Experiment showing the effect of a vacuum on sound. A beeping alarm is placed in an airtight vessel. Air is then pumped out of the vessel, as shown by the falling gauge, until the pressure within is around 150 hectropascals compared to 1013.25 hPa at normal atmospheric pressure. Sound is a compression wave that travels through air. At such low pressure little sound is transmitted, and the alarm is barely audible. When the valve is released air rushed back into the vessel, allowing the alarm to be heard again.