Sound and Heat -- Whats the difference When we hear sound from a metal object ( like a tuning fork or a huge bell with gong) when they are striked by an external means we say that the atoms of the metal vibrate about their mean positions and so set the air around them also in compressive mode. This creates compressions and rarefactions in the air producing sound waves. Fine, this theory does make sense and we do hear sound due to the vibrations in the metal.( a bell, a tuning fork or any string instrument like guitar is a perfect example of this). BUT, Isnt it true that the atoms similarly vibrate around their mean positions when the metal is subjected to an external heat source. When we say that we supply heat to a metal the atoms absorb the heat energy and start to vibrate around their mean positions. And the amplitude and freqeuncy of the vibration are the indication of its thermal energy and tempreature. The point is that in both the cases ( that of heating and striking the metal with some external means to produce sound) both involve the same action ie the vibrations of atoms around their mean positions. 1. Then what is the exact difference between these vibrations that we hear sound in one case and feel the heat in the other when in both cases the atoms do the same thing ie they vibrate. 2. So why dont we hear sound from a iron bar which is being heated to red hot but still we dont hear any sound from it, though the atoms are vigorously vibrating. 3. Similarly why dont we feel heat when we strike a tuning fork but hear sound alone, again the atoms only being vibrating. What is the differnce in the vibrations that differentiates between the production of Sound and heat despite the basic behaviour of the atoms being the same in both cases. What produces sound and what produces heat.