# Is sound a classical mechanic phenomenon or is it a quantum effect?

I am just curious because sound is longitudinal waves, meaning the energy is passed from one particle to another nearby particle and produces a wave that continues on until it reaches our ear as a sound wave or hits our skin as a pressure wave, so is this considered quantum because of atoms and molecules?

• Sound is classical, and so is classical stat mechanics of atoms and molecules. Dec 10, 2021 at 3:27
• By your logic, everything is quantum. Which is not wrong, but also not a very practical point of view. Dec 10, 2021 at 14:50
• Everything can be potentially reduced to quantum mechanics, but most everyday phenomena are emergent effects resulting from averaing over large numbers of quantum particles, and classical physics describes these. Dec 10, 2021 at 15:49
• Raman scattering, Brillouin scattering, infrared absorption spectra, and many other phenomena relating to sound are not classical. Dec 11, 2021 at 23:02
• @AnonymousPhysicist: I think sound becomes a transverse wave when it travels in solid Dec 11, 2021 at 23:57

The detailed physics of sound wave transmission through air were mathematically worked out and found to be accurate before the invention of QM. No quantum-mechanical effects need to be taken into account to accomplish this; it is solidly in the domain of classical physics.

• At least in certain temperature, pressure ranges, and density ranges. Dec 13, 2021 at 5:29
• @Shufflepants, of course. Dec 13, 2021 at 5:46

Well, niels answer is correct of course. Sometimes you can have also sound in QM system, it is called phonons where a mechanical classical vibration is traveling along the atomic lattice of some materials similar to sound inside a medium.

It is also one of this rare cases where you can treat your quantum system classically.

• The quantum-mechanical phonon theory is only useful once you get to the energy scales of the lattice's vibrational modes, isn't it? Only then it makes a difference whether you regard the atoms as classical or quantum harmonic oscillators. What one would typically refer to as "sound" happens on much larger energies.
– m93a
Dec 12, 2021 at 15:54

Sound is successfully described using continuum mechanics of solids and liquids (that is the theory of elasticity and hydrodynamics). That is, one need not even descend to the level of molecules/atoms, although it is possible and some interesting phenomena appear at this level.

To summarize:

Sound < Continuum mechanics < Molecular theory < Quantum theory


Quantum sound
As another answer points out, one can speak of quantized sound waves, such as phonons in solids. Even more interesting are such essentially quantum phenomena as zero sound and second sound.