OA图书馆可以的。免费医学文献。 pubmed 上有的文献免费 像我们学校的图书馆内网就给期刊网站付费了，只要用馆内的网络就可以下载，你可以找找看有]
文献一般个事都是PDF的，电脑上下载需要一个阅览器用起来很不方便，我知道有管个手机软件叫云医，你试试方便携带。
通过数据库来查或者网络数据库主要有：中文数据库：万方，CNKI（中国知网），维普，读秀，中国中医药数据检索系统 中文生物医学期刊文献库，生物医学视频资源库，化学专业数据库群等 外文数据库：PubMed，ScienceDirect，Nature，Cell， 康健临床决策循证医学数据库，西文生物医学期刊文献数据库等如果你的学校有买过这些数据库的使用权的话，可以再图书馆的网页上免费使用，但如果没有的话，就需要自己付钱下载文献 另外，我们可以通过googol搜索的学术搜索平台查找相关的文献或者百度一下，]
1、打开百度，进入百度首页。准备进行相关网站的搜索。2、在百度的对话框里面输入相关的网站地址3、进入相关的网站后，就可以看到下面的界面。4、在搜索菜单里面输入要搜索论文文献的主题。5、会看到论文文献的种类有好多种，我们可以选择其中地的一种或者几种都是可以的。6、在使用文献资料时，一般都是使用最新的，那么也可以设置时间。7、最后，设置完成后，点击搜索键即可实现对相关资料的查找。
自19世纪末到20世纪初，在物理学上发现了压电效应与反压电效应之后，人们解决了利用电子学技术产生超声波的办法，从此迅速揭开了发展与推广超声技术的历史篇章。1922年，德国出现了首例超声波治疗的发明专利；1939年发表了有关超声波治疗取得临床效果的文献报道。20世纪40年代末期超声治疗在欧美兴起，直到1949年召开的第一次国际医学超声波学术会议上，才有了超声治疗方面的论文交流，为超声治疗学的发展奠定了基础。1956年第二届国际超声医学学术会议上已有许多论文发表，超声治疗进入了实用成熟阶段。声波是物体机械振动状态（或能量）的传播形式。超声波是指振动频率大于20000Hz以上的，其每秒的振动次数（频率）甚高，超出了人耳听觉的一般上限（20000Hz），人们将这种听不见的声波叫做超声波。由于其频率高，因而具有许多特点：首先是能量集中，其波长比一般声波短得多，因而可以用来切削、焊接、钻孔等。再者由于它频率高，波长短，衍射不严重，具有良好的定向性，工业与医学上常用超声波进行超声探测。超声和可闻声本质上是一致的，它们的共同点都是一种机械振动模式，通常以纵波的方式在弹性介质内会传播，是一种能量的传播形式，其不同点是超声波频率高，波长短，在一定距离内沿直线传播具有良好的束射性和方向性，1MHz=10^6Hz，即每秒振动100万次，可闻声的频率在20~20000Hz之间）。
自19世纪末到20世纪初，在物理学上发现了压电效应与反压电效应之后，人们解决了利用电子学技术产生超声波的办法，从此迅速揭开了发展与推广超声技术的历史篇章。1922年，德国出现了首例超声波治疗的发明专利；1939年发表了有关超声波治疗取得临床效果的文献报道。20世纪40年代末期超声治疗在欧美兴起，直到1949年召开的第一次国际医学超声波学术会议上，才有了超声治疗方面的论文交流，为超声治疗学的发展奠定了基础。1956年第二届国际超声医学学术会议上已有许多论文发表，超声治疗进入了实用成熟阶段。声波是物体机械振动状态（或能量）的传播形式。超声波是指振动频率大于20000Hz以上的，其每秒的振动次数（频率）甚高，超出了人耳听觉的一般上限（20000Hz），人们将这种听不见的声波叫做超声波。由于其频率高，因而具有许多特点：首先是能量集中，其波长比一般声波短得多，因而可以用来切削、焊接、钻孔等。再者由于它频率高，波长短，衍射不严重，具有良好的定向性，工业与医学上常用超声波进行超声探测。超声和可闻声本质上是一致的，它们的共同点都是一种机械振动模式，通常以纵波的方式在弹性介质内会传播，是一种能量的传播形式，其不同点是超声波频率高，波长短，在一定距离内沿直线传播具有良好的束射性和方向性，1MHz=10^6Hz，即每秒振动100万次，可闻声的频率在20~20000Hz之间）。
可以这样说：Zhang D Basic Ultrasonic M Beijing: Science Publisher, 希望能帮到你，满意请采纳，别忘了点个赞!
