the 'ringing' energy).

  1. If enough air is forced out of the space between the two bodies as they approach sound can be made either directly (the displacement is the initiating event) or a secondary effect of that out-rushing air interacting with the edges of the bodies. This mechanism is rare enough in everyday life that I can't think of an example off the top of my head.

Hand clapping and a squash ball hitting a wall are two examples. Unfortunately, there are two sources of sound

  1. the 'ringing' as described in dmckee's 1st point. I.e. the vibration of the skin that isn't between the two hands, the vibration within the squash ball.

  2. the sudden expulsion/ratification of the air. I.e. the air between the two hands, the deformation of the squash ball against a wall.

dmckee's 2nd point are not elastic collisions. In fact, I would say that any such collision is not close enough to being ideally elastic to actually qualify to be called elastic. Such a collision will cause too much energy to be lost fighting against suction forces and thermal heat loss.

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The sound is generated by pressure wave of air and collision provides energy to transform into sound.

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    $\begingroup$ Please expand... $\endgroup$
    – Neuneck
    Commented Jul 12, 2013 at 7:04

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