Listen and Read 62- Submarine communications

 TQT LISTEN AND READ 62 ~ GOOD VIBRATIONS 
LISTEN AND READ 62 
Submarine communications 
Good vibrations 
 RADIO WAVE do not travel well underwaer. That is why ships employ sonar rather 
than radar to plumb the briny depths. Messages broadcast through the ocean need to be 
sonic, too. For that purpose people often use acoustic modems, which can turn 
electronic signals into sound, and vice versa, like an old-fashioned acoustic coupler for 
a telephone. Such instruments need power, though. And if they are sitting on the seabed, 
replacing their batteries is a serious chore. But Fadel Adib of the Massachusetts Institute 
of Technology (MIT) may have the answer. A device he has created and tested not only 
broadcasts and receives sound—it is powered by sound as well. 
 The core of Dr Adib’s invention is called a broadband resonator. Typically, an 
object resonates strongly at only one or a few frequencies. This is why a singer can 
shiver a wineglass into fragments by holding a particular note—but only that note, and 
no other. A broadband resonator, by contrast, can receive or transmit sound across a 
range of frequencies. 
 Dr Adib’s resonator consists of two nested hollow ceramic cylinders with a layer of 
polymer sandwiched between them. This structure has many interacting resonance 
modes. It is this that gives it its frequency range. The trick that turns it into a power 
source is that the ceramics are piezo-electric—meaning they can convert the vibrations 
of acoustic energy into electrical energy and vice versa. And, the ocean being a noisy 
place, there is a lot of acoustic energy around to convert. A device powered by a 
piezoelectric broadband resonator can thus constantly replenish its batteries without 
them having to be changed. 
 The resonator also, though, has a second use. It acts as an acoustic modem that 
receives instructions to and broadcasts data from the instrument it is part of. To prove 
this works, Dr Adib and his colleagues used a resonator-based acoustic modem to 
communicate 60 metres across the Charles river, which separates mit’s home town of 
Cambridge from Boston—and, indeed, flows directly past the front of the institute. The 
Charles is nowhere near as noisy as the open ocean, so they had to supply the sound to 
power the resonator artificially, using an underwater loudspeaker. Thus supplied, 
however, the device was able to transmit data at a rate of 20 kilobits a second. This is 
about the same as a conventional acoustic modem. 
 Dr Adib has also, by attaching the resonator to an appropriate sensor, used it to 
transmit information about water temperature, acidity and salinity. Indeed, he sees 
sensors as an important market for the new devices. One application would be 
monitoring conditions in fish farms. Another would be in tracking tags for sea 
creatures—though the current minimum size of a resonator means this would, for the 
moment, be practical only for large animals such as whales. 
 Ring my chimes 
 Resonators could be employed, as well, as nodes in underwater communications 
networks—extending the range over which a message can be sent. And they might be 
used in underwater navigation beacons that would provide precise location data to 
submersibles unable (because signals from satellites are radio waves) to employ the 
global positioning system or one of its equivalents for the purpose. More specifically, 
America’s navy, which is sponsoring the project, has plans to use resonator-powered 
devices as sentries. An array of such devices could calculate the range and direction of a 
source of sound such as a ship or submarine and send it back to base. 
 Dr Adib and his team are now working on extending the devices’ capabilities. Their 
immediate goals include communicating between pairs of them over a distance of a 
kilometre, and building networks that have hundreds of nodes. [The Economist US, 17.10.2020] 
 Notes: 
 - piezo-electric: Hiện tượng xảy ra như sau: người ta tìm được một loại chất có tính chất hóa học gần giống gốm (ceramic) và 
nó có hiệu ứng thuận nghịch: khi áp vào nó một trường điện thì nó biến đổi hình dạng, và ngược lại khi dùng lực cơ học tác 
động vào nó thì nó tạo ra điện tích trên bề mặt xác định. 
 Nó như một máy biến đổi trực tiếp từ năng lượng điện sang năng lượng cơ học và ngược lại. Nếu như theo chiều hướng thuận, 
có nghĩa là tác dụng lực lên vật thì sẽ sinh ra điện và ngược lại là áp điện nghịch: tác động hiệu thế vào vật thì sẽ sinh ra công 
biến dạng làm biến đổi lực. Một vật được cấu tạo bởi ba yếu tố PZT (chì Pb, zorconi, titan) sẽ có tính chất áp điện. (VD: thạch 
anh).