It was right after 1:00 p.m. on July 3, 1863, during the third day of the Battle of Gettysburg, when Confederate artillery commander Colonel Edward Porter Alexander gave the order to fire all 140 Southern guns at the same time. Historian Bruce Catton referred to the cannonade as "the loudest sound ever heard on the North American continent."
Experts estimate that the sound levels near the cannons would have reached approximately 160 to 180 decibels (dB). That's about what you'd hear if you were dumb enough to stand next to a jet engine. At 150 dB, the eardrum will rupture. Indeed, it was an occupational hazard for Civil War gunners to lose their hearing for a time after a large battle. Some gunners became permanently deaf.
Now, suppose you were within a couple of miles of the Krakatoa volcano on the morning of Aug. 27, 1883. Krakatoa sits between Java and Sumatra in the Indonesian archipelago.
At 10:02 a.m. local time, the volcano exploded. The explosion was heard 3,000 miles away on the Indian Ocean island of Rodrigues, near Mauritius. The blast was so massive that the sound wave, traveling at 700 MPH, shredded hundreds of villages along with the people inhabiting them — estimates of the dead range from 40,000 to 120,000.
Krakatoa was not the loudest sound ever heard on Earth. About 75,000 years ago, the Toba Caldera, also in Indonesia, erupted in a massive explosion. Using the Volcanic Explosivity Index (VEI) to measure the magnitude of volcanic eruptions, the Toba eruption came in at a VEI of 8.
Other, much earlier volcanic eruptions were even more massive. The La Garita Caldera (Colorado, USA) ancient eruption, about 28 million years ago, is considered the largest explosive eruption known in Earth's history, producing a massive volume of ash and rock.
The largest volcanic explosion in modern history was the April 10, 1815, eruption of Mount Tambora in Indonesia. With a confirmed VEI of 7, Tambora takes the prize as the loudest sound made by Earth in recorded history.
Krakatoa was also around a VEI of 7, but the difference is in the fact that the blast was the most recorded volcanic event in human history to that point. Hundreds of seismographs, barometers, and other instruments worldwide recorded it.
"In all, it was heard by people in over 50 different geographical locations, together spanning an area covering a thirteenth of the globe," reported Aatish Bhatia in Nautilus.
In general, sounds are caused not by the end of the world but by fluctuations in air pressure. A barometer at the Batavia gasworks (100 miles away from Krakatoa) registered the ensuing spike in pressure at over 2.5 inches of mercury1,2. That converts to over 172 decibels of sound pressure, an unimaginably loud noise. To put that in context, if you were operating a jackhammer you’d be subject to about 100 decibels. The human threshold for pain is near 130 decibels, and if you had the misfortune of standing next to a jet engine, you’d experience a 150 decibel sound. (A 10 decibel increase is perceived by people as sounding roughly twice as loud.) The Krakatoa explosion registered 172 decibels at 100 miles from the source. This is so astonishingly loud, that it’s inching up against the limits of what we mean by “sound.”
Indeed, the sound wave, moving at supersonic speed, would have done extraordinary damage if it had been on land. Think of a nuclear bomb going off, with the resulting sound wave flattening buildings and killing people up to 50 miles away.
When you hum a note or speak a word, you’re wiggling air molecules back and forth dozens or hundreds of times per second, causing the air pressure to be low in some places and high in other places. The louder the sound, the more intense these wiggles, and the larger the fluctuations in air pressure. 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.
"Six hours and 47 minutes after the Krakatoa explosion, a spike of air pressure was detected in Calcutta. By 8 hours, the pulse reached Mauritius in the west and Melbourne and Sydney in the east," writes Bhatia. "By 12 hours, St. Petersburg noticed the pulse, followed by Vienna, Rome, Paris, Berlin, and Munich."
For as many as five days after the explosion, Earth's barometers were able to measure the shock wave as it made its way around the globe, being detected every 36 hours like clockwork.
Here's a video of a very small eruption followed by a minor shockwave.
When contemplating the stupendous forces that threw up the millions of tons of rock and ash in these volcanic eruptions, I'm reminded of how pitifully small we are.
Macbeth said it best.
"Tomorrow, and tomorrow, and tomorrow, Creeps in this petty pace from day to day, To the last syllable of recorded time; And all our yesterdays have lighted fools The way to dusty death. Out, out, brief candle! Life’s but a walking shadow, a poor player That struts and frets his hour upon the stage, And then is heard no more. It is a tale Told by an idiot, full of sound and fury, Signifying nothing."
(Macbeth, Act V, Scene 5)