可以在以下文献网站中检索需要的医学文献：UpToDate数据库用于协助临床医生进行诊疗上的判断、决策的循证医学数据库。Thieme是一家拥有百年历史的医学和科学出版社。Karger出版的学科领域涵盖了医学的传统领域及现代医学的最新发展。美国医学会（American Medical Association, AMA）是世界三大医学会之一HighWire Press是全球最大的提供免费全文的学术文献出版商。涵盖生物科学、人文、医学、物理科学、社会科学等大类。标为free的可免费访问全文。Ovid（Ovid Technologies）在国外医学界被广泛应用。其Databases@Ovid包括300多种医学外文文献数据库，并可直接链接全文期刊和馆藏。Pubmed是美国国家医学图书馆（NLM）所属的国家生物技术信息中心（NCBI）于2000年4月开发的一个基于WEB的生物医学信息检索系统。seek68文献馆整合了大量的中外文献数据库资源。涉及各科领域，包括医学的各个分类。EMBASE内容涉及药学、临床医学、基础医学、预防医学、法医学和生物医学工程等。Clinical trials提供了网站临床试验注册辅导，登记了各种观察性研究和干预性研究，包括药物、器械和手术等干预方式。其注册和查询临床试验均为免费。ClinicalKey临床精钥是信息分析公司爱思唯尔（Elsevier）推出的一个临床决策支持工具，帮助医生快速获取准确、简洁、世界前沿的循证医学知识。
回答 您好，我这边正在为您查询，请稍等片刻，我这边马上回复您~ 您好，很高兴为您解答。方法一STEP 1 在知网官网搜索主题、关键词、题名等信息。 STEP 2 点击【外文文献】，就可以看到搜索词下的全部外文文献。 知网可以自动识别中英文对应搜索内容。 方法二 运用 CNKI学术搜索 中国知网与世界100多家国际出版社达成合作，整合出版了数百个重要的学术数据库，3亿多篇中外文文献。 STEP 1 在主页进行搜索，点击【全文获取】。 STEP 2 点击 Get Access ，可以看到数据库该文章是否免费开放下载权限，可以通过 Check Access 或者单独付费获得文章。 希望以上回答对您有所帮助~ 如果您对我的回答满意的话，麻烦给个赞哦~ 更多11条
怎么长这个的话找的话他通过一个那个，文字的后缀就可以找到了。
可以这样说：Zhang D Basic Ultrasonic M Beijing: Science Publisher, 希望能帮到你，满意请采纳，别忘了点个赞!
原文 Ultrasonic distance meter Document Type and Number:United States Patent 5442592 Abstract:An ultrasonic distance meter cancels out the effects of temperature and humidity variations by including a measuring unit and a reference In each of the units, a repetitive series of pulses is generated, each having a repetition rate directly related to the respective distance between an electroacoustic transmitter and an electroacoustic The pulse trains are provided to respective counters, and the ratio of the counter outputs is utilized to determine the distance being Publication Date:08/15/1995 Primary Examiner:Lobo, Ian J 一、BACKGROUND OF THE INVENTION This invention relates to apparatus for the measurement of distance and, more particularly, to such apparatus which transmits ultrasonic waves between two Precision machine tools must be In the past, this has been accomplished utilizing mechanical devices such as calipers, micrometers, and the However, the use of such devices does not readily lend itself to automation It is known that the distance between two points can be determined by measuring the propagation time of a wave travelling between those two One such type of wave is an ultrasonic, or acoustic, When an ultrasonic wave travels between two points, the distance between the two points can be measured by multiplying the transit time of the wave by the wave velocity in the medium separating the two It is therefore an object of the present invention to provide apparatus utilizing ultrasonic waves to accurately measure the distance between two When the medium between the two points whose spacing is being measured is air, the sound velocity is dependent upon the temperature and humidity of the It is therefore a further object of the,present invention to provide apparatus of the type described which is independent of temperature and humidity 二、SUMMARY OF THE INVENTION The foregoing and additional objects are attained in accordance with the principles of this invention by providing distance measuring apparatus which includes a reference unit and a measuring The reference and measuring units are the same and each includes an electroacoustic transmitter and an electroacoustic The spacing between the transmitter and the receiver of the reference unit is a fixed reference distance, whereas the spacing between the transmitter and receiver of the measuring unit is the distance to be In each of the units, the transmitter and receiver are coupled by a feedback loop which causes the transmitter to generate an acoustic pulse which is received by the receiver and converted into an electrical pulse which is then fed back to the transmitter, so that a repetitive series of pulses The repetition rate of the pulses is inversely related to the distance between the transmitter and the In each of the units, the pulses are provided to a Since the reference distance is known, the ratio of the counter outputs is utilized to determine the desired distance to be Since both counts are identically influenced by temperature and humidity variations, by taking the ratio of the counts, the resultant measurement becomes insensitive to such 三、BRIEF DESCRIPTION OF THE DRAWINGS The foregoing will be more readily apparent upon reading the following description in conjunction with the drawing in which the single FIGURE schematically depicts apparatus constructed in accordance with the principles of this 四、DETAILED DESCRIPTION Referring now to the drawing, there is shown a measuring unit 10 and a reference unit 12, both coupled to a utilization means The measuring unit 10 includes an electroacoustic transmitter 16 