Why and how is sound produced when two objects hit each other? 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?
 A: Sound is longitudinal pressure waves in the air, It can be produced in a couple of ways in a collision.


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*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.

*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.
A: 
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...
  
  
*
  
*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).


  
*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


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*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.

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