Now, a scientist named Stephan Gekle has found that you can make air move faster than the speed of sound by doing a simple little trick: throw a rock in a pond.

View video | As a disc representing a stone plunges into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in the video, the air is filled with smoke for visibility) shoots thro

Stephan Gekle/Physical Review Letters 2010

Gekle is a scientist at the University of Twente in the Netherlands who studies the physics of fluids. Physics is the study of forces and motion, and Geckle investigates how forces act on liquids, like water. In a recent study, he and his colleagues showed that after a rock drops into a body of water, a tiny jet of air shoots upward faster than the speed of sound.

This isn’t the first time Gekle has explored what happens when a rock sinks through water. In an earlier study, he and his team showed that as a rock falls into a flat surface of water, like a pond, it carves out a tiny tube of air. This tube connects the sinking rock to the air above the pond. The tube doesn’t exist for very long, though — almost immediately, the surrounding water pushes on the sides. This pressure is stronger in the middle than at the ends. As a result, the tube looks like an hourglass, where the middle gets smaller and smaller as the water forces the air out.

There’s not room in the hourglass for water and air, so as the water comes in the air escapes upward — and fast. These tiny jets of air can blast faster than the speed of sound, Gekle found.

To measure these air jets is trickier than it may seem. Gekle and his colleagues had to do more than stand at the edge of a pond with stopwatches. A careful science experiment requires a scientist to take multiple measurements of the exact same thing, to check and double-check the results. In this case, it would have been almost impossible for Gekle and his colleagues to throw a rock in a pond in the same way over and over again.

Instead, the scientists created a lab experiment that acted like a rock falling through water: They dragged a circular disc down through water at the same speed, over and over again, and watched what happened.

But there was another difficulty: It’s hard to see and measure air. To solve that problem, the scientists filled the air above the water with smoke and illuminated the smoke with a laser, which made the moving air easier to see. (To make the smoke, Gekle said, they used a smoke machine like the ones that provide the dramatic effects seen onstage at theaters.)

Finally, because everything happens so fast when the rock moves through water, the scientists had to find a way to slow down time. As the disc moved through the water, the scientists took pictures with a camera that captured 15,000 frames every second. (That’s faster than most movie cameras.) After the experiment, the researchers could slow down the movie and, aided by computer simulations, calculate the speed of air as it blew out of the hourglass-shaped tube.

But there’s one aspect of supersonic air that Gekle and his team didn’t observe. When a jet exceeds the speed of sound, the air around it produces a noise like thunder, called a sonic boom. So far, however, Gekle says the tiny air jets aren’t making even a teeny, tiny boom — but the researchers will keep listening.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

speed of sound About 760 miles per hour, through air at sea level.

supersonic Faster than the speed of sound.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

force The capacity to do work or cause physical change.

pressure Force applied uniformly over a surface, measured as force per unit of area.

Supersonic flows in action from Science News on Vimeo.

As a stone plows into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in this video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speeds.

Credit: Stephan Gekle/Physical Review Letters 2010

Going Deeper:

People don’t have to go to outer space to make music in new ways. Technologies using computers and sensors are being created that will let people do that right here on Earth.

The musical instruments of the future may be right in front of your eyes and on the tables, walls and windows around you. All it takes to use them is the right hardware, and a little imagination.

In Switzerland, a team of scientists and artists are working together on new technology that can transform almost any surface into a musical instrument. The technology is called MUTE, short for Multi- Touch Everywhere. Using MUTE, a person can use a computer to translate taps on different parts of a table or a wall as different sounds.

For example, you may record and save different sounds on a computer — anything from a snare drum or trumpet to clapping hands or a sneeze. Then, you program your computer to play one of these recorded sound snippets whenever you tap a certain spot on a table top or wall. The left side of a table might play snare drum beats, the right side a melody on a trumpet. If you tap the two sides at the same time, you’ll hear both sounds come together as a song. The system uses a camera and lasers to see where you’ve tapped on the table.

This is the computer program used to assign different sounds to different parts of a surface. Sensors tell the computer when someone taps on one of the colored regions.

