WHY does the doppler effect occur?

Question

Can someone tell WHY this effect occurs? And some good examples of it.

Thanks

Answer 1

When the distance between an observer and a wave source changes, the waves will reach the observer at a faster or slower rate (depending on the direction of movement), and will be perceived as a higher or lower frequency. This shift in frequency is called the Doppler effect.

Imagine you are standing beside a conveyor belt which is moving along at a constant speed. Each second sixty cans of cola go by a stationary point on the conveyor. If you reach out with a stick and let it brush against the cans you will generate a sixty cycle sound. If you move toward the head of the conveyor as you do this the number of cans you brush each second will increase. So will the frequency of the sound that you generate. And if you walk toward the tail of the conveyor then the converse will occur. This would demonstrate the Doppler effect.

I can give you a true example of something that happened to me when I was a teenager and brought home the meaning of the Doppler effect to me in a very personal way.

At the time I lived in a very rural area and did not drive. So, I had walked several miles to see a girlfriend. And after my date I began walking the several miles back home (Yes, I was crazy).

In the middle of the night with nobody around, I was about half way home and had to walk down a long, lonely stretch of road which had electric poles beside it. It was very quiet and about all I could hear was my footsteps on the pavement and the humming of a power transformer on one of the poles that I had just walked past.

I had only gotten a few steps away from the pole when I started hearing a strange and creepy sound. It was a low, repetitive sound that went waaah-waaah-waaah. I stopped to look around, trying to see what might be making such a weird noise. But when I stopped, so did the sound. I felt the hair stand up on my neck because knew that someone must be watching me in the dark.

So, I began walking again slowly and the sound restarted. Waaah-waaah-waaah-waaah. The further I walked, the louder it got. Honestly, I was about to pee in my pants. I walked a little faster and the wah-wah-wah too became faster. I was about to panic now and could hear another sound. It was the sound of my heart beating loudly in my ears as I ran down the dark road like a madman in the middle of the night, trying to get the wah-wah out of Dodge.

After a few moments, I ran out of breath and started to slow. And when I did, the waah-waah-waah sound slowed too, even though it was a bit louder. I finally stopped when I realized what the sound was. And even though I was all alone in the middle of nowhere, I couldn't help but to laugh very loudly with relief as I understood my pursuer.

Just ahead of me I could make out the shape of another power pole beside the road. And on that pole was another transformer. The creepy sound that had frightened me so badly was merely a beat sound between the two 60 cycle transformers. As I walked towards one and away from the other, the Doppler effect caused these identical frequencies to reach my ears as slightly different frequencies. And when you combine two different frequencies they will heterodyne, or beat, against each other making sum and difference frequencies to go along with the originals.

What I was hearing was a difference beat frequency in the 0-3 cycle range which varied with my speed. When I stopped, the beat stopped too because then the two frequencies reaching my ears were once again identical.

It turns out that the only person watching me that night was old Mr. Doppler himself. I hope he got a good laugh because I sure did!

WK

Answer 2

The usual example is waves in water. If you repeatedly touch the water in the same place, it makes continuous ripples that move out from that point in ever widening circles. The wavelength is the distance between any two waves. Now, instead of touching the same place over and over, touch a spot a little to one side each time. The second touch is an inch to the right, the third touch is another inch to the right and so on. Now each of the spreading circles has a different center, each one an inch to the right of the one before. The result is that the distance between waves on the right is less and the distance between waves on the left is more.

So the effect is due to the source of the waves (your finger touching the water) moving with respect to the medium through which the waves are propagating (the water). The effect is easy to understand with sound, where the medium is the air, but not so easy to understand with light, where there is no medium. At one time, people assume there was a medium for light, called the aether, but the Michelson-Morley experiment shows that there is no medium. Still, the idea of the aether lives on, as in radio broadcasts sometimes being described as "over the aether"

Answer 3

The doppler effect is created when a wave (sound, light, water, etc) bounces off a moving object. If the object is moving away from the source of the wave, the speed of the wave bouncing back has the speed of the object moving away subtracted. So, if it was a sound wave the sound would come back at a lower pitch than when it went out. By analogy, the same sound wave bouncing off the same object moving at the same speed but moving toward the source of the sound wave would bounce back at a higher pitch.

Of course, the sound could be produced by the moving object itself and as it comes toward you, the sound it makes would be at a higher pitch until it is just opposite to you (when you might briefly hear the exact sound), but as it moves away from you the pitch of the sound it makes would be lower. We can all recognize this in sound effects of a train passing through a town.

