What is the difference between sound and heat?


Sound is the vibration of particles; heat is the vibration of particles. So why can we not hear hot things or feel the temperature of sounds?

In: Physics

Sound is a wave. It means it’s a perturbation that propagates over time and space without (in general) a macroscopical displacement of particles. If I talk to you, you’ll ear me but none of the molecules I had in my lungs have made their way into your ear. A wave also has non-random properties : direction, etc.

In a hot gaz, particles move and their speed is linked to their temperatures. But, in average, at every moment (e.g. the average speed of all the particles at time t) and for every particles (that is, the average speed for ONE particle between time t and a bit later), this speed (accounting direction) is zero.

So they are two very different concepts.This : [https://www.physics.byu.edu/research/acoustics/images/gifs/LongitudinalWave.gif](https://www.physics.byu.edu/research/acoustics/images/gifs/LongitudinalWave.gif)

is a wave. You see that “something” moves from the left to the right (without the particles themselves following it).

This : [http://scenari2.irem.univ-mrs.fr/lexiqueINEFLP-2BPS/res/Translational_motion.gif](http://scenari2.irem.univ-mrs.fr/lexiqueINEFLP-2BPS/res/Translational_motion.gif)

is temperature (particles moving at random).

So, when a sound wave hits your tympanun, it vibrates, turns the vibration into an electrical signal that is sent to your brain.

->”Yeah, but the particles moving due to the temperature can also hit the tympanun, so why can’t we ear that too ?”. True, they do. But so do the ones on the other side, that ALSO make it vibrates, and they cancel each other (in average).

Interestingly, loud enough sounds can heat things up like [this](https://www.youtube.com/watch?v=3FpXQ3kFrNk) dude does at the end for his egg.

But the ELI5 of it is that sound is organized and neat, what we call “coherent”, and the air (or other material) compresses and un-compresses (rarefies) in a regular fashion. All the particles work together in unison in these waves to all hit your eardrum at the same time and create a sound!

Heat on the other hand is much more random and there’s no coherency, it’s just particles travelling every which way and banging up against you (or your ear). Because all the particles vibrating and hitting you are all random, they’re not working together enough to create a sound loud enough for you to actually hear.