Alain Crevoisier / Future Instruments

What’s more, the programmed surface doesn’t even have to be solid — it can float right in front of you, explains musician and MUTE developer Alain Crevoisier. “It can even work in the air,” he says. “Since the lasers are creating a plane of light, what we actually detect is when you cross this plane with either the hands or sticks or mallets.” Imagine, for example, a virtual piano hovering in front of your face.

When the MUTE system is installed on a surface, it also uses acoustic sensors to track the location of a performer’s tap. (For more information on acoustics, see the sidebar below story, “What is acoustics?”) “When you tap the table you generate vibrations,” says Crevoisier, a researcher at the Music Conservatory of Geneva. The vibration travels through the surface as an acoustic wave, and when the vibration strikes the sensor, the sensor sends an electric signal to the computer.

The device is not a musical instrument in the way we normally think about instruments. But that’s part of the beauty of it, says Crevoisier. It allows a person to be creative. “It’s more like we are providing a means for people to design their own instruments,” he says. His system adds a layer of music to already existing sounds. On a regular drum, for example, a drum beat is just the sound of the drum. But on a drum outfitted with MUTE technology, a drum beat could be both the sound of the drum and a control for some other sound layered on top of that.

Crevoisier’s work on new musical instruments grew out of his participation in a project called TAI-CHI (pronounced ty-chee), which stands for “tangible acoustic interfaces for computer-human interactions.” An interface is a device, or a lot of devices working together, that allow people to communicate with machines. A tangible object is one that you can touch. The keyboard of a personal computer, for example, is a tangible interface between you and your computer. So is your mouse.

Sound waves are measured by height, called amplitude, and width, called frequency. At top, the sound waves have a lower frequency and, to our ears, a lower pitch. The bottom image shows a sound wave with a higher frequency, which we would hear as high-pit

NOAA Ocean Explorer

In the TAI-CHI project, Crevoisier and his colleagues showed that acoustics, or the science of sound, could be used to turn any surface into a musical instrument. They also showed that the technology could lead to a new kind of interface.

Here’s how: The sensors pinpoint the place on a surface where a person taps.

One way to do this requires at least three sensors on the surface. When a person taps the table, the sound waves travel to the sensors, and each sensor records the exact time when the waves reached it. By knowing where the sensors are located and what the surface is made of, a computer program can use the waves’ arrival times to figure out exactly where the surface had been tapped.

Another method to pinpoint a tap uses only one acoustic sensor, but it is more complicated. A user needs to fine-tune the device very carefully, and provide the computer with lots of information about the surface material itself.

Acoustic sensors could be used to build new kinds of computers that look nothing like traditional desktop models. Unlike a keyboard and mouse, which require a user to remain in front of a computer screen, acoustic sensors would allow a user to interact with the computer almost anywhere. You could use your fingers to draw a picture on a wall, for example, and record the drawing with your computer.

This device, built with TAI-CHI technology, is called the Sound Rose because when a person taps on the table, a colorful flower appears. A person’s taps are tracked using acoustic sensors, and the images are projected from the ceiling.

Alain Crevoisier / Future Instruments

Or, imagine a restaurant owner, who could glue menus to the top of his tables and install acoustic sensors underneath. Diners could then order simply by tapping on the menu. The vibration from the tap would be picked up by the sensors, which would be able to figure out where the tap came from. A computer could match that location to a dish on the menu and send the order to the kitchen.

In another example, perhaps someone in a wheelchair could mark a spot on the wall, a table top or the arm of a chair to serve as a switch. It might be for turning on or off a light, for turning up or down the volume on a television or even for sending out a distress alarm. A simple tap on the spot could trigger the sensors, which could relay the information to a computer. The technology would allow people to make any surface into an interface to control some action.

Crevoisier isn’t the only one looking at ways to use acoustic sensors in future devices. Another European company, for example, is finding ways to use them to make a “smart apartment,” where any surface — mirrors, tables, counters, walls — can be used to interact with the house computer, to do tasks like change the lighting, turn on the television or raise the temperature.

The Touch Wall, created by Microsoft Corporation, made its debut in May of this year.

