53_doppler - Lesson 53: Doppler Effect Youve probably...

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Lesson 53: Doppler Effect You’ve probably noticed that when an ambulance passes you with its siren going, or a plane flies overhead, you can hear the sound change. I'm not talking about louder or quieter, but rather the pitch of the sound. As it comes closer to you the pitch (frequency) of the siren will seem to increase . When it is moving away, it seems like the frequency decreases . When the true frequency of a source of sound is changed so that an observer hears a different apparent frequency as they move relative to each other, it is known as the Doppler Effect . Christian Doppler described the Doppler Effect in 1842. In his version of the experiment, he had trumpet players ( source of sound ) stand on a flatbed train car and move past a stationary observer . The observer recorded the notes that he thought the trumpet players were playing. Doppler used the differences in the frequencies to help create a formula that could be used to calculate the changes (more on that later). “Bob the Swimming Bug” ( aka Why Frequency Changes ) To see why this change in frequency happens, I want you to imagine a very special bug named “Bob.” I have trained Bob to do a very special trick. He can tread water! When I first trained him to do this, he could only do it while staying in exactly one spot. If we were to look down on Bob as he treads water, we would see waves that look like Figure 1… Notice how Bob is in the middle and all the waves are spreading out evenly from him in the water. If I asked you to stand on the right side of Bob, and then later on the left of Bob, and measure the frequency of the waves (how many pass you per second) you would give the same number. Let’s say that Bob is bobbing up and down at exactly 4 Hz, and he always treads water at this
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53_doppler - Lesson 53: Doppler Effect Youve probably...

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