and an electroacoustic receiver The transmitter 16 includes piezoelectric material 20 sandwiched between a pair of electrodes 22 and Likewise, the receiver 18 includes piezoelectric material 26 sandwiched between a pair of electrodes 28 and As is known, by applying an electric field across the electrodes 22 and 24, stress is induced in the piezoelectric material If the field varies, such as by the application of an electrical pulse, an acoustic wave 32 is As is further known, when an acoustic wave impinges upon the receiver 18, this induces stress in the piezoelectric material 26 which causes an electrical signal to be generated across the electrodes 28 and Although piezoelectric transducers have been illustrated, other electroacoustic devices may be utilized, such as, for example, electrostatic, electret or electromagnetic As shown, the electrodes 28 and 30 of the receiver 18 are coupled to the input of an amplifier 34, whose output is coupled to the input of a detector The detector 36 is arranged to provide a signal to the pulse former 38 when the output from the amplifier 34 exceeds a predetermined The pulse former 38 then generates a trigger pulse which is provided to the pulse generator In order to enhance the sensitivity of the system, the transducers 16 and 18 are resonantly There is accordingly provided a continuous wave oscillator 42 which provides a continuous oscillating signal at a fixed frequency, preferably the resonant frequency of the transducers 16 and This oscillating signal is provided to the modulator To effectively excite the transmitter 16, it is preferable to provide several cycles of the resonant frequency signal, rather than a single pulse or single Accordingly, the pulse generator 40 is arranged, in response to the application thereto of a trigger pulse, to provide a control pulse to the modulator 44 having a time duration equal the time duration of a predetermined number of cycles of the oscillating signal from the oscillator This control pulse causes the modulator 44 to pass a "burst" of cycles to excite the transmitter When electric power is applied to the described circuitry, there is sufficient noise at the input to the amplifier 34 that its output triggers the pulse generator 40 to cause a burst of oscillating cycles to be provided across the electrodes 22 and 24 of the transmitter The transmitter 16 accordingly generates an acoustic wave 32 which impinges upon the receiver The receiver 18 then generates an electrical pulse which is applied to the input of the amplifier 34, which again causes triggering of the pulse generator This cycle repeats itself so that a repetitive series of trigger pulses results at the output of the pulse former This pulse train is applied to the counter 46, as well as to the pulse generator The transmitter 16 and the receiver 18 are spaced apart by the distance "D" which it is desired to The propagation time "t" for an acoustic wave 32 travelling between the transmitter 16 and the receiver 18 is given by: t=D/V s where V s is the velocity of sound in the air between the transmitter 16 and the receiver The counter 46 measures the repetition rate of the trigger pulses, which is equal to 1/ Therefore, the repetition rate is equal to V s /D The velocity of sound in air is a function of the temperature and humidity of the air, as follows: ##EQU1## where T is the temperature, p is the partial pressure of the water vapor, H is the barometric pressure, Γ w and Γ a are the ratio of constant pressure specific heat to constant volume specific heat for water vapor and dry air, Thus, although the repetition rate of the trigger pulses is measured very accurately by the counter 46, the sound velocity is influenced by temperature and humidity so that the measured distance D cannot be determined In accordance with the principles of this invention, a reference unit 12 is The reference unit 12 is of the same construction as the measuring unit 10 and therefore includes an electroacoustic transmitter 50 which includes piezoelectric material 52 sandwiched between a pair of electrodes 54 and 56, and an electroacoustic receiver 58 which includes piezoelectric material 60 sandwiched between a pair of electrodes 62 and Again, transducers other than the piezoelectric type can be The transmitter 50 and the receiver 58 are spaced apart a known and fixed reference distance "D R " The electrodes 62 and 64 are coupled to the input of the amplifier 66, whose output is coupled to the input of the detector The output of the detector 68 is coupled to the pulse former 70 which generates trigger The trigger pulses are applied to the pulse generator 72 which controls the modulator 74 to pass bursts from the