How Come?: The sweet sounds of shifting sands

Wow, this beach is noisy today! But it's not boom boxes, or beachgoers splashing and screaming in chilly water. It's the sand itself, which can act like a musical instrument . . . played by your feet.

Even the whitest tropical beach is really a strip of sediment, a vacation version of the debris at the bottom of rushing rivers. Most of the sand on ocean beaches comes from inland mountains. Blowing wind and pelting rain erode mountain rock over time. The chipped-off grains wash into rivers, and are carried down to the sea.

Once in the ocean, currents ferry the bits of rock to the shoreline, where they are tossed up onto the beach by waves. When a wave breaks on the shore, rivulets of sand-laden water run up the beach. Some of the water sinks down through the beach; some backwashes down the beach and into the ocean. If the wave wasn't too strong, more sand is deposited on the beach than flows away. Besides rocks and minerals, beach sand usually includes fragments of broken shells.

In just one cubic foot of sand, there are about a million individual grains. With all those small, shifting particles, it's not surprising that sand can make a racket. As you shuffle through beach sand, you may hear it squeak or croak. You may even hear a short, high-pitched bark, as from a very small, very excitable dog.

The noisiest sand is hot, dry, and made up of smooth grains, such as rounded quartz. In other words, an easily shiftable sand. And you don't need to tromp through sand to make it sing. According to physicist Jearl Walker, of Cleveland State University, you can also evoke the squeak by pushing an object - like a toy shovel, or your hand - into the sand at about a 45-degree angle.

No one knows exactly why beach sand squeaks, but there are some good theories. As the sand shifts and flows, layers slide over other layers, producing sound waves. (Beaches with more irregular, sharp, grains - plus lots of broken shells - don't flow as easily, and don't squeak.) Some scientists suggest thinking of beach sand shifting as a mini-avalanche, as when layers of snow noisily slip over other layers. Another theory suggests sand sounds may erupt as air is shoved out between colliding grains, making the beach one big wind instrument.

Researchers say that the sounds produced by walking on (or poking at) beach sand are high-pitched, often at a frequency of about 500 Hz, and last only a fraction of a second. But Jearl Walker notes one mystery: Sand washed in fresh water mysteriously loses its squeak, even after drying or re-soaking in saltwater. It's as if, Walker says, beach sand has a removable crust that makes it noisier.