# How does sound imparts vibrations to molecules in solids

How does sound imparts vibrations to molecules in solids which are tightly bounded. I understand the molecular bounding is very strong that is why it is solid and tough.

So how does mere a tap on a wall keeps the molecules in vibrations ? In fact they vibrate more than the molecules in liquid that is why sound travels faster.

Please help me understand this fallacy.

Thanks in advance

• The reason why vibrations travel faster in solids than liquids is because there’s greater order in solids leading to more directional transmission. It’s easier to lose directionality in fluids. – Superfast Jellyfish Apr 16 '20 at 22:39
• There is no fallacy here. You need to demonstrate the fallacy for us to pick it apart. – ggcg Apr 17 '20 at 14:05

## 2 Answers

You can think of a chunk of (for example) solid metal as being a bunch of very tiny pieces of mass (atoms) connected together to their nearest neighbors by very stiff and strong springs (the interatomic binding forces).

If you suddenly apply a push to one corner of that chunk by for example striking it with a hammer, two things happen: you set the atoms right next to the hammer in motion, and you compress all those springs in the vicinity of the hammer strike. Those springs then transmit the push from the hammer to the neighboring atoms, which are then set into motion as well.

Those moving atoms then press against all the springs to which they are attached, which then push on the next nearest atoms, and so forth.

In this way, a sound wave gets propagated through the chunk.

• Hammer is fine. But when I just scratch or gently touch the wall and put my ears on wall I can hear the sound. It means mere a gentle touch pushed the atoms in motion. This is what I want to know how come such a small force disturbs those strong and stubborn atoms? – gpuguy Apr 16 '20 at 23:57
• those atoms are strong, but because they exhibit elasticity, they can and do transmit sound. – niels nielsen Apr 17 '20 at 3:51

What matters is the speed of the applied stress. As you said, molecules (or atoms) are strongly bounded. But even a gently scratching with the end of a nail, has a big energy compared to the eV scale of that individual bindings.

So, if the stress increases slowly, it can reach the yield or rupture point of the material in silence, because it is distributed to the $$10^{23}$$ atoms or so.

But a sudden touch must be handled only by some atoms at the point of contact. And it can easily perturbate their equilibrium location, what causes a pressure wave to all the solid.