This figure shows two wave forms. The top one is three cycles of a sine wave. The bottom one consists of many cycles of a sine wave but it's amplitude is not constant. At the start of the graph, on the left, where the upper sine wave is zero the lower wave has its starting amplitude. As the top wave increases from zero the amplitude of the lower wave increases. Each cycle has a higher amplitude than the previous one. The increase continues until the top wave reaches its peak. As the top wave starts down the amplitude of the bottom wave decreases. This continues until the top wave reaches its negative peak at which point the bottom wave has been reduced to zero amplitude. When the top wave returns to zero the bottom wave returns to its generic amplitude. As the top wave repeats, the amplitude variations of the bottom wave also repeat. End verbal description.