想要提升自己的托福听力反应速度，除了不断做好托福托福听力练习和词汇累积之外，一些相关的背景知识也会为大家的备考带来很大的帮助。那么，在以下的内容中就为大家整理了托福听力必备背景知识。
噪音影响
I'm grad to see so many of you here. We've become really alarmed over the health center by the number of students we are seeing, who are experiencing hearing loss. First, I want to go over some basic about hearing. Then we can take a look at our school environment and see if we can figure out some ways to protect hearing. The leading cause of preventable hearing loss is excessive noise. Too much moderate noise for a long time or some types of intense noise for even a short time can damage hearing. Loudness is measured in units called decibels. One decibel is the lowest sound that the average person can here. Sounds up to 80 decibels generally aren't harmful. That's noise like traffic on a busy street. But anything louder than 80 decibels, especially with continuous exposure, may eventually hurt your hearing. Once you are up to around 140 decibels, that's like a jet plane taking off, then you might even feel pain in your ears. And pains are sure sign that your hearing's at risk. Even one exposure to a really loud noise at close range can cause hearing loss. So what you need to do is limit your exposure to harmful levels. If you pass along this handout, we can take a look at the decibel level of some common campus sounds. Notice how loud those horns are that people take to football games. They are really dangerous if blown right behind you. Now, let's try to generate a list of damaging noises
