三基试题名词解释（Three questions Glossary）.doc

三基试题名词解释（Three questions Glossary） 12 transducer: a device that allows electrical energy and mechanical energy to change with each other, also called probes. 13, positive piezoelectric effect: some specific materials, caused by relative deviation of positive and negative charge center inside the material in the original contact external tension or pressure under the action of surface charge of opposite sign appeared in the surface of the materials by mechanical force generated electric field, this kind of mechanical energy into electrical energy effect it is called positive piezoelectric effect. The ultrasonic receiving transducer uses a positive piezoelectric effect to convert the reflected (scattered) ultrasonic waves from the human body into voltages. 14, the inverse piezoelectric effect of piezoelectric materials: voltage on the surface along the axis direction and, due to the electric field of the materials of positive and negative charge center displacement, the displacement of the polarization of the material to produce stress, which leads to geometric deformation of the macro, the electrical energy into mechanical energy is called the inverse pressure effect electric effect. The ultrasonic emission transducer adopts an inverse piezoelectric effect, converting the voltage to sound pressure and transmitting it to the human body. 15 and near field: the high frequency ultrasonic waves emitted by planar piezoelectric crystals in the probe propagate in plane waves at some distance in the beginning, and the ultrasonic fields in the bunch are not called near fields. 16, the far field: in the near field at the sound beam began to spread for the far field. 17, visualization: the smallest diameter of an object that can be detected by an echo is the apparent force of an ultrasonic wave. In theory, the largest visible force is the 1/2 of the wavelength. The higher the frequency, the shorter the wavelength, the smaller the object can be detected, the higher the display po