continuous wave oscillator 76 to the transmitter The trigger pulses from the pulse former 70 are also applied to the counter Preferably, all of the transducers 16, 18, 50 and 58 have the same resonant Therefore, the oscillators 42 and 76 both operate at that frequency and the pulse generators 40 and 72 provide equal width output In usage, the measuring unit 10 and the reference unit 12 are in close proximity so that the sound velocity in both of the units is the Although the repetition rates of the pulses in the measuring unit 10 and the reference unit 12 are each temperature and humidity dependent, it can be shown that the distance D to be measured is related to the reference distance D R as follows: i D=D R (1/t R )/(1/t) where t R is the propagation time over the distance D R in the reference unit This relationship is independent of both temperature and Thus, the outputs of the counters 46 and 78 are provided as inputs to the microprocessor 90 in the utilization means The microprocessor 90 is appropriately programmed to provide an output which is proportional to the ratio of the outputs of the counters 46 and 78, which in turn are proportional to the repetition rates of the respective trigger pulse trains of the measuring unit 10 and the reference unit As described, this ratio is independent of temperature and humidity and, since the reference distance D R is known, provides an accurate representation of the distance D The utilization means 14 further includes a display 92 which is coupled to and controlled by the microprocessor 90 so that an operator can readily determine the distance D Experiments have shown that when the distance between the transmitting and receiving transducers is too small, reflections of the acoustic wave at the transducer surfaces has a not insignificant effect which degrades the measurement Accordingly, it is preferred that each transducer pair be separated by at least a certain minimum distance, preferably about four Accordingly, there has been disclosed improved apparatus for the measurement of distance utilizing ultrasonic While an illustrative embodiment of the present invention has been disclosed herein, it is understood that various modifications and adaptations to the disclosed embodiment will be apparent to those of ordinary skill in the art and it is intended that this invention be limited only by the scope of the appended 译文 超声波测距仪 文件类型和数目：美国专利5442592 摘要：提出了一种超声波测距仪来抵消的影响温度和湿度的变化，包括测量单元和参考资料。在每一个单位，重复的一系列脉冲的产生，每有一个重复率，直接关系到各自之间的距离，发射机和接收机。脉冲提供给各自的主机，和比例的反产出是利用确定的距离被衡量的。 出版日期： 1995年8月15日 主审查员：罗保伊恩 一、背景发明 本发明涉及到仪器的测量距离，更特别是，这种仪器传送超声波两点之间。 精密机床必须校准。在过去，这已经完成利用机械设备，如卡钳，微米等。不过，使用这种装置并不容易本身自动化技术。据了解，该两点之间距离才能确定通过测量传播时间的浪潮往返那些两点。这样一个类型的波是一种超声波，或声，海浪。当超声波旅行两点之间，距离两个点之间可以衡量乘以过境的时间波由波速，在中期分开两点。因此，这是一个对象本发明提供仪器利用超声波准确测量两点之间距离。 当中等两个点之间的间距是被衡量的是空气，声速是取决于温度和空气相对湿度。因此，它是进一步对象的，现在的发明，提供仪器的类型所描述的是独立于温度和湿度的变化。 二、综述发明 前述的和额外的对象是达到了根据这些原则的这项发明提供距离测量仪器，其中包括一个参考的单位和测量单位。参考和测量单位是相同的，每个包括一电发射机和接收机一电。间隔发射器和接收器的参考股是一个固定的参考距离，而间距之间的发射机和接收机的测量单位是距离来衡量。在每一个单位，发射机和接收机是再加上由一个反馈环路导致发射机产生的声脉冲是由接收机和转换成一个电脉冲这是然后反馈到发射机，使重复一系列脉冲的结果。重复率脉冲是成反比关系之间的距离发射器和接收器。在每一个单位，脉冲提供一个反。由于参考的距离是众所周知，比例反产出是利用，以确定所期望的距离来衡量。由于这两方面都是相同的影响，温度和湿度的变化，采取的比例罪状，由此产生的测量变得麻木等变化。 三、简要说明图纸 前述将更加明显后，读下列的说明，在与该绘图并在其中单一数字schematically描绘仪器兴建根据这些原则的这项发明。 四、详细说明 谈到现在的绘图，有结果表明，测量单位和10个参考单位12个，均加上一个利用的手段14 。测量单位包括1 10电发射机16日和1电接收机18 。变送器16包括压电材料20夹心阶层之间的对电极的22日和24日。同样，接收机18个，包括压电材料26夹心阶层之间的对电极的28日和30日。作为众所周知，采用电场整个电极22日和24日，强调的是，诱导，在压电材料20 。如果该字段各有不同，如所申请的一个电脉冲，声波是32所产生的。为进一步众所周知，当声波影响到接收器18 ，这诱导应力，在压电材料26 ，导致一种电信号，以产生全国电极28日和30日。虽然压电传感器已说明，其他电声装置，可利用，例如，静电，驻极体或电磁类型。 如表所示，电极28日和30日的接收18岁以下的耦合的投入一34放大器，其输出耦合输入一个探测器36 。探测器36是安排提供一个信号，脉冲前38时，输出放大器34已经超过预定的水平。脉冲前38 ，然后产生一个触发脉冲，这是提供给脉冲发生器40 。在为了提高灵敏度，该系统，传感器16和18岁以下的共振兴奋。有相应的提供了一个连续波振荡器42提供了一个连续振荡信号在一个固定的频率，最好是共振频率的传感器16和18 。这个振荡信号是提供给调制器44 。要有效地激发发射机16 ，可取的做法是提供几个周期的共振频率信号，而不是一个单脉冲或单周期。因此，脉冲发生器40是安排，在回应的应用存在的一个触发脉冲，提供一个控制脉冲调制器44有一个时间的平等的时间，时间预定人数的周期振荡信号从振荡器42 。这个控制脉冲调制器的原因， 44个通过了“水管爆裂”的周期，以激发发射机16 。 当电力是适用于所描述的电路，有足够的噪音在输入到放大器34 ，其输出触发脉冲发生器40至造成了一片叫好声，振荡周期，以提供整个电极22日和24日的发射器16 。变送器16因此产生声波32条，其中影响到接收器18 。接收器18 ，然后产生一个电脉冲，这是适用于输入放大器的34 ，这再次触发原因的脉冲发生器40 。这个周期重演，使重复一系列的触发脉冲结果的输出脉冲前38 。这脉冲列车是应用到46个柜位，以及向脉冲发生器40 。 变送器16日和接收18岁以下的间隔，除了由距离的“ D ” ，它是理想的衡量。传播时间的“ T ”为一声波32往来变送器16日和接收18所给予的： = D的吨/视频s 凡v s是声速在空气中之间的发射机16日和接收18 。柜台46措施重复率触发脉冲，这是平等的1 /汤匙因此，重复率是平等的一至中五的S /四该声速空气中是一个功能的温度和湿度的空气，内容如下： ＃ ＃ ＃ ＃ equ1其中T是温度， P是局部的压力，水汽， H是该气压， γ瓦特和γ一顷的比例不断的压力，具体的热不断货量具体的热水汽和干燥的空气，分别。因此，虽然重复率触发脉冲测量非常准确地反46 ，声速的影响，温度和湿度，使测量的距离d无法确定准确。 根据这些原则的这项发明，参考单位提供的是12 。参考单位12是相同的建设为测量单位的10个，因此，包括一电发射机50个，其中包括压电材料52夹心之间的一对电极的54和56 ，和一电接收机58 ，其中包括压电材料60夹心阶层之间的一对电极60,61,62和64 。再次，传感器以外的其他类型压电可以利用。变送器50和接收五十八顷间隔，除了已知的和固定的参考距离“博士” 。电极60,61,62和64耦合到输入的放大器66 ，其输出是耦合的投入探测器68 。