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  • 多普勒效果

    Police car with flashing lights parked at night, illustrating the Doppler effect concept.

    Has this ever happened to you? You hear a siren from a few blocks away. The source is a police car that is racing in your direction. As the car approaches, zooms past you, and then speeds off into the , the sound of its siren keeps changing in pitch . First the siren gets higher in pitch, and then it suddenly gets lower. Do you know why this happens? The answer is the Doppler effect.
    ::这难道没有发生在你身上吗?你听到从几条街外传来的警笛声。源头是一辆朝你方向跑来的警车。随着汽车靠近你,放大你,然后加速进入,它的警笛声在投球中不断变换。首先,警报声在投球中上升,然后突然下降。你知道为什么会这样吗?答案是多普勒效应。

    What Is the Doppler Effect?
    ::多普勒效应是什么?

    The Doppler effect is a change in the frequency of that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.) The Figure shows how the Doppler effect occurs. The sound waves from the police car siren travel outward in all directions. Because the car is racing forward (to the left), the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch. 
    ::多普勒效应是声波源相对于固定听力器移动时发生频率的变化。 (声音源与静态听力器相对移动时也可能发生。 ) 图显示多普勒效应是如何发生的。 警车的声波向外发出警报。 由于汽车正在向前( 向左) , 声波会集中在汽车前面, 分散到车后面。 更近的声波频率更高, 距离更远的声波频率较低。 声波的频率反过来决定声波的声势。 高频声波产生声波, 高声波产生声波, 高声波产生声波, 低声波产生声音, 低声波产生声波。

    A police car emitting sound waves that bunch up in front and spread out behind.

    Experiencing the Doppler Effect
    ::在发生 " 多普勒效应 " 和 " 多普勒效应 " 的

    As the car approaches listener A, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the siren get higher. As the car speeds away from listener B, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the siren get lower. 
    ::当汽车接近听众A时,声音波会越来越近,增加频率。这个听众听到警报器的声势会越来越高。随着汽车远离听众B,声音波会变得更远,减少频率。这个听众听到警报器的声势会越来越低。

    Q: What will the siren sound like to listener A after the police car passes him?
    ::问:警笛在警车经过A号后,

    A: The siren will suddenly get lower in pitch because the sound waves will be much more spread out and have a lower frequency.
    ::A:警报器在投放时会突然降低, 因为声波会分散得多, 频率会更低。

    Launch  the PLIX Interactive below to  further explore how the noise produced by a moving car changes depending on the position of the observer as a result of the doppler effect:
    ::进一步探索移动汽车产生的噪音如何改变, 取决于观察者因多普勒效应所处位置:

    Summary
    ::摘要

    • The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener.
      ::多普勒效应是声波频率的变化,当声波源相对于静止的听众移动时,声波频率就会发生变化。
    • As the source of sound waves approaches a listener, the sound waves get closer together, increasing their frequency and the pitch of the sound. The opposite happens when the source of sound waves moves away from the listener.
      ::随着声波的源头接近听众,声波的频率和声波的声势越来越接近。 而当声波的源头远离听众时,情况正好相反。

    Review
    ::回顾

    1. Define the Doppler effect.
      ::定义多普勒效应。
    2. Explain why the Doppler effect occurs.
      ::解释为什么多普勒效应会发生
    3. Does the siren of a moving police car seem to change pitch to the police officers inside the car? Why or why not?
      ::动动警车的警笛似乎会改变对车内警察的姿态吗?