Click here to purchase the entire book in PDF format. Travelling waves and reflections Go find a long piece of rope, tie one end of it to a solid object like a fence rail and pull it tight so that you look something like Figure 3.35 Figure 3.35: You, a rope and a fence post before anything happens. Then, with a flick of your wrist, you quickly move your end of the rope up and back down to where you started. If you did this properly, then the rope will have a bump in it as is shown in Figure 3.36. This bump will move quickly down the rope towards the other end. Figure 3.36: You, a rope and a fence post just after you have flicked your wrist to put a bump in the rope. When the bump hits the fence post, it can't move it because fence posts are harder to move than rope molecules. Since the fence post is at the end of the rope, we say that it terminates the rope. The word termination is one that we use a lot in this book, both in terms of acoustic as well as electronics. All it means is the end of the system - the rope, the air, the wire... whatever the ``system'' is. So, an acoustician would say that the rope is terminated with a high impedance at the fence post end. Remember back to the analogy in Section 3.2.2 where the person in the front was pushing on a concrete wall. You pushed the person ahead of you and you wound up getting pushed in the opposite direction that you pushed in the first place. The same is true here. When the wave on the rope refelects off a higher impedance, then the reflected wave does the same thing. You pull the rope up, and the reflection pulls you down. This end result is shown in Figure 3.37. Figure 3.37: You, a rope and a fence post just after the wave has reflected off the fence post (the high impedance termination).