The 1883 eruption of Krakatoa was an event of staggering geological power. On August 27, the climactic explosion shattered over two-thirds of the island, creating a sound wave that traveled farther than any other in recorded history. Generating a blast equivalent to hundreds of megatons of TNT, this single event redefined the limits of sound propagation. A central mystery arose: how could the sound be heard thousands of miles away yet be silent in areas much closer to the source?
The Maximum Auditory Range
The maximum distance the Krakatoa eruption was heard was verified by historical maritime logs and colonial records. The sound traveled an astonishing 4,800 kilometers (nearly 3,000 miles) to the island of Rodrigues, near Mauritius, where listeners mistook the low rumble for cannon fire. The blast was also clearly heard in Perth, Western Australia, over 3,100 kilometers away. These extreme auditory ranges were confirmed by compiling hundreds of reports from over 50 geographical locations.
Closer to the volcano, however, a phenomenon known as the “Zone of Silence” occurred. In a region extending to about 160 kilometers from Krakatoa, the sound was much less noticeable or completely unheard. This acoustic anomaly meant that people closer to the eruption experienced silence while those thousands of miles away heard a distinct roar. This acoustic shadow zone provided the first circumstantial evidence that the sound was being manipulated by the atmosphere itself.
Atmospheric Conditions and Sound Propagation
The extraordinary reach of the sound waves was possible due to specific atmospheric conditions that caused the sound to be refracted, or bent, back down to Earth. Sound travels faster in warmer air; normally, temperature decreases with altitude, causing sound waves to refract upward and away from the ground. The Krakatoa eruption injected massive energy into the upper atmosphere, creating a temperature inversion in the stratosphere where temperature increases with height. When the sound wave reached this warmer layer, the difference in speed caused the wave to be refracted downward. This created a “sound duct” that guided the sound back toward the Earth’s surface, allowing it to skip the Zone of Silence and be heard thousands of kilometers away.
The Global Tracking of the Barometric Wave
Separate from the audible sound was the extreme low-frequency pressure wave, or infrasound, generated by the explosion. This massive atmospheric disturbance was detected globally by barographs, instruments that measure changes in atmospheric pressure. The barometric wave radiated outward at the speed of sound, and observatories across the planet recorded the pressure spike. This powerful pulse continued to travel, circling the entire planet three and a half times before finally dissipating. Tracking this global disturbance allowed scientists to study the structure of the atmosphere.
Eyewitness Descriptions of the Noise
The subjective experience of the Krakatoa blast varied dramatically based on the listener’s distance. Closer to the source, the blast was a destructive pressure phenomenon, not merely a sound. At 160 kilometers, the pressure wave was calculated to be over 172 decibels, an intensity level that ruptured eardrums and shattered windows in Batavia (now Jakarta). Sailors near the Sunda Strait reported the sound as an overwhelming concussive force, causing hearing loss. At the farthest confirmed distances, the sound was reduced to a faint, low-frequency boom, consistently described in historical reports as a “distant roar of heavy guns.”