The Loudest Sound

On 27 August 1883, reverberations from the northwest disturbed the morning serenity of sheep ranchers outside Alice Springs, Australia.

The men later described the sound as “a series of loud reports, resembling those of artillery.” No war raged in central Australia in 1883; no military exercises took place. Were these ranchers under attack from some mystery force?

The force was indeed mysterious, but these humans were in no danger. Instead, their eardrums had just processed the loudest sound ever experienced in recorded history. Amazingly, that vibration emanated from 2,233 miles away!

The eruption of Krakatoa, and subsequent phenomena - lithograph by Parker & Coward

What the Australian sheep ranchers had heard was the explosion of Krakatoa, a volcano in Indonesia.

It was a calamitous event, one of the deadliest and most destructive volcanic eruptions in history. Tsunamis that feature 100-foot waves pummeled the region. Seventy percent of the island of Krakatoa was obliterated, forming a massive caldera. At least 36,000 people perished, though many estimates significantly dwarf this total.

And the sound it created was loud.

Forty miles away in the Sunda Strait, a group of sailors sat in the path of the pressure waves. Eardrums instantly burst. The captain wrote, “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.”

A coral block blown onto the island of Java from the Krakatoa blast - Tropenmuseum

Eardrums can be shattered because sound is a mechanical phenomenon. Sound is caused by changes in the pressure of a medium. For most human life, that medium is air. The louder a sound is the greater the fluctuation in air pressure caused by the sound. Make the pressure-wave large enough and the physical membrane of an eardrum can puncture. Ouch.

Sound waves propagate through a medium by bumping into other particles (this nature is why you can’t hear sound in space, which is a vacuum; there are no particles to bump into). Maggie Koerth of fivethirtyeight describes sound in an easy-to-understand way:

“Think of being on a crowded train car. If you were to hip check the person standing next to you…they would tense up and scoot away from you. In the process, they’d probably bump into the next person, who would tense up and shimmy away from them…Meanwhile, though, that original person you bumped into has now relaxed. The pattern travels through the crowd — bump-tense-wiggle-sigh, bump-tense-wiggle-sigh. That’s what a sound wave looks like.”

The following video might help visualize the science:

There are various ways to measure sound, but the most common way to think about loudness comes via the decibel. When a wave is measured in decibels we are essentially measuring the pressure level in the medium of choice. Decibels are a logarithmic scale, not linear. So if a noise comes in a 5 dB and another one clocks 15 dB, the second one is not three times louder but 10 times! A difference in 20 decibels means the louder sound is 100 times louder.

Interestingly, there is a theoretical upper bound to how loud a sound can be in the air, or water, or solids. For air, this Iimit is 194 dB. Loudness is dictated by how high the amplitude of a wave is in relation to the ambient pressure. For air at sea level at 0 degrees Celsius, pressure measures at 101.325 kilopascals. At 194 decibels, the pressure deviation of the wave is 101.325 kilopascals. If the wave got “louder” from that point, it would create a vacuum and would move from a soundwave into the realm of a shockwave.

Technically, things can get “louder” than the upper bounds we just set forth. Louder than the threshold, though, the wave doesn’t really move through the air anymore and the energy distorts the wave. Instead, it pushes the air itself forward, producing pressurized bursts known as shockwaves.

Here are the decibel levels of some common sounds:

Rainfall – 50 dB
Conversation – 60 dB
Noisy restaurant – 70 dB
Ringing Telephone – 80 dB
Electric Drill – 95 dB
Chainsaw – 110 dB
Thunder – 120 dB
Shotgun – 140 dB
Jet Engine – 140 dB

Sounds above 125 decibels produce acute pain in human ears. The loudest sound a rock band hit is reportedly 139 decibels. The atomic bombs in World War II are estimated to come in around the 170 dB range. NASA claims the Saturn V rocket used to go to the moon wracked up loudness levels of 204 decibels, which technically puts it past the “sound” level and into the “shockwave” stratosphere. The largest bomb ever detonated on earth, a Soviet hydrogen bomb, nicknamed Tsar Bomba, supposedly hit 224 decibels, which means it was 100 times louder than the Saturn V because the scale is logarithmic.

Yet none of these incidents comes close to the Krakatoa eruption.

Tsar Bomba's mushroom cloud from 100 miles away

As we noted earlier, the sound of the explosion from Krakatoa traveled thousands of miles. Humans as far away as 3,000 miles heard it (that’s farther than the distance from New York to Los Angeles).

Measuring devices 100 miles from the island registered decibel levels of 170! That’s atomic-bomb level from 100 miles away. The shockwave from Krakatoa circled the earth 3.5 times, based on measurements from around the globe.

Though no one who heard the sound at ground zero lived to tell the tale, taking the 170-decibel measurement and working backward, scientists estimate the Krakatoa blast produced a pressure wave of 310 decibels. Anyone who “heard” this wave would be dead.

Remember, kids, always wear earplugs to the show!

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