输出探测器68是耦合的脉搏，前70产生触发脉冲。触发脉冲应用到脉冲发生器的72个控制调制器74通过扫射从连续波振荡器76至变送器50 。触发脉冲从脉冲前70也适用于反78 。 最好是，所有的传感器16 ， 18 ， 50和58具有相同的共振频率。因此，振荡器42和76都在运作，频率和脉冲发电机40和第72条提供平等的输出脉冲宽度。 在用法上，测量装置10和参考资料股一十二顷在接近，使该声速在这两个单位是相同的。虽然留级率的脉冲在测量单位， 10和参考资料股十二顷每个温度和湿度的依赖性，能证明的距离D来衡量。 其中T R是传播时间超过距离博士在参考股12 。这种关系是独立于双方的温度和湿度。 因此，产出的柜台46和78所提供的投入微处理器的90个利用的手段14 。微处理器90是适当的程序提供了一个输出是成正比的比例，产出的柜台46和78 ，这反过来又是成正比的重复率分别触发脉冲列车的测量单位， 10和参考资料股12 。作为描述，这个比例是独立的温度和湿度，由于参考的距离，博士，是众所周知的，提供了一个准确的代表性距离四，利用手段， 14日还包括一个显示92这是耦合和控制的微处理器，使90一个经营者可以随时确定的距离四 实验表明，当之间的距离发射和接收传感器是太小了，思考的声波在传感器的表面有一个不小的作用，降低了测量精度。因此，最好是每换一双分开，至少由某一个最小距离，最好是约四英寸。 因此，已披露的改善仪器的测量距离，利用超声波。而一个说明性的体现，本发明已披露者外，据了解，各种修改和适应所披露的体现，将是显而易见的那些普通的技巧与艺术，这是打算把这个发明只限于由范围所附的索赔。
医学硕士研究生论文参考文献肯定是越多越好，这样更能够支持你的观点，而且查重的时候不容易出问题。具体数目没有统一规定，各个学校自己掌握。我是山东大学的，我的毕业论文参考文献是41篇（含外文文献9篇，港台地区文献2篇）
Introduction vibrations of frequencies greater than the upper limit of the audible range for humans—that is, greater than about 20 The term sonic is applied to ultrasound waves of very high Hypersound, sometimes called praetersound or microsound, is sound waves of frequencies greater than 1013 At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 25 × 1013 hertz, it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly TableMany animals have the ability to hear sounds in the human ultrasonic frequency Some ranges of hearing for mammals and insects are compared with those of humans in the T A presumed sensitivity of roaches and rodents to frequencies in the 40 kilohertz region has led to the manufacture of “pest controllers” that emit loud sounds in that frequency range to drive the pests away, but they do not appear to work as Transducers An ultrasonic transducer is a device used to convert some other type of energy into an ultrasonic There are several basic types, classified by the energy source and by the medium into which the waves are being Mechanical devices include gas-driven, or pneumatic, transducers such as whistles as well as liquid-driven transducers such as hydrodynamic oscillators and vibrating These devices, limited to low ultrasonic frequencies, have a number of industrial applications, including drying, ultrasonic cleaning, and injection of fuel oil into Electromechanical transducers are far more versatile and include piezoelectric and magnetostrictive A magnetostrictive transducer makes use of a type of magnetic material in which an applied oscillating magnetic field squeezes the atoms of the material together, creating a periodic change in the length of the material and thus producing a high-frequency mechanical Magnetostrictive transducers are used primarily in the lower frequency ranges and are common in ultrasonic cleaners and ultrasonic machining By far the most popular and versatile type of ultrasonic transducer is the piezoelectric crystal, which converts an oscillating electric field applied to the crystal into a mechanical Piezoelectric crystals include quartz, Rochelle salt, and certain types of Piezoelectric transducers are readily employed over the entire frequency range and at all output Particular shapes can be chosen for particular For example, a disc shape provides a plane ultrasonic wave, while curving the radiating surface in a slightly concave or bowl shape creates an ultrasonic wave that will focus at a specific Piezoelectric and magnetostrictive transducers also are employed as ultrasonic receivers, picking up an ultrasonic vibration and converting it into an electrical Applications in research One of the important areas of scientific study in which ultrasonics has had an enormous impact is When water is boiled, bubbles form at the bottom of the container, rise in the water, and then collapse, leading to the sound of the boiling The boiling process and the resulting sounds have intrigued people since they were first observed, and they were the object of considerable research and calculation by the British physicists Osborne Reynolds and Lord Rayleigh, who applied the term cavitation to the process of formation of Because an ultrasonic wave can be used carefully to control cavitation, ultrasound has been a useful tool in the investigation of the The study of cavitation has also provided important information on intermolecular Research is being carried out on aspects of the cavitation process and its A contemporary subject of research involves emission of light as the cavity produced by a high-intensity ultrasonic wave This effect, called sonoluminescence, is believed to create instantaneous temperatures hotter than the surface of the S The speed of propagation of an ultrasonic wave is strongly dependent on the viscosity of the This property can be a useful tool in investigating