Zones of silence

The railway gun used during WWI to shell Paris 75 miles away created a very loud noise. A noise gets fainter as listeners are farther away, but with very loud noises it was sometimes noted that there was a "zone of silence" where no noise could be heard but surprisingly the sound became audible again further away. Several explanations were offered to explain this apparent anomalous sound. After WWI, scientists used loud guns to study the effect and from the results were able to show that the atmospheric temperature in the stratosphere increases up to an altitude of about 30 miles (50 km). >