Case Study 1.1: The Loudest Sound in History

The 1883 Krakatoa Eruption and What It Teaches Us About Sound

Overview

At 10:02 on the morning of August 27, 1883, the volcanic island of Krakatoa in the Sunda Strait between Java and Sumatra produced the largest explosion in recorded human history. The eruption was heard over 4,800 kilometers away — in Rodriguez Island near Mauritius, in Sri Lanka, in Australia. Barometers around the world registered a pressure pulse so powerful that it circumnavigated the globe not once but at least four times, in each direction. The atmospheric shock wave that the eruption produced was, by any physical measure, the loudest sound ever documented.

This case study examines what the Krakatoa eruption tells us about the physics of sound — about wave propagation, energy decay, atmospheric ducting, the difference between what instruments measure and what humans hear, and the extraordinary scale at which acoustic physics operates when freed from the small rooms and concert halls where we usually encounter it.

The Sound That Circled the Earth

The eruption of Krakatoa released an estimated equivalent energy of 200 megatons of TNT — roughly 13,000 times the yield of the atomic bomb dropped on Hiroshima. The acoustic energy released was a small fraction of that total, but even a small fraction of 200 megatons is staggering.

Witnesses on the island of Rodrigues, some 4,800 km from Krakatoa, reported hearing what they described as the sound of "heavy guns fired at intervals" — rhythmic booming that continued for hours. The population of Rodriguez had no idea they were hearing a volcano. They assumed a ship in distress was firing distress signals. Police were dispatched to the coast to investigate.

In Alice Springs, Australia (approximately 3,500 km away), residents reported hearing the distant detonations. In Colombo, Sri Lanka, nearly 2,000 km away, the sound was described as cannon fire.

The physics of this long-range sound transmission is not straightforward. Sound in the open air dissipates rapidly — following the inverse square law, the intensity should fall as 1/r². At 4,800 km, if sound radiated freely from a point source, the energy per unit area should be vanishingly small. How did anyone hear it?

Atmospheric Ducting: The Sound Superhighway

The key mechanism is called atmospheric ducting (also known as the acoustic waveguide effect). The atmosphere is not a uniform medium. Temperature varies with altitude: generally cooler at higher altitudes, but with inversions where warm air sits above cooler air. The speed of sound depends on temperature, and because the speed of sound varies with altitude, sound waves undergo refraction — they bend toward regions of lower sound speed, just as light bends at a glass surface.

At certain altitudes — particularly in the stratosphere (around 20–50 km altitude) — temperature inversions create a channel where sound speed is a local minimum. Sound rays that enter this channel at shallow angles are continually bent back toward it, trapped in the channel much as light is trapped in an optical fiber. This is the acoustic stratospheric waveguide.

Krakatoa's explosion was powerful enough to inject acoustic energy into this stratospheric channel. Once there, the energy was not spreading freely in all directions — it was constrained to propagate roughly along the channel, losing energy far more slowly than the inverse square law would predict for a free-field point source. This is why barometric pressure pulses from Krakatoa were detected in cities thousands of miles away.

The same mechanism operates in the ocean — the SOFAR channel (Sound Fixing and Ranging channel) is an ocean acoustic duct at about 700–1,000 meters depth where sound speed is minimized. Whale calls can travel through the SOFAR channel for thousands of kilometers. The U.S. Navy used this channel during the Cold War to track submarines.

Infrasound: Below the Threshold of Hearing

The Krakatoa explosion produced energy across a vast frequency spectrum, from the audible range down into the very low frequency region called infrasound: sound below approximately 20 Hz, below the threshold of human hearing.

Barometric pressure measurements from weather stations around the world in 1883 recorded pressure pulses far below any frequency humans could hear — waves with periods of tens of minutes. These infrasound waves circled the globe multiple times, detectable not by the ear but only by sensitive barometers.

Infrasound has particular advantages for long-range propagation. The absorption of sound in air increases with frequency — high-frequency sounds lose energy much more rapidly than low-frequency sounds over long distances. A 10,000 Hz tone loses energy rapidly to molecular collisions in the air; a 0.001 Hz atmospheric pressure wave (with a period of 1,000 seconds) loses almost no energy to absorption. This is why the lowest-frequency components of Krakatoa's explosion traveled furthest and were detectable in instruments that could measure sub-Hz pressure variations.

The audible portion of the explosion (that 20–20,000 Hz range that humans could actually hear) dissipated much more rapidly with distance. At 3,000–4,000 km, even with atmospheric ducting, the audible energy had fallen to levels barely above ambient noise. What witnesses heard at those distances was, in physical terms, the tip of a much larger iceberg of infrasound energy that their ears could not detect at all.

What Barometers Measured vs. What Humans Heard

This discrepancy — between the vast infrasonic energy recorded by instruments and the modest audible disturbance noticed by humans — is a perfect illustration of the ear's frequency limitations.

