Understanding Doppler Shift: Impact on Sound Waves and Airplane Movement

In summary, when an aeroplane emits a sound wave and it reflects off a stationary or moving plane, the frequency of the reflected sound will either increase or decrease depending on the direction the plane is traveling in relation to the reflector. This is due to the Doppler effect, where the frequency of a sound wave changes when the source or observer is in motion. The closer the plane is to the observer, the higher the frequency of the reflected sound, and the farther away the plane is, the lower the frequency.
  • #1
jsmith613
614
0
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
 
Physics news on Phys.org
  • #2
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.
 
  • #3
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?
 
  • #4
Is "doppler effect" a good answer? If not, then do you know the doppler shift equation?
 
  • #5
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" [Broken]
 
Last edited by a moderator:
  • #6
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" [Broken]

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
 
Last edited by a moderator:
  • #7
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).
 
  • #8
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.
 
  • #9
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?
 
Last edited by a moderator:
  • #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!
 

1. What is Doppler shift and how does it affect sound waves?

Doppler shift is a phenomenon that occurs when there is relative motion between a source of waves (such as a sound wave) and an observer. It causes a perceived change in the frequency and wavelength of the wave, resulting in a shift in pitch or tone. For example, as a sound source moves towards an observer, the frequency of the sound waves increases, making the sound appear higher in pitch.

2. How does Doppler shift impact the movement of airplanes?

Doppler shift plays a crucial role in the movement of airplanes. As an aircraft moves through the air, it creates sound waves that propagate in all directions. These sound waves are affected by the relative motion between the aircraft and an observer on the ground. This can result in a change in the perceived pitch of the sound, making it difficult for air traffic controllers to accurately determine an aircraft's position and speed.

3. Can Doppler shift be used for aircraft tracking and monitoring?

Yes, Doppler shift can be utilized for aircraft tracking and monitoring. By measuring the change in frequency of sound waves emitted by an aircraft, ground-based radar systems can accurately determine an aircraft's speed and direction of movement. This information is crucial for air traffic controllers to safely guide aircraft and maintain safe distances between them.

4. How does air temperature affect Doppler shift?

Air temperature can have a significant impact on Doppler shift. As temperature increases, the speed of sound also increases. This means that the frequency of sound waves emitted by an aircraft will change differently at different temperatures, resulting in a varying amount of Doppler shift. This can make it challenging to accurately measure an aircraft's speed and position.

5. How does the Doppler effect differ from Doppler shift?

The Doppler effect and Doppler shift are two terms that are often used interchangeably. However, there is a subtle difference between the two. The Doppler effect refers to the perceived change in frequency and wavelength of any type of wave, while Doppler shift specifically refers to the change in frequency and wavelength of sound waves due to relative motion between a source and an observer.

Similar threads

Replies
1
Views
1K
Replies
6
Views
2K
  • Other Physics Topics
Replies
24
Views
3K
  • Introductory Physics Homework Help
Replies
14
Views
1K
Replies
8
Views
1K
  • Mechanics
Replies
13
Views
2K
Replies
6
Views
1K
  • Quantum Physics
Replies
2
Views
1K
Replies
1
Views
2K
  • Other Physics Topics
Replies
27
Views
6K
Back
Top