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Rubber is often regarded as having one of the slowest transmission speeds while aluminum and steel have some of the highest. Density alone can't account for this as air is less dense than rubber but has a faster transmission speed. Metals tend to be more rigid with a specific lattice structure, so the difference seems to be some measure of softness and/or structural organization, but how do you compare rubber to air in order to explain why air has a faster transmission speed than rubber?

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The velocity of sound in a medium goes like velocity = square root of (stiffness/density). The highest sound speed hence requires high stiffness and low density in combination. Air is not very stiff but is nondense; steel is very stiff but much denser, diamond is somewhat dense but extremely stiff, rubber is not as dense as steel but is very nonstiff. these characteristics put their sound speeds all over the map.

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  • $\begingroup$ Seems reasonable at first, but, how do you compare stiffness to density in order to explain steel's high transmission speed? How do you say steel is more stiff than it is dense? Lead is also dense and fairly soft, but has a higher transmission speed than air. $\endgroup$
    – Vane Voe
    Oct 5, 2018 at 5:43
  • $\begingroup$ wikipedia has a pretty good explanation of this under "speed of sound". the entry includes notes on the mathematical derivation and other useful details and is well-written. $\endgroup$ Oct 5, 2018 at 5:46

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