The Sound That Rounded Earth Four Times – The Krakatoan Incident
As the following is about an extraordinarily loud sound, I will have to clarify what sound, in general and in short, is.
Sounds are caused by swayings in air pressure. For example, when you talk, sing or even just whisper you're moving air molecules back and forth many times per second, changing the air pressure where the sound occurs and beyond. The louder the noise is, the stronger the movements and changes in air pressure will be – and, as it happened on Krakatoa – the farther its effects will be noticed.
But there’s a limit to how loud a sound can get. At some point, the fluctuations in air pressure are so large that the low pressure regions hit zero pressure – a vacuum – and you can’t get any lower than that. This limit happens to be about 194 decibels for a sound in Earth’s atmosphere. Any louder, and the sound is no longer just passing through the air, it’s actually pushing the air along with it, creating a pressurized burst of moving air known as a shock wave.
Just to give you some numbers: A construction site, jackhammers involved, can produce 100 decibels – mind you, for the one operating the tool, making ear protections a necessary given. Standing next to a jet engine? – Bad idea, as this exposes you to 150 decibels, much higher than any human can bear (the tolerance limit is set at around 130 decibels). An increase of 10 decibels is perceived by human ears as sounding approximately twice as loud.
Event and Aftermath
On August 27, 1883, Earth gave birth to a sound yet to be outdone – in intensity and effect.
At 10 am local time a loud noise emerged from Krakatoa, an Indonesian island located between Java and Sumatra. It was heard even 3,000 miles away in Rodrigues, an Indian Ocean island near Mauritius.
[...] coming from the eastward, like the distant roar of heavy guns.
But what had happened? A volcano had just erupted with such force that it devastated the island, ripping Krakatoa to pieces in the process. The material that was spat out from the guts of the volcano exceeded speeds of over half a mile per second – more than twice the speed of sound.
The aftermath was desastrous and fatal.
The Norham Castle, a British ship, was located 40 miles from Krakatoa as the catastrophe took place.
"So violent are the explosions that the ear-drums of over half my crew have been shattered. My last thoughts are with my dear wife. I am convinced that the Day of Judgement has come." (The captain's log)
In addition, the shock wave of high pressure air caused a powerful tsunami that cost the lives of between 36,000 and over 120,000 (the estimates differ wildly) unfortunate souls living in the coastal villages of Indonesia at that time.
As the sound moved onward from the volcano and the island, crossing Australia and the Indian Ocean (locals were hearing "gunshots" as several sources express), its intensity wore down and after 3,000 miles it could not be heard by human ears, but still be measured by instruments of as much as 50 weather stations all around the globe. The wave spread radially in every direction and because of its reverberating nature collided with itself and caused detectable spikes in air pressure for five days after the explosion – repeating in intervalls of around 34 hours at any affected place.
India, England and San Francisco witnessed a significant rise in ocean waves simultaneous to these air movements.
It was a sound that could no longer be heard but that continued moving around the world, a phenomenon that people nicknamed “the great air-wave.”
The Krakatoan Incident was something else entirely. It caused an enormously loud sound way above the threshold. Converted to today's measurement standards the volcanic eruption reached over 172 decibels – 100 miles away from the source – spawning a gigantuan shock wave measured all around the world – multiple times.
This is so astonishingly loud, that it’s inching up against the limits of what we mean by "sound".
Providing A Glimpse
The clip above is an amateur video captured from a boat showing a volcanic eruption in Papua New Guinea. As the eruption took place, it caused a sudden increase in air pressure. One can observe it moving through the air and condensing water vapour into clouds. The couple recording the video is lucky to be located far enough from the volcano – about 4.4 kilometers or 2.7 miles – so the blast took a while to reach their boat. This is similar to what happened in Krakatoa, except that the Krakatoan "bang" was heard not three but 3,000 miles away.
Judd, J.W., et al. The Eruption of Krakatoa, and Subsequent Phenomena Trübner & Company, (1888).
Winchester, S. Krakatoa: The Day the World Exploded Penguin, London, United Kingdom (2004).
Simkin, T. & Fiske, R.S. Krakatau, 1883, the Volcanic Eruption and Its Effects Smithsonian Institution Scholarly Press, Washington, D.C. (1983).
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