the viscosity of Because the various parts of a living cell are distinguished by differing viscosities, acoustical microscopy can make use of this property of cells to “see” into living cells, as will be discussed below in Medical Ranging and navigating Sonar (sound navigation and ranging) has extensive marine By sending out pulses of sound or ultrasound and measuring the time required for the pulses to reflect off a distant object and return to the source, the location of that object can be ascertained and its motion This technique is used extensively to locate and track submarines at sea and to locate explosive mines below the surface of the Two boats at known locations can also use triangulation to locate and track a third boat or The distance over which these techniques can be used is limited by temperature gradients in the water, which bend the beam away from the surface and create shadow One of the advantages of ultrasonic waves over sound waves in underwater applications is that, because of their higher frequencies (or shorter wavelengths), the former will travel greater distances with less Ranging has also been used to map the bottom of the ocean, providing depth charts that are commonly used in navigation, particularly near coasts and in shallow Even small boats are now equipped with sonic ranging devices that determine and display the depth of the water so that the navigator can keep the boat from beaching on submerged sandbars or other shallow Modern fishing boats use ultrasonic ranging devices to locate schools of fish, substantially increasing their Even in the absence of visible light, bats can guide their flight and even locate flying insects (which they consume in flight) through the use of sonic Ultrasonic echolocation has also been used in traffic control applications and in counting and sorting items on an assembly Ultrasonic ranging provides the basis of the eye and vision systems for robots, and it has a number of important medical applications (see below) The Doppler effect If an ultrasonic wave is reflected off a moving obstacle, the frequency of the resulting wave will be changed, or Doppler- More specifically, if the obstacle is moving toward the source, the frequency of the reflected wave will be increased; and if the obstacle is moving away from the source, the frequency of the reflected wave will be The amount of the frequency shift can be used to determine the velocity of the moving Just as the Doppler shift for radar, an electromagnetic wave, can be used to determine the speed of a moving car, so can the speed of a moving submarine be determined by the Doppler shift of a sonar An important industrial application is the ultrasonic flow meter, in which reflecting ultrasound off a flowing liquid leads to a Doppler shift that is calibrated to provide the flow rate of the This technique also has been applied to blood flow in Many burglar alarms, both for home use and for use in commercial buildings, employ the ultrasonic Doppler shift Such alarms cannot be used where pets or moving curtains might activate Materials testing Nondestructive testing involves the use of ultrasonic echolocation to gather information on the integrity of mechanical Since changes in the material present an impedance mismatch from which an ultrasonic wave is reflected, ultrasonic testing can be used to identify faults, holes, cracks, or corrosion in materials, to inspect welds, to determine the quality of poured concrete, and to monitor metal Owing to the mechanism by which sound waves propagate in metals, ultrasound can be used to probe more deeply than any other form of Ultrasonic procedures are used to perform in-service inspection of structures in nuclear Structural flaws in materials can also be studied by subjecting the materials to stress and looking for acoustic emissions as the materials are Acoustic emission, the general name for this type of nondestructive study, has developed as a distinct field of High-intensity applications High-intensity ultrasound has achieved a variety of important Perhaps the most ubiquitous is ultrasonic cleaning, in which ultrasonic vibrations are set up in small liquid tanks in which objects are placed for Cavitation of the liquid by the ultrasound, as well as the vibration, create turbulence in the liquid and result in the cleaning Ultrasonic cleaning is very popular for jewelry and has also been used with such items as dentures, surgical instruments, and small Degreasing is often enhanced by ultrasonic Large-scale ultrasonic cleaners have also been developed for use in assembly Ultrasonic machining employs the high-intensity vibrations of a transducer to move a machine If necessary, a slurry