Why don't we hear sonic booms because of gas molecules? I was running some calculations of ideal gas molecules in a box for $O_2$ molecules at $T=300K$ and $V = 100 cc$, and I saw that $v = 445 m/s$.
My question is, why don't we hear sonic booms all the time when we are outside on a nice day? Is it because gas molecules are constantly colliding? Even if we have a box of gas molecules, you don't hear crackling sounds coming from it. Is it purely a size issue, where gas particles are too small?
 A: A sonic boom is produced when an macroscopic object moves through a gas faster than the speed of sound in that gas. It is due to a shock wave, which is an abrupt change in pressure in the gas. This is a behaviour of the gas in bulk.
The thermal motions of gas molecules do not create sonic booms because (a) they are in random directions, not correlated, and (b) an individual molecule is too small to create bulk behaviour such as a sonic boom.
A: Macroscopic bodies like planes are moving through air. But what are molecules moving through? The answer is empty space, because that's what separates molecules. So the means by which macroscopic bodies cause a sonic boom (formation of a shock wave in air) is missing in the case of molecules.
The existence of so much empty space in the air doesn't stop sound waves from travelling, because the wave can be thought of as small co-ordinated velocities superimposed on the random motion of many molecules and these small velocities are passed on when molecules collide.
A macroscopic object can give these organised (co-ordinated) velocities to molecules across a finite area, and so waves can propagate from it. The random motions of the molecules themselves are clearly not co-ordinated and cannot form a wave, still less a shock wave.
