丁-chaptr 3 sound wave2.pptVIP

波的产生和传播 波线和波面 波在介质中的传播 exercise intensity level ---eg Intensity level and intensity of some common sounds exercise exercise Ultrasonic wave (超声波) and applications A型显示是最基本的超声显示方式,属于幅度调制,即回波的脉冲大小决定显示器中脉冲的幅度.脉冲之间的距离正比于反射界面之间的距离(深度).A超主要用于颅脑的占位性病变的诊断.一维信号 彩色多普勒血流现象仪(简称“彩超”)多采用脉冲超声多蹼勒成像系统.其原理是：利用多道选通技术可在同一时间内获得多个采样容积的回波信号进行频谱或自相关处理,获得速度大小、方向及方差信息,同时滤去迟缓部位的低频信号,再将提取的信号转变为红色、蓝色、绿色的色彩显示. summary In this section, we would like to introduce the the features and applications of ultrasonic wave. It is really not my job to introduce all details about the productions of ultrasonic waves and the principles of all ultrasonic machines. Students who are interested in this part could get more information from our Chinese text book. Wave length: 16.6cm ~ 0.67×10-7m in air 74.0cm ~ 0.30 ×10-6m in water The properties of ultrasonic waves The frequency of ultrasonic waves is between 2×104 and 5×109 Hz. Ultrasonic waves not only have the features of sound wave but also possess the characteristics of high frequencies or short waves and other properties: Directionality (方向性) : Since high frequency corresponding to short wavelength， high frequency waves hardly (几乎不) diffract (衍射: deviation of a wave line at the edge of an obstacle (障碍物)) and propagate in line. 2. High transmission (高透射本领): This property is very important as it can penetrate high density materials like water, fat, muscle (肌肉) , soft tissue (软组织) , bone, lung (肺) tissues and so on but not air. Using these property, the ultrasonic wave can be used to the medical treatments. 3. Reflection (反射): Ultrasonic waves will reflect at the boundary (边界) of medium. Only if the dimension (尺度) of the object is a few times of the wavelength, can the ultrasonic waves be reflected. Due to the short wavelength of the ultrasonic waves, a pretty small object can cause reflection, such as an air bubble (气泡) in steel, lesion (损伤/li: ? ?n/) in biological tissue (生物组织). Echo (回声) generates the ultrasonic image. Ultras