containing carborundum grit may be used; diamond tools can also be A variation of this technique is ultrasonic drilling, which makes use of pneumatic vibrations at ultrasonic frequencies in place of the standard rotary drill Holes of virtually any shape can be drilled in hard or brittle materials such as glass, germanium, or Ultrasonic soldering has become important, especially for soldering unusual or difficult materials and for very clean The ultrasonic vibrations perform the function of cleaning the surface, even removing the oxide layer on aluminum so that the material can be Because the surfaces can be made extremely clean and free from the normal thin oxide layer, soldering flux becomes Chemical and electrical uses The chemical effects of ultrasound arise from an electrical discharge that accompanies the cavitation This forms a basis for ultrasound's acting as a catalyst in certain chemical reactions, including oxidation, reduction, hydrolysis, polymerization and depolymerization, and molecular With ultrasound, some chemical processes can be carried out more rapidly, at lower temperatures, or more The ultrasonic delay line is a thin layer of piezoelectric material used to produce a short, precise delay in an electrical The electrical signal creates a mechanical vibration in the piezoelectric crystal that passes through the crystal and is converted back to an electrical A very precise time delay can be achieved by constructing a crystal with the proper These devices are employed in fast electronic timing Medical applications Although ultrasound competes with other forms of medical imaging, such as X-ray techniques and magnetic resonance imaging, it has certain desirable features—for example, Doppler motion study—that the other techniques cannot In addition, among the various modern techniques for the imaging of internal organs, ultrasonic devices are by far the least Ultrasound is also used for treating joint pains and for treating certain types of tumours for which it is desirable to produce localized A very effective use of ultrasound deriving from its nature as a mechanical vibration is the elimination of kidney and bladder Diagnosis Much medical diagnostic imaging is carried out with X Because of the high photon energies of the X ray, this type of radiation is highly ionizing—that is, X rays are readily capable of destroying molecular bonds in the body tissue through which they This destruction can lead to changes in the function of the tissue involved or, in extreme cases, its One of the important advantages of ultrasound is that it is a mechanical vibration and is therefore a nonionizing form of Thus, it is usable in many sensitive circumstances where X rays might be Also, the resolution of X rays is limited owing to their great penetrating ability and the slight differences between soft Ultrasound, on the other hand, gives good contrast between various types of soft Ultrasonic scanning in medical diagnosis uses the same principle as Pulses of high-frequency ultrasound, generally above one megahertz, are created by a piezoelectric transducer and directed into the As the ultrasound traverses various internal organs, it encounters changes in acoustic impedance, which cause The amount and time delay of the various reflections can be analyzed to obtain information regarding the internal In the B-scan mode, a linear array of transducers is used to scan a plane in the body, and the resultant data is displayed on a television screen as a two-dimensional The A-scan technique uses a single transducer to scan along a line in the body, and the echoes are plotted as a function of This technique is used for measuring the distances or sizes of internal The M-scan mode is used to record the motion of internal organs, as in the study of heart Greater resolution is obtained in ultrasonic imaging by using higher frequencies—, shorter A limitation of this property of waves is that higher frequencies tend to be much more strongly Because it is nonionizing, ultrasound has become one of the staples of obstetric During the process of drawing amniotic fluid in testing for birth defects, ultrasonic imaging is used to guide the needle and thus avoid damage to the fetus or surrounding Ultrasonic imaging of the fetus can be used to determine the date of conception, to identify multiple births, and to diagnose abnormalities in the development of the Ultrasonic Doppler techniques have become very important in diagnosing problems in blood In one technique, a three-megahertz ultrasonic beam is reflected off typical oncoming arterial blood with a Doppler shift of a few kilohertz—a frequency difference that can be heard directly by a Using this