bell-in-vacuum experiment

The "bell-in-vacuum" experiment is a classic physics demonstration that has been used to explore the nature of sound. Early experiments, like those by Athanasius Kircher in the 17th century, attempted to show that sound could travel without air. However, his vacuum pump was not efficient enough, and he incorrectly concluded air wasn't necessary for sound transmission.1

Later, Robert Boyle improved vacuum technology and correctly deduced that a medium, such as air, is required for sound waves to travel. The experiment involves removing air from a container with a ringing bell. As air is pumped out, the sound diminishes, demonstrating that air is crucial for sound propagation. While often presented as a simple demonstration of sound needing a medium, the decrease in sound is also due to an impedance mismatch between the air and the bell/jar, not just the absence of air. A vacuum is defined as a space with no matter or extremely low pressure.12