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The team, led by professor Jian-chun Cheng, will describe its workat the Acoustics 2012 meeting in Hong Kong, May 13-18, a jointmeeting of the Acoustical Society of America (ASA), AcousticalSociety of China, Western Pacific Acoustics Conference, and theHong Kong Institute of Acoustics. Acoustic diodes are analogous to the electric diodes that produceunidirectional flow of current through electronic devices,protecting them from sudden and damaging reversals of flow.Electric diodes, which are akin to the check valves in car engines,work by providing nearly zero resistance to current flow in onedirection and very high resistance in another. However, saysassociate professor and team member Bin Liang, "there is noanalogous method to protect ultrasound sources from the disturbanceof backtracking acoustic waves. Indeed, such unidirectional flow isfar tougher to achieve with acoustic waves than with electriccurrent because sound waves travel just as easily in bothdirections along any given path." The acoustic diode consists of two parts. The first is anultrasound contrast agent (UCA), made from a suspension ofmicrobubbles. The UCA has a strong acoustic nonlinearity, whichmeans it converts the acoustic energy of an incident wave into awave with twice as many pulsations per second. Therefore, Liangsays, "sound waves enter such a material at a particularfrequency and leave with a frequency twice as great." The UCAmicrobubbles come in a broad range of sizes, so they can produceacoustic nonlinearity over a broad frequency range. The second part of the acoustic diode is a superlattice consistingof thin alternating sandwich-like layers of water and glass. Thesuperlattice acts like a filter that allows the sound waves withthe doubled frequency to pass through the material but not theoriginal sound waves. "Hence," Liang says, "if the sound comes from theside of the nonlinear material, it will hit that material first,creating doubled frequency sound that passes through the filter,while any sound coming from the other side at the originalfrequency is blocked before it reaches the doubling layer." In clinical medical imaging using ultrasound, acoustic waves aresent into the body and the reflected waves are received by thescanning instrument and the surrounding sensors to form theultrasound images of the internal organs. "However, some ofthe reflecting waves interfere with the ingoing waves, which maylower the brightness and the resolution of the image. Therefore,preventing waves from coming back toward the ultrasound sourcewould help to improve the quality of the ultrasound image,"Liang says. "In general," he adds, "we hope that the acousticdiode could apply to diverse situations where a special control ofacoustic energy flux is required, for example, to improve thequality and effect of medical ultrasound diagnosis and therapy, orthe design of unidirectional sound barriers.". I am an expert from mobilesecuritydvr.com, while we provides the quality product, such as Mobile Security DVR , China 3G Mobile DVR, Mobile Security DVR,and more.
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