technique, it is possible to monitor the heartbeat of a fetus long before a stethoscope can pick up the Arterial diseases such as arteriosclerosis can also be diagnosed, and the healing of arteries can be monitored following A combination of B-scan imaging and Doppler imaging, known as duplex scanning, can identify arteries and immediately measure their blood flow; this has been extensively used to diagnose heart valve Using ultrasound with frequencies up to 2,000 megahertz, which has a wavelength of 75 micrometre in soft tissues (as compared with a wavelength of about 55 micrometre for light), ultrasonic microscopes have been developed that rival light microscopes in their The distinct advantage of ultrasonic microscopes lies in their ability to distinguish various parts of a cell by their Also, because they require no artificial contrast mediums, which kill the cells, acoustic microscopy can study actual living Therapy and surgery Because ultrasound is a mechanical vibration and can be well focused at high frequencies, it can be used to create internal heating of localized tissue without harmful effects on nearby This technique can be employed to relieve pains in joints, particularly in the back and Also, research is now being carried out in the treatment of certain types of cancer by local heating, since focusing intense ultrasonic waves can heat the area of a tumour while not significantly affecting surrounding Trackless surgery—that is, surgery that does not require an incision or track from the skin to the affected area—has been developed for several Focused ultrasound has been used for the treatment of Parkinson's disease by creating brain lesions in areas that are inaccessible to traditional A common application of this technique is the destruction of kidney stones with shock waves formed by bursts of focused In some cases, a device called an ultrasonic lithotripter focuses the ultrasound with the help of X-ray guidance, but a more common technique for destruction of kidney stones, known as endoscopic ultrasonic disintegration, uses a small metal rod inserted through the skin to deliver ultrasound in the 22- to 30-kilohertz frequency Infrasonics The term infrasonics refers to waves of a frequency below the range of human hearing—, below about 20 Such waves occur in nature in earthquakes, waterfalls, ocean waves, volcanoes, and a variety of atmospheric phenomena such as wind, thunder, and weather Calculating the motion of these waves and predicting the weather using these calculations, among other information, is one of the great challenges for modern high-speed TableAircraft, automobiles, or other rapidly moving objects, as well as air handlers and blowers in buildings, also produce substantial amounts of infrasonic Studies have shown that many people experience adverse reactions to large intensities of infrasonic frequencies, developing headaches, nausea, blurred vision, and On the other hand, a number of animals are sensitive to infrasonic frequencies, as indicated in the T It is believed by many zoologists that this sensitivity in animals such as elephants may be helpful in providing them with early warning of earthquakes and weather It has been suggested that the sensitivity of birds to infrasound aids their navigation and even affects their One of the most important examples of infrasonic waves in nature is in Three principal types of earthquake wave exist: the S-wave, a transverse body wave; the P-wave, a longitudinal body wave; and the L-wave, which propagates along the boundary of stratified L-waves, which are of great importance in earthquake engineering, propagate in a similar way to water waves, at low velocities that are dependent on S-waves are transverse body waves and thus can only be propagated within solid bodies such as P-waves are longitudinal waves similar to sound waves; they propagate at the speed of sound and have large When P-waves propagating from the epicentre of an earthquake reach the surface of the Earth, they are converted into L-waves, which may then damage surface The great range of P-waves makes them useful in identifying earthquakes from observation points a great distance from the In many cases, the most severe shock from an earthquake is preceded by smaller shocks, which provide advance warning of the greater shock to Underground nuclear explosions also produce P-waves, allowing them to be monitored from any point in the world if they are of sufficient The reflection of man-made seismic shocks has helped to identify possible locations of oil and natural-gas Distinctive rock formations in which these minerals are likely to be found can be identified by sonic ranging, primarily at infrasonic