Sound waves and intensity

Sound is a mechanical pressure wave: a source vibrates, pushing neighbouring molecules together and pulling them apart in a travelling pattern of compressions and rarefactions. That mechanism is exactly why sound cannot cross a vacuum — with no molecules present, there is nothing to compress and nothing to pass the disturbance along, which is why explosions in space films are dramatic license rather than physics. The speed at which sound travels also depends entirely on the medium's own stiffness and density, not on how "thin" it feels: sound moves faster through denser, stiffer media, roughly 340 m/s in air but around 1500 m/s in water and over 5000 m/s in steel, the reverse of the everyday intuition that thin media should be faster.

Intensity measures how much power a wave delivers per unit area, and for a small source radiating equally in all directions, that power spreads out over the surface of an ever-expanding sphere as distance grows. Since a sphere's surface area grows as r², the same total power spread over a bigger and bigger area means intensity falls as 1/r² — an inverse-square law, the same mathematical shape met in gravity and in the electric field of a point charge, for exactly the same underlying reason: something spreading undiminished through three-dimensional space.

That inverse-square falloff is why doubling your distance from a speaker does not merely halve the loudness you experience — it quarters the intensity. Human hearing itself is roughly logarithmic in its response to intensity, which is why sound levels are reported in decibels rather than raw intensity units, but the physical intensity reaching your ear obeys the plain inverse-square law regardless of how your ear or brain then interprets it.