THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

484 17. Commercial and Medical Ultrasound ApplicationsFigure 17.4. Magnetostrictive delay line.attenuation is found from the decay of the maxima of the periodic amplitude plottedas the function of the distance x between the transmitter and the reflector increases.Ultrasonic Delay LinesDelay lines are used to store electrical signals for finite time periods. These are usedin computers to store information to be extracted for a later stage of calculation. Amethod for generating the delay is to convert those signals into ultrasonic wavesthat then travel through a material to be reconverted into their original forms. Thesimplest ultrasonic delay line is a crystal transducer radiating into a column ofliquid, such as mercury, that terminates at a reflector. An adjustment of the delaytime can be effected by changing the position of the reflector relative to the crystal.As liquid delay devices are not always convenient to use, solid delay lines aremore common. For delay times of a few microseconds, only a few centimeters ofa solid rod or block is sufficient. The delay time may be doubled by using shearwaves instead of longitudinal waves. If even longer delay times are required, say,in the order of a few milliseconds, very long acoustic paths are necessary. Thesecan be achieved by using materials in the form of polygons in which large numbersof multiple reflections can occur. The solid delay line has the disadvantage thatthe delay time usually cannot be varied. However, the magnetostrictive delay line,illustrated in Figure 17.4, can be varied in length and unwanted reflections areavoided by coating the ends of the rods with grease, which completely absorbs thesound waves. The line may be a wire, a rod, or some sort of ribbon of ferromagneticmaterial such as nickel. An electrical signal applied to coil A induces through amagnetostricive effect a sound pulse in the line. The pulse travels along the lineand induces an electrical signal in coil B by the reverse magnetostrictive effect.The permanent magnets C and D provide the requisite polarization.Measuring Thicknesses through the Pulse TechniqueWhen the pulse technique is used for gauging thicknesses, better results areachieved through the use of a variable delay line. Two pulses are generated simultaneously.One pulse is sent through the sample and the other through a delay