Physics

Sound speed depends on how quickly molecules can pass vibrations to their neighbors—not on how far apart those neighbors are. Iron's atoms are packed far more densely and bound much more tightly than air molecules, so they transmit vibrations almost instantaneously through the material. Think of it like a chain: a rigid, closely-linked chain passes a shake down its length faster than a loose, floppy one. The key insight is that $v = \sqrt{\frac{\text{stiffness}}{\text{density}}}$ captures this trade-off, and iron's enormous stiffness overwhelms any density advantage air might have.

Sound speed depends on how quickly molecules can pass vibrations to their neighbors—not on how far apart those neighbors are. Iron's atoms are packed far more densely and bound much more tightly than air molecules, so they transmit vibrations almost instantaneously through the material. Think of it like a chain: a rigid, closely-linked chain passes a shake down its length faster than a loose, floppy one. The key insight is that $v = \sqrt{\frac{\text{stiffness}}{\text{density}}}$ captures this trade-off, and iron's enormous stiffness overwhelms any density advantage air might have.