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When we are in an empty room with no one around, we don't hear any sound, but there are billions of atoms and molecules that are colliding at the same instant.

So my question is, when two molecules collide, does it produce a sound?

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    $\begingroup$ Sound isn't a type of energy that's "produced" in a collision (unlike light, which can be thought of that way). 'Sound' is the name we give to a specific oscillation of the particles of a given medium (like air or water), and such oscillations typically involve millions of particles. $\endgroup$
    – Crisco
    Aug 3 at 12:50
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    $\begingroup$ As an amateur audio engineer, I can assure you that there is a lot of sound in your room, you just cannot hear it without mechanical aid. $\endgroup$ Aug 3 at 21:37
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    $\begingroup$ As you know, there is no sound in space because there is no air. Sound is transmitted through the air by the molecules in the air bumping into each other. When 2 atoms or molecules bump into each other, they don't make a sound. To make a sound, they'd have to bump into more molecules and that chain of bumping molecules would have to reach your ears. $\endgroup$ Aug 4 at 9:25
  • $\begingroup$ I think defining sound is important here. The current answer is great, but it talks specifically of 'sounds' audible to the human ear. There are numerous inaudible 'sounds' - where does one draw the line? When you look at sound strictly as a vibration, wouldn't two molecules colliding technically create a sound wave within the local environment (the next, maybe 10 atom, radius?) - whole comment is questioning, not stating - idk $\endgroup$
    – TCooper
    Aug 5 at 18:28
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A sound wave is a synchronised movement of millions and millions of atoms or molecules. The random collisions of atoms or molecules are not synchronised and do not produce a sound wave.

A sound wave is like a stadium wave in a large sports stadium. You only get a wave if people move in a synchronised way, each person standing up just after their neighbour. If people just stand up and sit down at random then there is no wave.

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    $\begingroup$ You could make your answer more specific: of course, individual atoms will produce a sound, but it is so low that it will not be noticeable by our ears. Only when several thousands of Atom/molecules act in a synchronous way, like in an actual sound wave, the intensity is high enough to be noticeable by your ear. As far as I know there exists technical equipment (i.e microphones) that are so sensitiv that it can detect the patter of single molecules as white noise. $\endgroup$ Aug 3 at 11:42
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    $\begingroup$ @HartmutBraun I would say that the collision of individual atoms does not produce a sound wave because the collision does not produce a synchronised pattern of movement in neighbouring atoms. $\endgroup$
    – gandalf61
    Aug 3 at 12:12
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    $\begingroup$ @HartmutBraun, Sound in air is waves of air pressure. If you understand what "air pressure" really means, then you will understand how absurd it is to think that one molecule colliding with another makes a sound of any intensity. It's kind of like suggesting that if you can build a city from bricks, then it makes sense to say that a single brick is just a very small city--not a great analogy, but it's the closest I can think of this early in the morning. $\endgroup$ Aug 3 at 12:45
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    $\begingroup$ @SolomonSlow Everything that you hear is caused by individual air molecules colliding with the sensors in your ears. The idea that "air pressure" or "sound" is a smoothly varying function of time is just a (very useful) approximation to what is really happening. $\endgroup$
    – alephzero
    Aug 3 at 14:56
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    $\begingroup$ @HartmutBraun - This is such a cool example of a more physics-oriented version of the "if a tree falls in a forest..." problem - are we talking about the vibrations of the eardrum, or about a pressure wave in the air. I think the OP is assuming in their question that it's sensible to talk about two molecules colliding "out there" producing a pressure wave that then can travel to someone's ear - and others are trying to point out that it's not as that's too "zoomed in" to sensibly treat the medium as a continuum. $\endgroup$ Aug 3 at 22:53
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A 20 kHz sound wave assuming the speed of sound is 343 m/s has a wavelength of 17 mm ($f\lambda = v$), much larger than the mean free path of molecules in air (roughly 68nm), the wavelength you might ascribe to such a collision. This means that the frequency of such a collision would be higher than the upper limit of human hearing. Such a collision would release energy < $3k_{B}T \sim 10^{-20}$ J.

In practice, sound waves are a solution to hydrodynamic equations which assume a fluid approximation where we are already not allowed to treat the air as individual molecules, but as a fluid.

But even in a situation where you allow sound to be defined for individual molecules, the frequency (in any contrived way you'd be forced to define it) would be too high to detect, and the energy would be too low to detect, for a human ear. In a situation where you inject a single high energy collision, random collisions would quickly sap the energy. Think about how slowly heat diffuses through air, and how the energy has been spread out across the entire volume of atoms by the time it reaches your ear.

Indeed, we do not hear heat, especially because it manifests as a large number of collisions upon our eardrum which provide (in the fluid approximation) a constant pressure.

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No, not in the same way that collisions of larger objects do.

The reason for this is very simple: at the scale of individual molecules, we are effectively "in space", as in like what a spaceship flies through. If you want, this is inner space, by analogy with outer space.

Sound requires a medium to travel through. Air provides that medium for large objects. But air is made of molecules. At Earth sea level, those molecules are about 68 nm apart on average, but only maybe 0.3 nm in size. In between molecules is vacuum. Thus, the collision of two air molecules effectively happens in vacuum, and so no sounds are produced, for the exact same reason that if two asteroids smack each other in outer space, no sound will be produced.

Now that said, there is a caveat here: "molecules", technically, do not have a strict upper limit to their size. In theory, a diamond, of visible macroscopic size, could be constituted entirely by a single molecule (though realistic diamonds will likely have several). If one were to bump two such single-molecule diamonds into each other, one would then, of course, hear a sound.

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  • $\begingroup$ I think your explanation is also correct because a similar post occurred before about the fact that air is mostly empty. physics.stackexchange.com/questions/202381/… $\endgroup$
    – Preet
    Aug 5 at 3:04
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    $\begingroup$ conversely, extremely low frequency sounds can propagate through what we would consider "vacuum" in the interstellar medium $\endgroup$
    – Michael
    Aug 5 at 4:26

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