Understanding Doppler Shift: Impact on Sound Waves and Airplane Movement

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Discussion Overview

The discussion revolves around the Doppler effect as it applies to sound waves emitted by an airplane and reflected off a stationary surface. Participants explore the implications of the airplane's movement on the frequency and wavelength of the sound waves, addressing both theoretical and conceptual aspects of the phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants describe scenarios where an airplane emits sound waves towards a reflector, questioning how the frequency changes based on the airplane's direction of travel.
  • Others argue that when the airplane approaches a stationary reflector, the frequency of the reflected sound will be higher than the source frequency.
  • A participant acknowledges confusion regarding the roles of the source and reflector, clarifying that the airplane is the moving object reflecting sound waves emitted from a stationary source.
  • Some participants explain that as the airplane moves closer to the source, the reflected sound waves become compressed, resulting in a shorter wavelength and higher frequency.
  • Examples of the Doppler effect are provided, including scenarios involving police sirens and a person being swung around, illustrating the connection between frequency and the velocity of the sound-emitting object.
  • There is a discussion about the need to clearly define the positions of the source and observer to apply the Doppler effect correctly.

Areas of Agreement / Disagreement

Participants express some agreement on the basic principles of the Doppler effect, particularly regarding how the frequency changes based on the relative motion of the source and observer. However, there is also confusion and disagreement regarding the initial scenarios presented, leading to a need for clarification on the roles of the source and reflector.

Contextual Notes

Participants acknowledge that the discussion involves assumptions about the movement of the airplane and the stationary source, as well as the conditions under which the Doppler effect applies. There are unresolved aspects regarding the specific scenarios described and how they relate to the Doppler effect.

jsmith613
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Imagine this scenario:
An aeroplane emits a sound wave. The sound wave propagates in all directions.

a) If the plane is flying forward, it emits a sound towards a reflector in front of it. When the sound is reflected, its frequency is reduced, presuming the plane is traveling towards the reflector

b)If the plane is flying in the other direction, it emits a sound towards a reflectorbehind it. When the sound is reflected, its frequency is increased, presuming the plane is traveling away from the reflector

Why are a) and b) as they are
 
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Seems like you have those situations reversed. When the plane is flying toward a stationary reflector, the frequency of the reflected sound will be higher that the source frequency.
 
Doc Al said:
Seems like you have those situations reversed. When the plane is flying toward a stationary reflector, the frequency of the reflected sound will be higher that the source frequency.

Yes sorry, I confused it.
The question still remains. why does it happen?
 
Is "doppler effect" a good answer? If not, then do you know the doppler shift equation?
 
jsmith613 said:
Yes sorry, I confused it.
The question still remains. why does it happen?
That's just the basic Doppler effect. Think of succeeding wavefronts as getting bunched closer together when the source moves towards the observer (resulting in higher observed frequency). It's the reverse when the source moves away from the observer.

See: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp.html"
 
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Doc Al said:
That's just the basic Doppler effect. Think of succeeding wavefronts as getting bunched closer together when the source moves towards the observer (resulting in higher observed frequency). It's the reverse when the source moves away from the observer.

See: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp.html"

Brin - I do know what doppler effect is!

Doc Al - just to clarify then,
the transmitted wave will have a constant wavelength / speed / amplitude.
The wave will be reflected when the wave hits the plane. If the plane gets closer then the waves are reflected quicker so the reflected wavelength is smaller
 
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jsmith613 said:
Doc Al - just to clarify then,
the transmitted wave will have a constant wavelength / speed / amplitude.
The wave will be reflected when the wave hits the plane.
I thought the plane was the source? There's a moving plane emitting sound at some frequency approaching a stationary (with respect to the air) reflecting surface.
If the plane gets closer then the waves are reflected quicker so the reflected wavelength is smaller
Yes. If the plane approaches the reflecting surface, the reflected sound will have a higher frequency and shorter wavelength (compared to the source).
 
The doppler effect is that thing where the cop car is driving towards you with its sirens blaring and it sounds high pitched, then as it drives away it is lower pitched.

Or, when there is a man being swung around in a circle, and you are standing to the side watching. As you listen to him screaming for help, you'll notice that the pitch of the voice changes as he gets nearer than farther.

If a plane is flying towards you, it'll have a higher pitched engine noise than when it is flying away from you. You definitely hear the frequency change as it flies by you. All of these examples are to illustrate that for the frequency you hear, there is a connection between the frequency of a noise, and the velocity of the object emitting the noise.
 
Doc Al said:
I thought the plane was the source? There's a moving plane emitting sound at some frequency approaching a stationary (with respect to the air) reflecting surface.


No. The source emits the sound and it reflects off the plane. Let's get that bit clear.

That is why I was confused.

Even with this siutation applying, do the same principles work
 
  • #10
jsmith613 said:
No. The source emits the sound and it reflects off the plane. Let's get that bit clear.
OK, but that's different from the scenario you described in your first post. Please describe the situation you want to discuss: Where's the source? Where's the observer?

That is why I was confused.

Even with this siutation applying, do the same principles work
Sure, the Doppler effect still applies. Once you define the situation, we can apply it.
 
  • #11
Doc Al said:
OK, but that's different from the scenario you described in your first post. Please describe the situation you want to discuss: Where's the source? Where's the observer?


Sorry about the confusion:
Here is the situation:
A stationary SOURCE emits sound waves.
a MOVING plane reflects these waves
the SOURCE also detects the reflected waves.

If the plane is moving nearer then the returing wavelength is shorter
If the plane is moving further away then the returing wavelength is longer

So what is going on is that when the source emits the wavelength and the plane moves nearer the reflected sound waves sqash closer together because the plane is constanlty reflecting the waves as it gets nearer

Correct?
 
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  • #12
jsmith613 said:
Sorry about the confusion:
Here is the situation:
A stationary SOURCE emits sound waves.
a MOVING plane reflects these waves
the SOURCE also detects the reflected waves.
OK.

If the plane is moving nearer then the returing wavelength is shorter
If the plane is moving further away then the returing wavelength is longer
Yes. If the plane moves towards the source, then the observed wavelength is shorter.

So what is going on is that when the source emits the wavelength and the plane moves nearer the reflected sound waves sqash closer together because the plane is constanlty reflecting the waves as it gets nearer
Sounds good to me.
 
  • #13
Doc Al said:
OK.


Yes. If the plane moves towards the source, then the observed wavelength is shorter.


Sounds good to me.

Thanks for your help!
 

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