In the late 19th century, barometers used in meteorological stations were sensitive enough to record slow pressure variations. The Krakatoa pressure pulse was measured at stations from London to Sydney, arriving and re-arriving as the wave propagated around the globe. Meteorologists identified four complete global passages of the pressure wave. The total acoustic energy in this infrasonic pulse dwarfs anything that human ears perceived.

What the barometers recorded: pressure variations at periods of 20–120 minutes, with amplitudes measurable in millibars (one millibar = about 100 pascals). This is very large by any measure of atmospheric disturbance — comparable to a significant storm system.

What humans heard at great distances: faint, rumbling detonations in a narrow frequency range where the explosion's audible energy, attenuated over thousands of kilometers, just barely crossed the threshold of hearing under favorable atmospheric conditions.

The event beautifully illustrates that our perception of sound is a narrow window into a much broader physical reality. Most of the acoustic universe — the deep bass rumbles of earthquakes, the subsonic communications of elephants, the atmospheric pressure waves from volcanic eruptions — passes through us unheard.

Energy Decay and the Scope of the Event

The Krakatoa eruption generated an estimated acoustic energy equivalent to the entire global electrical generation capacity operating for several minutes. And yet, that energy spread over the surface of a sphere. The inverse square law applied in the audible range; the infrasound reached further through ducting. By the time the pressure wave had circled the globe and returned to Krakatoa, it had attenuated enormously — but remained measurable.

This illustrates a fundamental feature of wave physics: energy is not created or destroyed, only distributed. The energy that shook barometers in London and Sydney was the same energy generated in the Sunda Strait. It had simply been diluted over an enormous volume of atmosphere. Every sound you have ever heard — every note of music, every spoken word — follows the same principle. The energy leaves the source, spreads, and eventually is absorbed by the medium, converting to microscopic heat fluctuations too small to measure. The last whisper of Krakatoa's explosion became, eventually, an infinitesimal warming of the stratosphere that dissipated into the universal thermal noise floor.

Connections to Chapter Concepts

The Krakatoa case study illustrates multiple principles from this chapter:

Amplitude and intensity: The eruption produced sound pressure levels estimated at 180 dB or higher at distances of several kilometers. At this level, the pressure variation approaches a significant fraction of atmospheric pressure itself — the linearity assumptions underlying normal acoustics break down, and the wave propagates in the nonlinear regime.

Frequency spectrum and absorption: The infrasound components (below 1 Hz) traveled furthest because low-frequency sounds are absorbed less by the atmosphere. This is why the event was detected in barometers globally while the audible portion reached only a few thousand kilometers.

Atmospheric ducting: A physical mechanism by which sound can be guided by refractive index gradients (temperature variation → speed of sound variation → wave bending), dramatically extending the range of propagation beyond what free-field inverse-square-law decay would predict.

The ear as frequency filter: Human hearing, limited to 20–20,000 Hz, missed most of the physical event. Instruments measuring pressure variation in the sub-Hz range captured the bulk of the acoustic energy. This is a reminder that "sound" as humans experience it is a culturally and biologically specific slice of the mechanical wave spectrum.

Discussion Questions

If infrasound at 0.01 Hz (one cycle per 100 seconds) is essentially inaudible to humans, in what sense is it "sound"? Should the definition of sound be based on the physics of wave propagation or on the biology of human hearing? How might organisms sensitive to infrasound (elephants, whales, some birds) experience an event like Krakatoa differently?
The Krakatoa pressure wave circled the globe four times. Each time it passed a given location, it was weaker. Using what you know about wave energy dissipation, would you expect the wave to decay exponentially (losing a fixed percentage per unit distance), or would you expect a different decay pattern? What information would you need to test your prediction?
Atmospheric ducting allows infrasound to travel thousands of kilometers by trapping sound energy in temperature-inversion channels. The ocean's SOFAR channel does the same thing underwater. What acoustic properties would need to be true for a planet without oceans and with a much thinner atmosphere (like Mars) to support long-range acoustic communication? What challenges would musicians face performing on Mars?
Modern infrasound monitoring networks (such as those operated by the Comprehensive Nuclear-Test-Ban Treaty Organization, CTBTO) can detect explosions, volcanoes, and other large-scale events anywhere on Earth. These networks essentially do for atmospheric infrasound what seismographs do for ground vibration. What are the implications of this global acoustic surveillance for international security? For scientific understanding of geophysical events?
The Krakatoa eruption killed approximately 36,000 people, primarily through the tsunami waves it generated. The acoustic pressure waves, despite being the loudest sounds in recorded history, do not appear to have directly killed anyone at distance. Why might the acoustic waves be less lethal than the water waves, even though both involve wave propagation from the same source event? What does this comparison reveal about the energy-carrying properties of different wave types in different media?