听力原文： Today we are going to practice evaluating a main tool used when addressing groups—the voice. There are three main elements that combine to create either a positive or negative experience for listeners. They can result in a voice that is pleasing to listen to and can be used effectively. Or they can create a voice that doesn't hold attention. Or even worse, causes an adverse reaction. The three elements are volume, pitch and pace. When evaluating volume, keep in mind that a good speaker will adjust to the size of both the room and the audience. Of course, with an amplifying device like a microphone, the speaker can use a natural tone. But speaker should not be dependent on microphones. A good speaker can speak loudly without shouting. The second element, pitch is related to the highness or lowness of the sounds. High pitches are for most people more difficult to listen to. So, in general, speaker should use the lower registers of their voice. During a presentation, it's important to vary pitch to some extent in order to maintain interest. The third element, pace, that is how fast or slow words and sounds are articulated should also be varied. A slower pace can be used to emphasize important points. Note that the time spent not speaking can be meaningful too. Pauses ought to be used to signal transitions or create anticipation. Because a pause gives the listener time to think about what was just said or even to. predict what might come next, it can be very affective when moving from one topic to another. What I'd like you to do now is watch and listen to a videotape and use the forms I gave you to rate the speaking voices you hear. Then tonight, I want you to go home and read a passage into a tape recorder and evaluate your own voice. According to the professor, what can a speaker do to keep an audience's attention?