Microsoft

Other researchers around the world are developing other kinds of new tangible interfaces, though not all of them use acoustic sensors. The computer company Microsoft has developed a device called TouchWall, for example, which converts almost any surface to a computer interface by using sophisticated laser trackers, cameras and a projector.

Look around again. The future of computing and of musical instruments may be all around you.

Going Deeper:

News Detective: What is Acoustics?

Additional Information

Questions about the article

Certainly, many bats are active at night and asleep during the day. They have sharp teeth. A few species do feed on blood. These vampire bats, which are actually quite small and rare, typically target birds, cattle, pigs, and other animals. When most bats are hungry, however, they stick to insects or fruit.

A big brown bat shows its sharp teeth.

Public Health Image Library

Because they usually hunt at night, bug-eating bats have developed a special system for finding insects in the dark. In much the same way as dolphins use sound to locate objects underwater, bats use information from sounds to “see” their prey in the dark. This process is called echolocation.

Scientists are studying bats and their use of echolocation to learn more about how bats process information to understand and adapt to the world around them.

High-pitched squeals

When bats use echolocation to hunt, they make high-pitched squeals that people can’t hear but that other bats can detect. If there’s a bug or another object nearby, the sound bounces off the object and comes back to the bat.

A gray bat in flight.

U.S. Fish & Wildlife Service

“They can tell where the bug is by listening to the difference in the sound at the left and right ears,” says Cindy Moss, a psychologist at the University of Maryland in College Park.

And a bat can tell how far away an object is by keeping track of the time between when it makes the sound and when the echo returns, she says. If an object is nearby, the sound comes back quickly. If the object is farther away, the reflected sound takes longer to travel back to the bat’s ear.

“They’re performing calculations in their minds all the time in much the same way that we perform calculations about what it is that we see,” says Ellen Covey. She studies bats at the University of Washington in Seattle.

Big brown bats

Moss is interested in big brown bats, which live in dark places such as attics and barns and under the eaves of buildings throughout most of the United States and Canada.

Big brown bats live throughout most of the United States and Canada.

Centers for Disease Control and Prevention

Using echolocation, a big brown bat can swoop down, capture a bug, and eat it—all in about 2 seconds.

Scientists studying big brown bats in the wild have observed that they change both the pitch and the timing of their calls depending on whether they’re looking for food or whether they’ve already found a bug that looks like a tasty dinner.

When they’re searching for food, bats tend to use longer sound pulses to locate bugs flying in the open. When they notice a bug, they begin to make fast, high-pitched noises that, in general, get faster and higher as they close in for the kill.

Using sound like a flashlight

Clearly, bats know how to zero in on their food. They not only make appropriate sounds but also appear to have no trouble analyzing the echoes that come back to their ears.

A big brown bat captures a mealworm dangling on a thread in this lab experiment.

Brown University photo by Steven P. Dear

To learn more about how bats process the information that they gather, Moss and her coworkers set up lab experiments to see how bats use echolocation while doing simple tasks.

The researchers tied a bug to a string and hung it from the ceiling of a dark room. Then, they used high-speed infrared cameras and special audio equipment to record a bat’s calls as it hunted for the bug.

Moss and her students found a close connection between where a bat “looks” and how it flies. A bat directs its sound beam, like a flashlight, ahead of its flight. As the bat moves its head in the direction in which it calls, its body turns in the same direction

This illustration shows the direction in which a bat’s sound beam points at different times, as seen from overhead, as it searches for (widely spaced signals), identifies (closely spaced signals), and captures a tethered insect (top).

Courtesy of Cindy Moss, University of Maryland

Bats convert what they see into what they do in much the same way that people convert what they see into action. For example, if you’re hungry and see an ice cream truck to your left, your head turns to the left. Your body then turns to the left as you begin walking to the truck.

The researchers also discovered that when a bat makes calls at a fast rate as it turns, it turns more quickly than it does when calling at a slower rate.

Obstacle course

Finding a bug that’s hanging from a string in the middle of an empty room is easy for a bat. That’s because bats normally hunt in environments in which insects flit among other objects, such as trees, buildings, or animals.

To make things harder for the bats in the lab, Moss and her coworkers tied bugs to a string and hung them in front of a plant. Now, the bats would have to sort out echoes that came not only from the bug but also from the plant.

