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Question:
Our Physics teacher asked me to forward the following to you Our textbook gives the following values for the speed of sound in air. 342 m/s in H2 1339 m/s in O2 316 m/s and in general the more dense the medium the faster the speed . H2 is not in line with the pattern...Why?



Replies:
Yes it is in line: for gases, the more dense the medium in terms of mass per unit volume (at constant pressure), the SLOWER the speed of sound. Pure O2 is actually heavier than air (which is mostly N2) while H2 is much much lighter. The reason is that the lighter a molecule is, the faster it moves (at given temperature), and the speed of sound in gases is simply proportional to the speed (on average) of the molecules. Sound speed following density is an often quoted fact that is really not followed by nature. The speed in solids is determined by the elastic constants of the material which have very little relation to density - they are more directly related to the strength of the bonding between the atoms of the material, so that a very strongly bonded material has a faster sound speed.

Arthur Smith



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