SlowBob
FastBob
If the waves are sound or light, since frequency is perceived as pitch or color, the pitch or color changes!
<A HREF="carhorn.au"><b>Play Sound</b></a> Which brings us to the inevitable example that's in all the books or web pages: If a car passes you with horn or siren on continuously, you'll hear the pitch of the sound drop as the car passes. [see note below] This is just what the previous paragraph said: the pitch is high as the car approaches you and low as it moves away after passing. You've probably heard (of) this, but you may not know that the idea was first thought of on purely theoretical grounds, long before there were car horns. Christian Doppler was born in Salzburg, Austria at No. 1 Makart Square (a few doors down from Mozart's place at No. 8 ... the Salzburg Tourist Bureau will be happy to tell you more) in 1803 as the second son of a master stone-mason. He became a math professor and presented his eponymous effect in a study on changes in color due to motion of double stars in 1842. The first experiment verifying Doppler's idea, for sound, was performed in 1845 by the Dutch meteorologist Christoph Buys-Ballot using a train carrying trumpeters! They would play a note while other musicians listened on the side of the track. (This was pre-electronics, after all. How would you have measured small differences in pitch?) [Note to Art Dept: please find clip-art of train carrying trumpeters] BB later got his own name immortalized in 1857 in the law of wind direction in relation to atmospheric pressure in the Northern Hemisphere: if you stand with the wind at your back, the low pressure area will be to your left ... Buys Ballot's Rule...well, maybe not immortalized, but ... You can use a little electronic sound-maker to show the Doppler effect more conveniently but less musically than Buys-Ballot. The idea is just to tie it to one end of a string and whirl it around you. Other people will hear the pitch changing up and down as the source moves in a circle, alternately approaching and retreating from them, like the duck's experience above. This is more than a classroom demo of the Doppler Effect, it's a close analogy to what Doppler was suggesting for colors of starlight. One detail seems different in this real-world demo: the sound source is moving in a circle, not just in and out from the duck like the animation above. Does the side-to-side part of of the circle motion bother the receiver? For once it's simple: only the in-and-out part of the motion has a Doppler effect. This motion is officially called radial velocity since it's along an imaginary ray projecting out from the receiver. Any source velocity can have a component that's radial, which you can find by drawing little triangles around the velocity direction. The other component has to have a more respectable name than side-to-side, too, so it's called transverse. What we've seen so far is that a moving source changes the frequency a stationary observer sees. The useful part comes in when you don't know how fast the source is moving, but you can figure it out from how the frequency is changing: then you've got a technique with a lot of applications, astronomy included, as we'll see. It isn't that hard to take the numbers you can measure for the frequencies and get the speed of the source, which you're trying to find out. You'll probably accept that there is such a method based on what we've seen, but how can you get the formula? (Yeah, I know...look it up! but you won't understand it, and that's the point on this page) Our crack staff of multiple personalities has been hard at work, producing different web pages to let you really understand Doppler quantitatively. The editor couldn't decide which of their efforts was best, so we're letting you decide! Take a look at the candidates, and send us an e-mail with your vote! One final point: what counts is motion of the source or receiver, so the source can be stationary and the receiver moving, or the source moving and the receiver fixed, or any combination. Doppler doesn't care. He's cool. O.K., he cares a little. What the receiver receives depends on the two motions differently. The math becomes a little more complicated than in the voting pages, but you can derive a formula for that case too by similar means. If this is too easy, consider the case of very high velocities, as with light, when you need the Relativistic Doppler Effect! Catch the next wave! [Picky Technical Footnote from above] The car horn passing you makes a simple and dramatic demo, but the situation is more complicated than it looks, and what you hear isn't just the Doppler effect. Your perception of light and sound is complex, with a lot of subconscious brain processing going on, and color and pitch are more than just frequency (though that's the big part). The increase and decrease in loudness of the sound as the car passes also causes pitch changes, and that means you hear more than just the Doppler effect. In fact, even if the sound source wasn't moving, just growing lounder and then softer, there would be a slight rise and then fall of pitch. At great difficulty, we have made a new sound clip, which is the same as the one above except that the loudness is constant in the middle, to let you, our valued audience, hear just the Doppler shift. (OK, actually it's easy with a variety of sound manipulation programs, like Goldwave or Acid Wav which you can download as shareware, or SoundHack for the Mac.) <A HREF="carhornf.au"><b>Play Sound</b></a> What you hear is also more complicated in that the horn's sound is not just a single regular series of waves, but a combination of waves at different frequencies. It turns out that your ear can interpret this combo as a certain pitch, and that since all the frequencies are Doppler shifted by the right amount, the pitch you perceive is Doppler shifted. This is true for most natural sounds, even Buys Ballot's trumpeters, so his experiment was also not as clean as he would have liked. It would be better to have a pure tone of just one frequency and see how it changes, but it turns out our ear can handle the complex tones of instrument horns and car horns. Thus the standard Doppler example is legit. Copyright © 1999, Steve Donnelly Return to CPO Home Page
Christian Doppler was born in Salzburg, Austria at No. 1 Makart Square (a few doors down from Mozart's place at No. 8 ... the Salzburg Tourist Bureau will be happy to tell you more) in 1803 as the second son of a master stone-mason. He became a math professor and presented his eponymous effect in a study on changes in color due to motion of double stars in 1842.
The first experiment verifying Doppler's idea, for sound, was performed in 1845 by the Dutch meteorologist Christoph Buys-Ballot using a train carrying trumpeters! They would play a note while other musicians listened on the side of the track. (This was pre-electronics, after all. How would you have measured small differences in pitch?) [Note to Art Dept: please find clip-art of train carrying trumpeters] BB later got his own name immortalized in 1857 in the law of wind direction in relation to atmospheric pressure in the Northern Hemisphere: if you stand with the wind at your back, the low pressure area will be to your left ... Buys Ballot's Rule...well, maybe not immortalized, but ...
