Michel André

Listening to the sounds of silence

April 25, 2017

Two Rolex Laureates from very different scientific fields are working together in one of the most silent places on the planet – the caves below the Amazon’s table-top mountains – to record the whispers that speak volumes about our natural order.

By Julian Cribb

Rolex Laureates Francesco Sauro and Michel André propose to explore the most silent places on Earth, pioneering an entirely new field of science.

Sauro became a Rolex Young Laureate in 2014 for his ambitious plan to investigate, document and interpret the unexplored caverns beneath the remote tepuis, or table-top mountains, of the Amazon region. André won a Rolex Award 2002 to develop a technology to prevent whales and ships colliding. In the ensuing years, he has become one of the world’s foremost authorities in ocean bioacoustics – the sounds of the sea – and how animals use them for survival, and how human noise has interfered in that process.

The two first met at a Rolex ceremony in 2014 and then again at a gathering of Rolex Laureates at the programme’s 40th anniversary celebration in Los Angeles in 2016, which included a summit organized by Los Angeles Times and UCLA’s Anderson School of Management. André recalls: “I was in the room when Francesco gave his talk at the Los Angeles Times Summit. He ended his description of the exploration of the Venezuelan tepui caves by saying that what impressed him most was the silence inside.

Sound sources

“As someone who studies sound, this immediately caught my attention. I went to ask him about this ‘silence’. He smiled and began to describe the different, tiny sound sources that are heard inside the caverns of the tepuis. And naturally, I offered to use our technology to help unravel them.”

These two remarkable scientists then decided to join forces, employing state-of-the-art technology to measure sound in some of the quietest places on the planet, so pioneering the novel field of speleo-acoustics.

“There are two sorts of caves,” Sauro explains. “Active caves, which are in the process of forming or changing, and so-called fossil caves, which have remained in a stable state, without outside disturbance, maybe for millions of years.

Faint sounds

“In active caves, there are faint sounds – running or dripping water, airflow, the movement of the rocks themselves, the activity of living creatures that inhabit this underworld.

“In fossil caves, there may exist the deepest silence it is possible to find on the Earth. By listening to both, we hope to learn new things about how this secret part of our world forms and functions,” he says.

Marine acoustics

André’s groundbreaking work in marine acoustics helped to reveal that, far from being the “silent deep”, the oceans are in fact a tumult of sound: the calls of whales and songs of fish, the noise of storms, the clicking of shrimp, the rumble of earthquakes and undersea volcanoes, and now, the omnipresent thunder of humanity, in ship engines, explosions, sonar and seabed construction. In 2010, he established LIDO (Listen to the Deep Oceans), a network of international hydrophone observatories that monitors and records underwater sounds around the planet – human, animal and geological.

“Being able to listen to some of the most silent places on the planet – the deep caves of the tepuis which Francesco is exploring – was an unparalleled opportunity for acoustic science,” André says.

The two plan, over coming years, to place a network of highly sensitive acoustic ‘ears’ in the caverns of the tepuis to record whatever sounds are there to be detected, in a place so remote that human noise has not yet contaminated it.

Lost world

“It will be like listening to a lost world, a world without humans or man-made sound,” André explains.

Until now, the main scientific role for acoustics in caves has been for the study of bats. Now André and Sauro plan to expand its role dramatically, to study water, rock movement, life forms and micro-climate within caves. All of these, they hope, can reveal much about how caves form, grow, change through time and interact with the human world.

Listening to the flow of an underground river, the groaning of rock or the drip from a stalactite might sound tedious work, but from tiny changes in tone, volume and timing, scientists can deduce much about the natural workings of an active cave, André says. Sound offers a new way to receive warnings of sudden floods that may fill caves or affect rivers on the surface. Over time, even tiny drips can reveal subtle changes in climate and rainfall at the surface.

Cave life

Apart from bats, cave life is generally thought of as silent – but in the pitch-dark, sound assumes a far greater importance than other sensory perceptions, the two researchers say. The chirping of insects, the grunting of blind fish and calls of cave-dwelling reptiles may reveal strange choruses hitherto unknown to the human ear. “The potential new applications for monitoring animal activities are absolutely promising… and open a completely new perspective on cave biology,” they say in an article they are preparing for publication.

Though caves are usually remote from outside weather, changes in air and water pressure, and in temperature, cause air to flow back and forth even in deep caverns, and these subterranean “winds” offer a new way to study the micro-climate of the Earth’s inner space, they say.

Finally, since “fossil” caves are devoid of most forms of activity, they may be as silent as any place on the planet. The scientists say this offers unprecedented opportunity to listen to the authentic voice of the Earth itself, the faint creaks and groans as masses of rock weighing millions of tonnes stress and fracture to the slow churning of the crust. This offers a new way to monitor seismic activity, both at the large scale – earthquakes – and at the small, via micro-seismic events. “The monitoring of these environments could reveal unexpected scientific information… including the study of earthquake precursors,” they say.

Symphony of music

By spreading networks of instruments through the caves of the tepuis, the researchers hope to create a three-dimensional ‘symphony’ of the music of the deep Earth. In the same way that the calls of whales and choirs of fish have become familiar to humans through the use of hydrophones placed in the oceans, Sauro and André hope that speleo-acoustics will open a fresh chapter in Earth system observation and monitoring and teach us about a world we have barely begun to explore.

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