Sound travels through air as a pressure wave — a pattern of compressions (air molecules pushed together) and rarefactions (air molecules spread apart) that moves outward from the source in all directions. When this wave reaches the balloon membrane, each compression pushes slightly against it, and each rarefaction pulls it back. The membrane vibrates in and out, matching the frequency of the incoming wave exactly.

The salt grains on the membrane's surface are carried by these vibrations. Each tiny inward movement of the membrane flicks the grains upward; gravity returns them. At the frequencies of a human voice, this happens dozens to hundreds of times per second — fast enough to look like continuous dancing, but slow enough to be visible to the naked eye.

Your real eardrum, the tympanic membrane, is a thin oval disc about 8–10 mm across, stretched across your ear canal. It behaves in exactly the same way as the balloon. Sound waves cause it to vibrate; those vibrations are then transferred to three tiny bones in your middle ear — the malleus, incus, and stapes — which amplify and carry the vibration to the cochlea in your inner ear, where it is finally converted into electrical signals your brain reads as sound.