You can use a little electronic sound-maker to show the Doppler effect more conveniently but less musically than Buys-Ballot. The idea is just to tie it to one end of a string and whirl it around you. Other people will hear the pitch changing up and down as the source moves in a circle, alternately approaching and retreating from them, like the duck's experience above. This is more than a classroom demo of the Doppler Effect, it's a close analogy to what Doppler was suggesting for colors of starlight.
One detail seems different in this real-world demo: the sound source is moving in a circle, not just in and out from the duck like the animation above. Does the side-to-side part of of the circle motion bother the receiver? For once it's simple: only the in-and-out part of the motion has a Doppler effect. This motion is officially called radial velocity since it's along an imaginary ray projecting out from the receiver. Any source velocity can have a component that's radial, which you can find by drawing little triangles around the velocity direction. The other component has to have a more respectable name than side-to-side, too, so it's called transverse.
What we've seen so far is that a moving source changes the frequency a stationary observer sees. The useful part comes in when you don't know how fast the source is moving, but you can figure it out from how the frequency is changing: then you've got a technique with a lot of applications, astronomy included, as we'll see.
It isn't that hard to take the numbers you can measure for the frequencies and get the speed of the source, which you're trying to find out. You'll probably accept that there is such a method based on what we've seen, but how can you get the formula? (Yeah, I know...look it up! but you won't understand it, and that's the point on this page)
Our crack staff of multiple personalities has been hard at work, producing different web pages to let you really understand Doppler quantitatively. The editor couldn't decide which of their efforts was best, so we're letting you decide! Take a look at the candidates, and send us an e-mail with your vote!
One final point: what counts is motion of the source or receiver, so the source can be stationary and the receiver moving, or the source moving and the receiver fixed, or any combination. Doppler doesn't care. He's cool.
O.K., he cares a little. What the receiver receives depends on the two motions differently. The math becomes a little more complicated than in the voting pages, but you can derive a formula for that case too by similar means. If this is too easy, consider the case of very high velocities, as with light, when you need the Relativistic Doppler Effect!
Catch the next wave!
[Picky Technical Footnote from above] The car horn passing you makes a simple and dramatic demo, but the situation is more complicated than it looks, and what you hear isn't just the Doppler effect. Your perception of light and sound is complex, with a lot of subconscious brain processing going on, and color and pitch are more than just frequency (though that's the big part). The increase and decrease in loudness of the sound as the car passes also causes pitch changes, and that means you hear more than just the Doppler effect. In fact, even if the sound source wasn't moving, just growing lounder and then softer, there would be a slight rise and then fall of pitch. At great difficulty, we have made a new sound clip, which is the same as the one above except that the loudness is constant in the middle, to let you, our valued audience, hear just the Doppler shift. (OK, actually it's easy with a variety of sound manipulation programs, like Goldwave or Acid Wav which you can download as shareware, or SoundHack for the Mac.) <A HREF="carhornf.au"><b>Play Sound</b></a> What you hear is also more complicated in that the horn's sound is not just a single regular series of waves, but a combination of waves at different frequencies. It turns out that your ear can interpret this combo as a certain pitch, and that since all the frequencies are Doppler shifted by the right amount, the pitch you perceive is Doppler shifted. This is true for most natural sounds, even Buys Ballot's trumpeters, so his experiment was also not as clean as he would have liked. It would be better to have a pure tone of just one frequency and see how it changes, but it turns out our ear can handle the complex tones of instrument horns and car horns. Thus the standard Doppler example is legit.
The car horn passing you makes a simple and dramatic demo, but the situation is more complicated than it looks, and what you hear isn't just the Doppler effect. Your perception of light and sound is complex, with a lot of subconscious brain processing going on, and color and pitch are more than just frequency (though that's the big part). The increase and decrease in loudness of the sound as the car passes also causes pitch changes, and that means you hear more than just the Doppler effect. In fact, even if the sound source wasn't moving, just growing lounder and then softer, there would be a slight rise and then fall of pitch. At great difficulty, we have made a new sound clip, which is the same as the one above except that the loudness is constant in the middle, to let you, our valued audience, hear just the Doppler shift. (OK, actually it's easy with a variety of sound manipulation programs, like Goldwave or Acid Wav which you can download as shareware, or SoundHack for the Mac.) <A HREF="carhornf.au"><b>Play Sound</b></a> What you hear is also more complicated in that the horn's sound is not just a single regular series of waves, but a combination of waves at different frequencies. It turns out that your ear can interpret this combo as a certain pitch, and that since all the frequencies are Doppler shifted by the right amount, the pitch you perceive is Doppler shifted. This is true for most natural sounds, even Buys Ballot's trumpeters, so his experiment was also not as clean as he would have liked. It would be better to have a pure tone of just one frequency and see how it changes, but it turns out our ear can handle the complex tones of instrument horns and car horns. Thus the standard Doppler example is legit.
What you hear is also more complicated in that the horn's sound is not just a single regular series of waves, but a combination of waves at different frequencies. It turns out that your ear can interpret this combo as a certain pitch, and that since all the frequencies are Doppler shifted by the right amount, the pitch you perceive is Doppler shifted. This is true for most natural sounds, even Buys Ballot's trumpeters, so his experiment was also not as clean as he would have liked. It would be better to have a pure tone of just one frequency and see how it changes, but it turns out our ear can handle the complex tones of instrument horns and car horns. Thus the standard Doppler example is legit.
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