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My wife and I are having a debate about the Doppler effect. We both know "what" happens but aren't sure "why" it happens. Can someone explain in plain English what is going on as the paverbial motorcycle passes?

Paul -

Never, ever argue against the wife!!  But, in an ideal world......

A sound at a constant pitch and sound pressure level coming from a stationary object will always stay the same as observed from a stationary listener.  When the sound source (again with constant pitch and SPL) is moving toward then away from the stationary listener the pitch is changed as the wavelength observed by the listener changes by a factor of the velocity of the moving source (as Hesh has pointed out above) dependent on the angle of approach and departure.

The visual analogy would be looking out the window of a helicopter that is flying over the ocean.  Watching a spot below the craft, if the waves are a constant height and distance apart as you fly over them at a 90 deg angle to the waves they will appear even but you will count more of them passing you by at a higher speed.  If the helicopter slows down you won't count as many in the same amount of time.

Astronomers use this effect with light waves to see if something is moving toward or away from us.  They see a color shift.  Weathermen use Doppler radar to see the winds/movement within storms.

I hope this helops promote marital harmony and bliss!

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Steve Walden
Aspiring Builder,
Bartlesville, OK

I just thought of another analogy:

A baseball pitcher is on the bed of a pickup.  as the pickup appraches you he throws a fastball.  From his side a man with a radar gun measures the speed of the ball at 90 mph.  You are down the street a block also with a radar gun and you measure 110 mph.  How fast is the truck going?

The difference in perceived speed is the speed of the truck.  The speed of sound in air at sea level is about (if my memory serves me well) about 740 mph.  If the ambulance is moving toward you at 50 mph that is ~7% higher pitch.

TMI?

Steve Walden

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Steve Walden
Aspiring Builder,
Bartlesville, OK

Paul, you need to check this web site out. Search for "hyperphysics". It really an incredible resource, I end up using it as a reference at my day job a lot.
Everyone should should know about this.
IMO

But the basic answer is because the item producing the sound is moving creating a longer distance between the sound waves as the item moves passed you.

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Jim Watts
http://jameswattsguitars.com

Yet another way of looking at it:

An object is making a sound and it is standing still. The pitch of that sound is determined by the amount of time it takes one cycle of that sound to go from one peak to another. The sound also takes time to travel through the air. So, when you hear the sound it is the sound wave that is hitting you some time after the sound source made that wave.

Now, suppose that the object is moving toward you. The time it takes the first peak of the sound wave to reach you is based on the distance the sound source is when the first peak is emitted. When the second peak of the wave was emitted, the object was closer to you and gets to your ear sooner than it would have if the object hadn't moved. Since the second peak got to your ear sooner, your ear perceives this as a higher pitch.

From your observation point the sound waves are being squeezed closer together as the motorcycle approaches, then stretched apart as the motorcycle travels away from you. This makes it appear to you that the pitch raises then lowers.

The biker hears it at the same constant pitch. It's a Harley, if he's lucky.

D..   

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D..

Also, Paul
It is caused by relative motion. It doesn't have to be the sound that is moving. If the sound remained constant and did not move, but the observer moved past the sound, the same doppler effect would be heard. A classic example of that is in old B&W movies where the scene is on a train that passes through a crossing and you can hear the bell ringing as the crossing arm lowers. The pitch of the bell drops a lot.

With a constant, non moving sound source, the sound waves come off at a fixed wavelength. When the sound is approaching you the wavelength gets shorter as the waves are compressed, as mentioned above, and a shorter wavelength means a higher frequency, which in sound, causes a higher pitch and in light causes a shift toward the blue end of the spectrum. As the sound moves away from the listener the waves are stretched farther apart which lowers the frequency and thus, the pitch.

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

So if I stand on the front of a train moving at the speed of light and turn my flashlight on... what happens?   

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

[QUOTE=Brock Poling]
So if I stand on the front of a train moving at the speed of light and turn my flashlight on... what happens?    [/QUOTE]

You'll get to your ultimate destination really fast!

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Jim Watts
http://jameswattsguitars.com

When I was in 6th grade, we watched a film (5 days, 2 hrs a day) called Einstein's Universe. It was, shall we say, a bit over a 6th grader's head. But the flashlight on the train moving at the speed of light was one of the theories it talked about. I'm pretty sure I was sleeping when they finally got around to saying what would actually happen.
Thanks for the help on the Doppler thing.

Paul, that was my favorite video to show in physics class. Wonderful video, narrated by Peter Ustinov. I looked for a copy awhile back and it isn't available anymore. Maybe I can find it on ebay.

Anyway, they illustrated the light thing with a motorcycle. It was like looking through a helmet cam and it showed how the surroundings changed as the motorcycle approached the speed of light. Great stuff!! One thing I remember well is as you approached a house beside the road, you could see the opposite side of the house at the same time that you saw the front side.

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

[QUOTE=old man] ... as you approached a house beside the road, you could see the opposite side of the house at the same time that you saw the front side.

Ron[/QUOTE]
It just seems easier to take drugs.

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

Did he say Physics or Psychics?

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http://www.dickeyguitars.com

Physicdelics...

What would happen if you took drugs at the speed of light? What would your flash backs be like?

   Where's Einstein when we really need him!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Wouldn't they be flash-forwards or flash-same times or something like that? Maybe we should try it (just for the sake of science of course) because it is the weekend and all.

Weekends would be even shorter than they are!

And, Brock, riding on the front of that train, with your light, you would be very heavy and very small, riding in the dark.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

     I never meant, to even intend, that anybody should take drugs in any form! Let alone at the speed of light!

Just say NO to drugs at the speed of light!

[quote]And, Brock, riding on the front of that train, with your light, you would be very heavy and very small, riding in the dark.[/quote]

   That's not really true! Brock and his flashlight as well as the train would have infinite mass! Brock's reference to the universe, itself, would be in reality non-existant. Brock could explode! We just don't know!

       And another thing, why is it always a "train" going the speed of light, why isn't it a plane, or a spaceship, or a giant twinkie?

      Brock on a giant twinkie, with a flash light, going the speed of light, making motorcycle noises, sober! What would happen

   

    I forgot, and Hesh eating cookies!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

"Beam me up Scotty!"    

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

[QUOTE=Billy T]

      Brock on a giant twinkie, with a flash light, going the speed of light, making motorcycle noises, sober! What would happen



[/QUOTE]

Most likely my ADD would kick in, I would get bored, and then eat the twinkie.   

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

[QUOTE=Brock Poling]
So if I stand on the front of a train moving at the speed of light and turn my flashlight on... what happens?    [/QUOTE]

I'm just guessing here, but if you were moving that fast on the front of a train (or anything else), you'd be blown apart mighty quickly, unless you were in a perfect vacuum, and would never get a chance to turn that light on. Sorry...but if you were in a perfect vacuum, you'd be dead before you had a chance to tunr the light on from suffocation anyway, so your point is moot. But I think you knew that, Mr. Astrophysics major....

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

[quote]Most likely my ADD would kick in, I would get bored, and then eat the twinkie. [/QUOTE]

         

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)