Einstein used this idea in coming up with his theory of relativity because at the speed of light some very interesting effects happen that would not be expected at the speed of sound.

Answer 4

if a car comes at you (or a police car with its siren on) you hear the pitch of the sound to be much higher. As it passes you the pitch becomes lower. This is the doppler effect.

Waves eminate from the souce with a constant frequency, and they travel at a constant speed. when the source is moving at you the waves have a shorter wave length, and when it moves away the waves have a longer wavelength.

Imagine a person standing out the sun roof of a car throwing baseballs at a stop sign at a rate of one base ball per second, and each base ball is a 90 mile per hour fast ball.
if the car is standing still the balls will hit the sign once every second.
If the car is traveling at the stop sign at 45 miles per hour the balls that are thrown would be traveling at 90+45 = 135 miles per hour. But with sound this is not true... sound always travel at 700 miles per hour(or something like that), it is independent of how fast the source is moving.
So lets imagin that those baseballs are like sound waves, and they always travel at 90 miles perhour no matter what speed the car is traveling.

now when the car is going toward the sign it is traveling half the speed of the balls, so though he is throwing the balls once per second the distance between balls is cut in half, so when the hit the sign they hit once every half second.

And when he is traveling away from the stop sign there is 1 and a half times as much space between each ball so when they hit the stop sign they strike once every 1.5 second.

This is what is happening with the sound waves from the car(or police siren) an it is enouph to make an autiable sound shift.

This is especial true if you watch nascar or something like that you can really hear the change in the pitch of the engine whine.

Answer 5

the best example of it is when a car approaches you the sound is higher pitch than when the car is heading away from you. same goes for an air plane, or the easiest to think of is the siren on a police car as it passes you.

to explain, think of sound waves as rings of wave heading out from a central point, like the ripples you get if you drop a pebble in a pond. an object moving towards you is like the first pebble being dropped in the center of the pond, and each pebble being dropped closer and closer to you. each circle ripple produced, then, is off center (closer to you) and so the ripples that reach you at the shore are closer together than if you just kept dropping them in the same place. this closer "togetherness" of the waves means higher frequency, which means you hear a higher pitch. Conversely, if you were standing on the other side of the pond with the pebble drops moving away from you, then the ripples reaching you are spread out and sound lower in pitch. Consequently, the faster an object moves (ie, the more distance between each pebble drop towards you) the greater the affect of the doppler shift (the change in the sound you hear).

Following this line, a breaking of the sound barrier occurs when an object is moving faster than the speed of sound... think of it as the pebble spacing being so great that each NEW pebble drop is actually outside the ring of ripples formed by the previous pebble drops. this forms a "cone" of sound following the object, but not around it, which is why you won't hear a plane moving faster than the speed of sound until long after it has already passed you. the sonic boom is where you cross from the area of no sound around the object into the cone shaped region of sound trailing the object.

Answer 6

Light or sound waves are compressed as they move toward you, and they're expanded as they move away from you. The compression or expansion changes the color of the light, or the pitch of the sound. You could tell, with the proper instruments, the speed at which the motion is occuring and the direction.

Answer 7

It happens because, when point X approaches point Y, the forward motion results in an increase in frequency and when point X moves away from point Y, there is a decrease in frequency.

The classic is a car horn that is blaring as the car comes towards you... the wavelength of the sound becomes slightly higher, resulting in a higher pitch... as the car move away, the wavelength lengthens and the pitch lowers.

Answer 8

Everything you need to know about the Doppler effect in one catchy tune: http://www.astrocappella.com/songs/doppler.mp3

Well, okay, maybe you'd like the page with the lyrics, as well, since it has links to the info you're looking for.
http://www.astrocappella.com/doppler.shtml

Answer 9

None of those answers is entirely correct, and one (C) could be entirely incorrect if both the source and the listener are moving in the same direction at the same speed. The only answer that is correct is: The source and/or listener must be moving in relation to each other. .

Answer 10

This refers to the movement of frequency waves. Sound and light. When that which emits the waves is moving toward you the waves compress and the frequency rises and as it moves away waves "decompress" and the frequency drops. See below:

/ / / / / / / / ////// You ////// / / / / / / / / / / / /

This applies to moving objects. When sound comes to you the pitch rises and drops as it moves away. A white light appears blue when coming toward you and red as it moves away.