Acoustics: The Sound of Sand Reveals its Source -- It’s all about the ancient shellfish
Lift a shell from the sand to your ear and everyone knows you can hear the sea. But listen carefully enough and you can hear shells in the sand too. Sand, it turns out, has a signature sound of its own, and now scientists have found a way to tune in.
To the untrained eye, one bucket of beach sand looks much like another but mixed into the multitude of microscopic minerals are carbonate chemicals left behind from the shells of long-dead sea creatures such as molluscs. The carbonate concentration varies according to local geology, and Saskia van Ruth, a researcher at Wageningen University in the Netherlands, and her colleagues say this leaves each batch of sand with its own distinctive noise. The results could extend forensic techniques, providing a quick way to determine the source of disputed sand.
After water, sand and gravel are the most used natural materials in the world. But a looming global shortage has led to a boom in clandestine sand mining and even outright theft. In the southern Indian state of Tamil Nadu, authorities are battling a so-called “sand mafia” who supply the construction industry through illegal dredging of riverbanks. Last year Malaysia became the latest country in the region to ban the sale and export of its sand, demand for which has soared as Singapore seeks to reclaim land from the sea. A decade ago an entire beach, 500 truckloads, was stolen from a resort in northern Jamaica and, it is believed, sold to rival operators.
Writing in Applied Acoustics, Dr van Ruth’s team show they can distinguish between sand samples retrieved (legally) from nine seaside spots along the Dutch coast. And armed with that information as a reference tool, they could work out which beach a given sand sample had come from.
The scientists used a technique called Broad Acoustic Dissolution Spectroscopy analysis (bards). It is done with a sensitive listening device that picks up changes in acoustic properties when a scoop of sand or other powder is dropped into a beaker of mild acid and mixed. Chemical changes, including the breakdown of carbonates to carbon dioxide, release bubbles that increase the compressibility of the liquid and therefore slow down the passage of sound through it. Tap the side of the beaker as the sand and acid are mixed together and the sound that emerges drops in frequency over time. After a few minutes, all of the carbonate is broken down so the production of gas slows and stops. In response, the frequency of sound passing through the liquid goes back to normal. This gives each sample of sand two distinct measurements: how quickly the sound changes pitch, and how much it does so.
Dara Fitzpatrick, a chemist at University College Cork who developed the bards technique, says the change, from high notes to low and then back again, can be heard when many powders dissolve and is known to physicists as the hot-chocolate effect. “You can do it in your kitchen,” he says. His team is selling the kit to drug companies as a quicker and cheaper way to analyse powders. It has also been used to distinguish expensive Himalayan table salt from inferior fakes.
With sand, the more carbonate there is to produce carbon-dioxide gas, the greater the acoustic shift. That is what allows the eavesdropping scientists to pinpoint its source. They can also pick up more subtle influences, including the effect of the remains of different-shaped shells, because variations in their thickness and surface area speed or slow the release of gas.
Follow the sand
In places like the Netherlands, forensic tracing could help track the effectiveness of coastal-management practices, such as the dumping of millions of tonnes of sand to bolster natural defences against the sea. Where that sand ends up is not always clear. Placing the sand directly onto beaches is believed to be wasteful because much of it is washed back into the sea. Newer methods drop the sand into the shallow water just off the beach, allowing the tides to deposit it onto the land over time. Dr Fitzpatrick says existing methods to distinguish sand samples are crude and slow as they rely on looking at the size and shape of individual grains under a microscope. He sells sea shells as a way to see more. ■
This article appeared in the Science and technology section of the print edition under the headline"Name that dune"
Sand from different beaches has its own unique sound—and researchers have figured out how to listen in, The Economist reports. Sand, which is a valuable commodity, is often taken illegally for construction projects and artificial beaches. (India even has its own “sand mafia.”) To find out where a sample of sand comes from, scientists drop it in acid, which breaks down the sand’s carbonate chemicals (made from the shells of long-dead sea creatures) into carbon dioxide bubbles. Using sensitive listening devices, the researchers measure how the bubbles change the way sound travels through the mixture—yielding a unique frequency. When scientists listened to sand samples from several different beaches in the Netherlands, they found that each had a unique signature, they report in Applied Acoustics. Future research might address a more pressing philosophical question: If sand falls in acid with no one there to hear it, does it still make a sound?