Sound Goes Faster Through a Wall Than Through Air

Step 1 of 5

Sound is a shove handed from one particle to the next.

Nothing flies across the room. One particle is pushed, it bumps its neighbour, and the bump travels. The speed of sound is really the speed at which that hand-off happens.

Step 2 of 5

Two things decide how fast the hand-off goes.

How close the neighbours are, and how stiff the connection is. In air the particles are far apart and there is a wait before contact. In a solid they are packed touching and bonded, so a push at one end is felt almost immediately at the other.

Step 3 of 5

Stiffness matters more than heaviness.

Lead is far denser than steel and sound moves through it more slowly, because lead is soft. What speeds sound up is a springy, unyielding bond — the material's refusal to be squashed. Density on its own actually slows sound down.

Step 4 of 5

Which is why people put an ear to the rail.

A train's noise reaches you through the steel long before it reaches you through the air, because the rail delivers it about fifteen times faster. Whales get the same benefit — sound in the ocean carries fast and far.

Step 5 of 5

Fast and loud are not the same thing.

A wall muffles because most of the sound bounces off the front of it instead of entering — the mismatch between air and brick reflects nearly all the energy back. The little that does get in crosses quickly. Quieter and sooner, at the same time.

This is why you hear the thump of a bass note through a wall but not the words. Low frequencies get through far more easily than high ones.