超声论文您经常发表吗？---超声世界

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Ultrasonic

A term refering to acoustic vibration frequencies greater than about 18,000 Hertz. Ultrasonic waves have a wide variety of applications over an extended range of intensity, with cutting, cleaning, and the destruction of tissue as one extreme and nondestructive testing (NDT) at the other end. The table of contents of the World Congress on Ultrasonics shows the wide range of application: Physical Acoustics - Acoustooptics/Photoacoustics - Saw Techniques - NDE/NDT - Materials Characterization - Acoustic Emission - Underwater Acoustics - Caviation - Sonochemistry - Processing - Motors - Positioning/Levitation - Macrosound/ Atomization - Bio-Effect Research - Exposimetry/Testing - Sonography - Contrast Enhancement - Tissue Characterization - Surgery and Therapy - Lithot**sy.

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Application of Ultrasound

超声波的应用


     

   我们的耳朵只能分辨频率为20至2万赫的声音，频率比人的听频范围高的声波就叫做超声波。不同的动物可听到的声波频率范围不尽相同。狗可以听到一些超声波，所以训练员可以用超声波哨子呼唤他的爱犬。超声波对於蝙蝠更为重要，这种动物是靠超声波来“看”世界的!
    蝙蝠先会发出一连串超声的尖叫声，声波遇到障碍物便会反射，就像我们向山谷拍手会听到回声一样。由于超声波的频率高，相对较少出现绕射现象，所以回声十分清晰。蝙蝠分析回声的方向和回传时间，便可以知道环境的精确图像。人们根据蝙蝠“看”事物的原理，发明了声纳探测器，用来测量水深。船只上的发射器先向海底发射超声波，再由另一些仪器接收和分析反射回来的讯息，从而得到整个海床的面貌。
    医生所用的超声波扫描术可说是超声波最重要的应用。超声波扫描不涉及有害的辐射，远比 X-射线等检验工具安全，所以常用于产前检查 。医生会将一个发出高频超声波 (频率为1-5 兆赫) 的手提换能器，贴着母亲的肚皮进行扫描。声波到达各种身体组织的边界时会有不同程的反射 (例如液体及软组织的边界、软组织及骨的边界)。***收到反射波，便可计算出反射的强度及反射面的距离，以分辨不同的身体组织，并得到胎儿的影像。***使用了压电的原理，把超声波所产生的压力转变成电子讯号，再输送到仪器分析。超声波扫描可以帮助医生测量胎儿的大小以确定产期，检查胎儿的性别、生长速度、头的位置是否正常向下、胎盘的位置是否正常、羊水是否足够，与及监察抽羊水的过程，以保障胎儿的安全等。此外，超声波扫描术也用于妇科检查，它可以帮助医生有效地把生长在**或卵巢中的恶性组织分辨出来。
    超声波扫描术的两个重要分支——多普勒超声波扫描术和立体超声波成像技术，更扩大了超声波在医学上的用途。
    多普勒超声波扫描术已应用了颇长的时间，这技术利用了波动的多普勒效应。反射超声波物体的运动，会改变回声的频率，当物体正向着***移动时，频率便会升高，相反当物体正在远去时，频率便会降低。从回声的频率改变，仪器便可计算到物体的运动速度。多普勒超声波扫描术主要用於检查血液在心脏及主要动脉中的流动速度。血液的流动情况会以一个颜色的影像显示出来，不同的颜色代表不同的流速。这有助医生及早发现胎儿先天性心脏毛病。
    立体超声波成像技术是很新的技术。检查员首先从多个不同角度拍摄胎儿的二维超声波影像，然后利用电脑技术合成胎儿的立体影像。利用这技术可清晰地显示胎儿的样貌 ，甚至摄录到胎儿细致如踢脚或转身等动态，实在为准父母带来不少惊喜。外表的缺憾如兔唇、多指甚至细如斑痣等都可以清楚地显示出来。立体成像技术将会成为未来超声波技术研究的重点。
    此外，高频的超声波带有强大的振动能。将超声波入射载满水的容器，再放入需要的清洗的物件，水的振动便可去除物件上的尘垢，而不需直接接触物件的表面。眼镜公司替我们洗眼镜时就是用这种方法。如果将高能超声波聚焦，能量甚至足以震碎石块，所以可以用来击碎体内结石，使患者免受手术之苦。

