Volcanic eruptions release immense amounts of energy as acoustic waves, in addition to heat and ash. How loud a volcano sounds depends entirely on the type of eruption, the size of the event, and the listener’s distance from the source. The resulting acoustic energy spans a spectrum far beyond human hearing, ranging from deafening audible booms to inaudible, globe-spanning pressure waves. Understanding this volcanic noise requires examining both the sound we can hear and the powerful low-frequency energy that travels unseen through the atmosphere.
Measuring the Audible Sound of an Eruption
The loudness of a volcanic explosion that humans can hear is quantified using the decibel (dB) scale, which is logarithmic, meaning a small increase in the number represents a massive increase in energy. Explosions from highly explosive eruptions can generate sound pressure levels that far exceed the threshold for human pain, which begins around 120 dB. For comparison, a typical rock concert might reach 120 dB, while a jet engine taking off nearby is approximately 150 dB.
A violent, explosive event generates sound levels that quickly turn into a physical shockwave instead of a traditional sound wave. The theoretical maximum for sound pressure in the atmosphere at sea level is approximately 194 dB, beyond which the wave physically breaks, creating a blast wave. During the 1883 eruption of Krakatoa, the sound was estimated at 172 decibels at a distance of 160 kilometers (100 miles) from the source. At closer range, this energy caused immediate physical harm, including ruptured eardrums for sailors on ships 64 kilometers (40 miles) away.
Smaller, less explosive events, such as Strombolian eruptions, produce loud, cannon-like bursts as individual gas bubbles escape the magma. These frequent, smaller explosions still generate significant noise, but the sustained, high-magnitude energy release of a Plinian eruption creates the most powerful acoustic signature. The explosive release of vast quantities of gas and fragmented rock is what pushes the sound intensity into the realm of physically damaging shockwaves near the vent.
Factors Affecting How Loud a Volcano Sounds
The acoustic magnitude of a volcanic event is determined by the specific dynamics of the eruption. A primary factor is the velocity at which volcanic gases and ash are expelled from the vent, forming a turbulent, high-speed jet. The sound power emitted is directly related to this exit velocity, where a minor increase in speed results in a substantial increase in the resulting noise. This mechanism of sound generation is scientifically compared to the noise created by a jet engine.
The type of magma involved also plays a significant role in dictating the explosivity and, consequently, the noise level. Magma with high viscosity traps gases more effectively, allowing enormous pressure to build up before the eventual violent and loud fragmentation. Conversely, low-viscosity, Hawaiian-style eruptions allow gases to escape relatively easily, resulting in much quieter effusive flows.
Beyond the source, atmospheric structure influences how the sound travels across the landscape. Changes in wind speed and temperature at different altitudes can channel or refract the sound waves, sometimes directing them toward the ground far from the volcano in an effect known as acoustic focusing. This atmospheric channeling can cause a powerful, audible boom to be heard clearly hundreds of kilometers away, while areas in between may experience a relative quiet zone.
The Power of Infrasound
Much of the immense acoustic energy produced by a large eruption exists at frequencies too low for human hearing, a phenomenon known as infrasound. Infrasound waves are defined as having frequencies below the 20 Hertz (Hz) limit of human perception, with the most energetic volcanic band typically falling between 0.5 and 20 Hz. Unlike audible sound, which quickly dissipates over distance, these low-frequency waves are less attenuated by the atmosphere and can travel vast distances.
The ability of infrasound to propagate with minimal energy loss is what makes it such a powerful tool for global volcano monitoring. These acoustic waves can travel thousands of kilometers, even circling the entire globe, because they are efficiently channeled by natural atmospheric layers, like the stratosphere. Scientists use specialized instruments called microbarometers, often deployed in large arrays, to detect and measure these subtle pressure fluctuations.
The measurement of volcanic infrasound provides near-real-time data on the intensity and duration of an eruption, particularly for remote or unmonitored volcanoes. For instance, the Plinian eruption of Manam Volcano in Papua New Guinea was detected by an infrasound array over 10,000 kilometers away. The analysis of these signals helps researchers determine characteristics of the eruption plume, such as gas exit velocity and volume, which is not possible using traditional seismic monitoring alone.
Historical Records of Extreme Volcanic Noise
The sheer power of volcanic acoustic energy has been recorded in the historical accounts of the world’s largest eruptions. The 1815 eruption of Mount Tambora in Indonesia, the largest eruption in recorded history, produced an explosion heard over 2,600 kilometers (1,600 miles) away. Witnesses across the region mistakenly reported the noise as distant cannon fire, illustrating the powerful projection of the acoustic energy.
The 1883 eruption of Krakatoa in the Sunda Strait set the modern benchmark for extreme volcanic noise. The sound from the final, cataclysmic explosion was heard clearly by people living on the island of Rodrigues, near Mauritius, which is approximately 4,800 kilometers (3,000 miles) from the volcano. The acoustic pressure wave from this single event was so immense that it was recorded on barographs, instruments designed to measure atmospheric pressure, as it circumnavigated the Earth multiple times over the following days. These historical events offer tangible proof of the colossal forces involved in the largest volcanic explosions.