Don’t expect to play a game of Marco Polo by shouting from under the pool’s surface. No one will hear you. Normally, a tiny fraction of one percent of a sound will move from water into the air. But a new type of device might one day greatly up that amount.
Researchers have created a device that frees underwater sound to move through the water’s surface and into the air. It lets 30 percent of the sound come through. That’s 160 times more than usual!
Playing Marco Polo isn’t the only thing for which this device could be used. A future version might help eavesdrop on noisy marine life or spy on submarines. The research was done by a team at Yonsei University in Seoul, South Korea. Eun Bok and his colleagues described their new technology in the January 26 issue of Physical Review Letters.
At the heart of the new device is a rubber sheet stretched across a metal frame with a weight at its center. That weight helps make sure that when it vibrates, it does so at the right frequency. This whole system floats atop the water. As underwater sound waves hit it, the device begins to vibrate. Its frame and rubber sheet vibrate at just the right frequency to help transmit that subsurface sound into the air.
Oliver Wright also worked on the study. He is an applied physicist at Hokkaido University in Sapporo, Japan. “A ‘hard’ surface like a table or water reflects almost 100 percent of sound,” he notes. In other words, it would send the underwater sound back away from the surface.
Both water and air resist the flow of sound. But because of its density, water resists sound some 3,600 times more than air does. Scientists have figured out how to transmit some sound waves from underwater. To do this, they place onto the surface of the water a thick layer of material that has no air bubbles. (Glass, metal or plastic would work.) Instead of blocking the soundwaves, this bubble-free material now helps those sounds pass through. But to cover a large area, this would be costly, and therefore not practical.
Bok’s team developed an alternative approach. Even though it is thin and lightweight, their membrane transmits sound as well as the thick layer of material.
“It’s a tour de force of experimental demonstration,” says Oleg Godin. This physicist, who was not involved in the new study, works at the Naval Postgraduate School in Monterey, Calif. Still, he says, “I’m less impressed by the suggestions and implications about its uses.” He describes them as mostly “wishful thinking.”
The device is not yet ready for prime time, its developers note. For instance, it doesn’t work on sound that arrives at the device from an angle. On the surface of a lake or ocean, therefore, it would transmit sounds only from the small area directly below it. The device also works for only a small range of sounds. Those with a higher or lower pitch would still reflect off the underside of it instead of passing through the water’s surface.
Bok’s team thinks it can overcome these problems and even link many of these devices together into a big sheet. We’ll be keeping an ear on their progress.
acoustics The science of sound.
angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
audible Something that can be heard, usually with ears or other sound-sensing structures.
density The measure of how condensed some object is, found by dividing its mass by its volume.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
marine Having to do with the ocean world or environment.
Marco Polo A game of tag played by children in a pool. The person who is “it” must close his or her eyes and then reach out to others, attempting to find and tag them. Whoever is tagged now becomes “it” and goes through the same motions. To help find the other players, the person who is it can call out “Marco.” All others must then respond from wherever they are, saying “Polo.” In this way, children use acoustics to sense where other players are.
membrane A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.
metal Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
physicist A scientist who studies the nature and properties of matter and energy.
pitch (in acoustics) The word musicians use for sound frequency. It describes how high or low a sound is, which will be determined by the vibrations that created that sound.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
sound wave A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.
submarine A term for beneath the oceans. (in transportation) A ship designed to move through the oceans, totally submerged. Such ships — especially those used in research — are also known as submersibles.
tour de force The term for some achievement that is extremely impressive or successful. It connotes that the accomplishment required great effort or skill. Originally a French expression, it is now widely used in many languages.
transmit (n. transmission) To send or pass along.
vibrate To rhythmically shake or to move continuously and rapidly back and forth.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.