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Transducer

A device in which the application of an electric field across the active element produces mechanical deformation of the active element thereby generating ultrasonic vibrations and vice versa. The transducer forms the actual core in all non-destructive ultrasonic inspection procedures: The fact whether a workpiece can be inspected or not depends upon them. In numerous cases ultrasonic inspection becomes only feasible by use of transducers, which have appropriate acoustic properties. In any case the choice of the correct transducer is decisive for the quality and the reliability of inspection results.
Ultrasonic transducers may be classified as follows: Piezoelectric, Electromagnetic, Electrostatic, Magnetostrictive, Laser and other optical, Miscellaneous. Today, ultrasonic transducers work almost exclusively according to the piezoelectric effect.
There are four fundamental transducer types:
(1) straight beam -, (2) angle beam -, (3) delay line -, (4) twin crystal transducers.
Depending on the application, transducers also differ with respect to the size of the active piezoelectric elements, their frequency, bandwidth and the basic design. The sound field characteristics of a transducer, which are expected under normal inspection conditions, are generally derived from the diameter and the frequency of the piezoelectric element.

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WHAT IS ACOUSTIC EMISSION?

Acoustic Emission (AE) is the class of phenomena whereby an elastic wave, in the range of ultrasound usually between 20 KHz and 1 MHz, is generated by the rapid release of energy from the source within a material. The elastic wave propagates through the solid to the surface, where it can be recorded by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal. In this way ***rmation about the existence and location of possible sources is obtained. The basis for quantitative methods is a localization technique to extract the source coordinates of the AE events as accurately as possible.

AE differs from ultrasonic testing, which actively probes the structure; acoustic emission listens for emissions from active defects and is very sensitive to defect activity when a structure is loaded beyond its service load in a proof test.

AE **ysis is a useful method for the investigation of local damage in materials. One of the advantages compared to other NDE techniques is the possibility to observe damage processes during the entire load history without any disturbance to the specimen.

AE **ysis is used successfully in a wide range of applications including: detecting and locating faults in pressure vessels or leakage in storage tanks or pipe systems, monitoring welding applications, corrosion processes, partial discharges from components subjected to high voltage and the removal of protective coatings. Areas where research and development of AE applications is currently being pursued, among others, are process monitoring and global or local long-term monitoring of civil-engineering structures (e.g., bridges, pipelines, off-shore platforms, etc.). Another area where numerous AE applications have been published is fibre-re***rced polymer-matrix composites, in particular glass-fibre re***rced parts or structures. (e.g., fan blades). AE systems also have the capability of detecting acoustic signals created by leaks.

The disadvantage of AE is that commercial AE systems can only estimate qualitatively how much damage is in the material and approximately how long the components will last. So, other NDE methods are still needed to do more thorough examinations and provide quantitative results. Moreover, service environments are generally very noisy, and the AE signals are usually very weak. Thus, signal discrimination and noise reduction are very difficult, yet extremely important for successful AE applications.

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Lamb wave

A type of ultrasonic wave propagation in which the wave is guided between two parallel surfaces of the test object. For an object sufficiently thin to allow penetration to the opposite surface, e.g. a plate having a thickness of the order of a wavelenght or so, Rayleigh waves degenerate to Lamb waves, which can be propagate in a number of modes, either symmetrical or antisymmetrical. The velocity is dependent on the product of frequency and material thickness. Lamb waves are named for Horace Lamb, in honor of his fundamental contributions to this subject. Investigation on Lamb and leaky Lamb waves have been carried out continuously since their discovery and researchers have done theoretical and experimental work for different purposes, ranging from seismology and the ship construction industry to acoustic microscopy, non-destructive testing and acoustic sensors. Synonymous terms are guided wave and plate wave.

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Ultrasonic Fountain at 1 MHz

When an ultrasonic wave strikes a liquid surface and reflects at the air-liquid boundary, momentum is transfered to the fluid and the liquid is levitated. This principle is sometimes used to image ultrasonic sound fields but is usually very small. At high powers, however, the levitation is very dramatic and is often called an ultrasonic fountain. At high power cavitation is often also produced which can cause the fountain to appear to be boiling even though the liquid is not at the boiling temperature. The experiment below uses a 1 MHz bowl shaped transducer with a diameter of 20 mm and a focal length (radius) of 12.5 mm. The transducer case is flipped so that the transducer points up and water is held in the cavity formed by the crystal and the transducer case, as shown below.

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:(偶英语不好哦。呵呵，好多看不懂