You may not be able to hear it, but all solid materials make a sound. In fact, atoms—bound in lattices of chemical bonds—are never silent nor still: Under the placid surface of each and every object in our surroundings, a low hum hovers or a high-energy squeak titters.

As atoms vibrate in their lattices, they do so by either all moving in the same direction, in which case their collective vibration shows up as a low humming sound, or by moving in opposite directions from one another, giving rise to an energetic vibration that registers as a bright squeak or titter.