经过本章的学习，我们知道，人要听清声音，除了听觉系统要正常外，还必须满足下列条件：
一定要有正在______的物体，即发声体．发声体和人耳之间必须有传声的______．发声体振动的______一定在人的听觉范围内．声音到达人耳处的响度至少要达到______dB．
人要听清声音，除了听觉系统要正常外，还需要满足下列条件：
①要有正在振动的物体，即发声体；
②发声体和人耳之间必须有传声的介质．
③发声体振动的频率一定在人的听觉范围内，即人能够听到的声音频率范围是20Hz～20000Hz；
④声音到达人耳处的响度至少要达到0dB．
故答案为：振动、介质、频率、0．
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| Perhaps you ride a bicycle to school. Riding a bicycle can be great fun. Do you know how to ride a bicycle 1 ? Careful boys and girls learn to ride their bicycle well 2 they go out on the road. How can you tell 3 you are a good rider? If you can ride a bicycle in a straight line, you can ride well. If you can't ,you should practise riding 4 a safe place. To be safe on a bicycle, you 5 to use both hands and both feet. 6 your feet on the pedals and your hands on the handle bars at all times. Do you have a basket on your bicycle? If so, put your books and packages in it. Then you can 7 both hands on the handle bars. You know that your bicycle is built for one person. It is not safe to carry anyone 8 you. 9 is this so? Keep 10  safe by safety rules when you ride on a bicycle. Watch where you are going at all times. | |||
| ( )1. A. safe ( )2. A. as soon as ( )3. A. that ( )4. A. in ( )5. A. must ( )6. A. Keep ( )7. A. keep ( )8. A. to ( )9. A. Where ( )10. A. it | B. safety B. before B. how B. on B. need B. Make B. make B. for B. Why B. yours | C. safely C. after C. what C. at C. may C. Let C. let C. with C. What C. you | D. save D. until D. whether D. to D. can D. Get D. get D. by D. How D. yourself |