How did the bell-in-vacuum experiment help establish that sound is a wave?

The bell-in-vacuum experiment helped establish that sound is a wave by challenging the 17th-century belief that sound propagated via invisible particles emitted from a source. By demonstrating that the intensity of a sound diminishes as air is removed from a container, the experiment provided evidence that sound requires a medium to propagate, rather than traveling as independent particles through space.1

While early attempts by Athanasius Kircher were inconclusive due to limitations in vacuum technology, Robert Boyle later improved the apparatus to observe sound intensity decreasing toward zero as air was evacuated.1

It is important to note that the common textbook explanation—that sound vanishes because there is no medium—is technically misleading. Even in a high-quality vacuum, enough air remains to transmit sound; the actual reduction in volume occurs because of an impedance mismatch. As air is removed, the bell cannot efficiently transfer its vibrations to the remaining thin air, which in turn cannot efficiently transfer those vibrations to the glass walls of the container. Despite this nuance, the experiment remains a foundational demonstration that sound propagates as a longitudinal wave through a medium.12