A bat tries to locate an insect tethered beside a plant.

Courtesy of Cindy Moss, University of Maryland

A plant’s echoes can serve as camouflage for a bat’s prey. In that case, a bat has a more difficult task and has to use different strategies to find the prey, Moss says.

The researchers found that, when the hanging bug was 20 centimeters from a plant, a bat located the bug about 80 percent of the time. When the bug was 10 centimeters from the plant, the bat needed more time and caught a meal about half as often.

When the bug was hanging 40 centimeters away, the bat hunted almost as efficiently as if the plant weren’t in the room.

So, even with extra, misleading echoes, bats can find food. But, it turns out, they adjust the timing of their signals to help make up for the clutter.

Decoding sounds

Many bat researchers have studied both the length and pitch of individual sounds that bats make as they hunt. In their studies, Moss and her coworkers observed something new. Bats can adjust their sound output to respond to information they receive by echolocation. What’s more, they produce pulses in distinctive patterns.

When bats hunt for a bug against a background object, they repeat particular patterns of sound, Moss says. She calls these patterns strobe groups. They resemble the repeated flashes of a strobe light.

“We started seeing sound groups again and again and again,” Moss says. “And we did some recording in the field, and there they were.”

Such groups of sounds may allow a bat to sharpen its view of a space and help it recognize the difference between an insect and shrubs or grass in the background.

The work that Moss and her students have done in identifying these patterns is a useful step in understanding how a bat processes information, Covey says.

“It’s important to look at the context of the sounds and not just the individual sounds,” she adds. Sounds mean different things depending on what went before and what comes after.

So, in the seconds that it takes a bat to swoop down and capture a meal, it’s using a complicated sonar system that scientists are only beginning to understand. There might even be similarities between the way that bats process their echolocation patterns and the way that people process sound patterns to understand speech.

That’s something to ponder the next time you’re out at night during the spring or summer and happen to glimpse a bat on the hunt.

Going Deeper:

Additional Information

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Word Find: Bat Echolocation

The Hyper-X recently broke the record for air-breathing jet planes when it traveled at a hypersonic speed of seven times the speed of sound. That’s about 5,000 miles per hour. At this speed, you’d get around the world—flying along the equator—in less than 5 hours.

Powered by its scramjet engines (shown in gold), the black, unmanned X-43A flew at a record speed for an air-breathing jet plane. The experimental plane is only 12 feet long.

NASA

The Hyper-X is an unmanned, experimental aircraft just 12 feet long. It achieves hypersonic speed using a special sort of engine known as a scramjet. It may sound like something from a comic book, but engineers have been experimenting with scramjets since the 1960s.

For an engine to burn fuel and produce energy, it needs oxygen. A jet engine, like those on passenger airplanes, gets oxygen from the air. A rocket engine typically goes faster but has to carry its own supply of oxygen. A scramjet engine goes as fast as a rocket, but it doesn’t have to carry its own oxygen supply.

A scramjet’s special design allows it to extract oxygen from the air that flows through the engine. And it does so without letting the fast-moving air put out the combustion flames. However, a scramjet engine works properly only at speeds greater than five times the speed of sound.

A booster rocket carried the Hyper-X to an altitude of about 100,000 feet for its test flight. The aircraft’s record-beating flight lasted just 11 seconds.

In the future, engineers predict, airplanes equipped with scramjet engines could transport cargo quickly and cheaply to the brink of space. Hypersonic airliners could carry passengers anywhere in the world in just a few hours.

Out of the three experimental Hyper-X aircraft built for NASA, only one is now left. The agency has plans for another, 11-second hypersonic flight, this time at 10 times the speed of sound.

Hang on tight!—S. McDonagh

Going Deeper:

Weiss, Peter. 2004. Soaring at hyperspeed: Long-sought technology finally propels a plane. Science News 165(April 3):213-214. Available at http://www.sciencenews.org/articles/20040403/fob6.asp .

You can learn more about NASA’s Hyper-X plane at oea.larc.nasa.gov/PAIS/FS-2003-07-77-LaRC.html and www.nasa.gov/missions/research/x43-main.html (NASA).