higher; louder The amplitude of a sound wave indicates its energy. And louder sounds have more energy. With that in mind, let's look at the options and see what makes sense. higher; louder * Looks good. Higher amplitude means more energy and that makes for a louder sound. This is completely correct. higher; quieter * Not right. We have more energy as indicated by a higher amplitude. But that should make the sound louder, not quieter. So this is definitely wrong. higher; weaker * This is just like the previous option instead of saying quieter, it says weaker. It didn't make sense then and it still doesn't make sense. Wrong answer. lower; louder * So a weaker signal means a louder sound? That doesn't make sense and it is wrong.