Register to reply 
The Doppler Effect (in general) 
Share this thread: 
#1
Jul1609, 10:01 AM

P: 696

I have a problem in regard to the doppler effect,which may be generalised to all wavessound,water etc.
Please explain why the observed frequency increases as the object approaches an observer and then decreases as the object passes the observer.Actually,I thought that since doppler effect depends only on the relative velocity between observer and source,the observed frequency should be constant throughout the process,as the relative velocity does not change as the distance between the source and observer changes. I also found on a website that it said that 'the observed frequency of an approaching object declines monotonically from a value above the emitted frequency, through a value equal to the emitted frequency when the object is closest to the observer, and to values increasingly below the emitted frequency as the object recedes from the observer'Does this mean the same thing as what I asked at the begginning or is it different? Also,does the intensity of the sound increase as an object approaches an observer and decrease once it passes and recedes from the observer? 


#2
Jul1609, 02:21 PM

Sci Advisor
HW Helper
Thanks
P: 26,148

In practice, it usually misses the observer, and so the relative speed does change continuously. 


#3
Jul1609, 02:29 PM

Mentor
P: 41,477

Adding to tinytim's comments:



#4
Jul1609, 02:31 PM

P: 113

The Doppler Effect (in general)
Well lets say it was heading stright for the observer, and the observer could see both forward and behind. He is right, relativley there is no difference, the doppler effect collapses... 


#6
Jul1609, 02:33 PM

P: 113




#7
Jul1609, 02:39 PM

Mentor
P: 41,477

What matters for the Doppler effect is your velocity relative to the source. In one case you move towards the source; in the other, you move away. Big difference. 


#8
Jul1609, 02:41 PM

P: 113

O well, it was fun while it lasted :)



#9
Jul1609, 02:50 PM

P: 513




#10
Jul1609, 02:58 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,565

I think Doc Al and tiny tim are talking about different situations. tiny tim is talking about a situation where the motion of the source of sound is slightly off to one side of the observer, the velocity is continuously changing so the frequency heard is continuously changing. That is also my interpretation of the original question. If the source is moving directly toward the observer, the frequency is constant (above that of the emitted signal) until the source passes then suddenly drops below the frequency of the emitted signal. That is the situation Doc Al is referring to. Note that in the first case, while the change in frequency is continuous, it is not linear. There will be relatively little change in the frequency when the source is farther off, most change when the source is nearest. That effect increases when the point of "closest approach" to the observer is closer to the observer that is when the source just misses the observer.



#11
Jul1609, 03:08 PM

Sci Advisor
HW Helper
Thanks
P: 26,148




#12
Jul1609, 03:13 PM

Mentor
P: 41,477




#13
Jul1609, 06:42 PM

P: 113

Wait a minute, with light it doesnt matter what your relative velocity is, light is only one speed c, invariant. So... If you had a light emitting diode travelling to you, then away from you, whats the difference? The light is traveling at speed c to you at all times.
Next time I get pulled over I know what my argument is going to be. "Its only a 4 cylinder" isnt working anymore. 


#14
Jul1609, 07:09 PM

Mentor
P: 41,477




#15
Jul1609, 11:20 PM

P: 696

What is the cause for this difference? Doc Al said that the observed frequency changes only when the relative motion changes from 'moving toward' to 'moving away'that's understandable,but I came accross a source which said that the observed frequency changes continuously during the interval that the source and observer move toward eachother and also during the interval that they move away from eachother. This is what I don't understandon these individual intervals,there is no change in relative velocity of 'moving toward' or 'moving away'. Again at the point of closest approach,there is still relative motion between source and observer,so shouldn't there be a different observed freqeuncy even here? I really don't understand it!! 


#16
Jul1609, 11:32 PM

P: 696




#17
Jul1709, 01:06 AM

P: 513

The doppler effect as you think holds in the ideal condition that the relative velocity is the same throughout, but in real cases it becomes a monotonically decreasing curve.
The doppler effect would be as you thought if you stood on linear track of an approaching train with constant velocity and the train went through you, but you stand by the side and the component of velocity vector towards you goes on decreasing and becomes zero when closest to you and agin goes on increasing like the velocity vector of a parabolic curve when a ball is thrown up, if you were to see it facing perpendicular to the earth in the air at the point of 0 velocity. The formulae still hold true but there is change in frequency as there is change in velocity. 


#18
Jul1709, 05:47 AM

Mentor
P: 41,477

Of course, that situation is unrealistic. Usually, the source doesn't come directly at you, otherwise you'll be hit. It passes by you. If you imagine a line drawn from you to the source, the length of that linewhich represents the distance between the source and observerchanges as the source approaches you (at an angle), passes by you, and then recedes from you. The rate of change of that distance is the "relative velocity" that we are concerned with. That rate of change varies continuously as the source moves. 


Register to reply 
Related Discussions  
Doppler Effect  General Physics  13  
Doppler effect  General Physics  1  
Doppler Effect...help!  Introductory Physics Homework  2  
Doppler effect  Introductory Physics Homework  9  
Doppler Effect  General Physics  5 