Doppler effect formula confusion

Click For Summary

Discussion Overview

The discussion revolves around the Doppler effect, specifically the perceived frequency changes when either a sound source moves towards an observer or when the observer moves towards a stationary sound source. Participants explore the implications of these scenarios in the context of inertial reference frames and the role of the medium through which sound propagates.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants express confusion regarding the differences in perceived frequency when the source moves versus when the observer moves, questioning the implications of the equivalence of inertial reference frames.
  • One participant suggests that the equations used may be misunderstood, indicating that there should only be one speed in the Doppler effect formula besides the speed of sound.
  • Another participant provides specific frequency equations for both scenarios, highlighting that the frequencies heard are different, which suggests that one can determine the motion of either the source or the observer.
  • Some participants argue that the differences in frequency are due to the role of the medium (air) in sound propagation, noting that the source's motion affects wavelength while the observer's motion affects the frequency heard.
  • There is a discussion about the nature of sound propagation and how the velocity of the air mass influences the Doppler effect, with some participants asserting that the physics remains the same across inertial frames, while others challenge this view.
  • One participant presents a thought experiment involving three bodies to illustrate the asymmetry in the Doppler effect, suggesting that the presence of the medium alters the outcomes in a way that is not symmetrical.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views regarding the interpretation of the Doppler effect and the implications of moving observers versus moving sources. Some participants agree on the importance of the medium, while others question the symmetry of the scenarios.

Contextual Notes

Limitations in understanding arise from assumptions about the equivalence of inertial frames and the role of the medium in sound propagation. The discussion highlights unresolved mathematical steps and differing interpretations of the Doppler effect.

Who May Find This Useful

This discussion may be of interest to those studying acoustics, relativity, or the Doppler effect, as well as individuals seeking to understand the nuances of sound propagation in different reference frames.

Silviu
Messages
612
Reaction score
11
Hello! I am a bit confused by the Doppler effect. Based on the formula it seems that if the source moves towards me with speed ##v## I hear a frequency ##\nu_1## but if I move towards the source with the same speed ##v## I hear a different frequency ##\nu_2##. However according to the equivalence of inertial reference frame (both classically and relativistically) I should not be able to tell if I am moving towards the source or if the source moves towards me (and based on doppler effect I can do an experiment to tell me if I am stationary or moving with constant velocity). So what am I missing here? Thank you!
 
Physics news on Phys.org
Silviu said:
Hello! I am a bit confused by the Doppler effect. Based on the formula it seems that if the source moves towards me with speed ##v## I hear a frequency ##\nu_1## but if I move towards the source with the same speed ##v## I hear a different frequency ##\nu_2##. However according to the equivalence of inertial reference frame (both classically and relativistically) I should not be able to tell if I am moving towards the source or if the source moves towards me (and based on doppler effect I can do an experiment to tell me if I am stationary or moving with constant velocity). So what am I missing here? Thank you!
Can you post the equation you are using? You are probably misunderstanding it...there should only be one speed in it besides the speed of sound.
 
russ_watters said:
Can you post the equation you are using? You are probably misunderstanding it...there should only be one speed in it besides the speed of sound.
Assume that the speed of sound is ##c## and the frequency of the source is ##\nu_0##. If I move towards the source with the speed ##v## and the source stand still, the frequency I hear is ##\nu_1 = \frac{c+v}{c}\nu_0## if the source moves towards me with the same speed ##v## and I stay still the frequency I hear is ##\nu_2 = \frac{c}{c-v}\nu_0##. ##\nu_1## and ##\nu_2## are not the same, so I can tell if it is me or the source that is moving.
 
The two situations are not the same. The difference is accounted for by the part the air, the media of propagation, plays. In the moving source scenario the source motion modifies the wavelength of the sound the observer hears the speed of propagation not being affected. In the case of the moving observer the wavelength is unaffected but the frequency is modified because the observer hears different cycles per second with is proportional to the relative velocity. of the observer and the sound.
 
gleem said:
The two situations are not the same. The difference is accounted for by the part the air, the media of propagation, plays. In the moving source scenario the source motion modifies the wavelength of the sound the observer hears the speed of propagation not being affected. In the case of the moving observer the wavelength is unaffected but the frequency is modified because the observer hears different cycles per second with is proportional to the relative velocity. of the observer and the sound.
Wait, I am not sure I understand. In both situations, the speed of propagation is unchanged (it is equal to c). And we have that ##\lambda = c \nu##. So in both cases the frequency (and hence the wavelength) is changed, just by a different amount. So what do you mean by "the wavelength is unaffected but the frequency is modified"? You can't have a change in frequency, without a change in wavelength.
 
Sorry I missed the object of your OP.I was describing the acoustical DE.
 
Silviu said:
based on doppler effect I can do an experiment to tell me if I am stationary or moving with constant velocity...
... relative to the medium (air), not in an absolute sense. You don't need Doppler to determine that.
 
A.T. said:
... relative to the medium (air), not in an absolute sense. You don't need Doppler to determine that.
I am not sure I understand. My question is, shouldn't the physics be the same in 2 inertial reference frames? For example in relativity, if 2 observers have a meter stick and one of them is moving and the other is stationary, the length of the meter stick saw from the other frame is the same for both of them (i.e. observer 1 sees that the meter stick of observer 2 has length L' but also observer 2 sees the length of observer 1 to be L') so they can't tell which one is moving (relative to a fixed reference frame). But in the case of Doppler effect, assuming that for example the street is the stationary reference frame I can tell if the car is moving towards me or I am moving towards the car (relative to the street).
 
Silviu said:
My question is, shouldn't the physics be the same in 2 inertial reference frames?
The physics is the same. The velocity of the air mass isn't, so neither is the sound propagation.

Silviu said:
(i.e. observer 1 sees that the meter stick of observer 2 has length L' but also observer 2 sees the length of observer 1 to be L')
That's a symmetrical situation, with two meter sticks, not with one air mass.
 
  • #10
A.T. said:
The physics is the same. The velocity of the air mass isn't, so neither is the sound propagation.That's a symmetrical situation, with two meter sticks, not with one air mass.
What do you mean by air mass? What does that have to do with frequency of the sound?
 
  • #12
OP, your question is a good one, and one that has been asked here before as well as elsewhere. In trying to find a good explanation for you, I came across a post on another forum, credited below, that I will elaborate on here.

Suppose you have three bodies, from left to right A, B and C in a frame of reference in which they are all at rest and consider two cases. In the first case, A starts moving at velocity v to the right, so it is approaching B. In the second case, instead have B move with velocity v to the left, approaching A. In both cases, A and B are approaching each other with the same relative velocity. So you would say these two cases are the same and all effects must be symmetric.

But what about C? In the first case, A is approaching C with velocity v and in the second case A is stationary with respect to C. So the two cases are not the same and if there is an effect that depends on the relative velocity of A and C, the results would be different. To relate this to the Doppler effect, let A = source, B = observer and C = air.

https://physics.stackexchange.com/a/257041
 

Similar threads

Undergrad Who's moving towards whom in Doppler effect situations?
  • · Replies 14 ·
Replies
14
Views
2K
High School Doppler effect/ both moving same direction
  • · Replies 6 ·
Replies
6
Views
11K
Undergrad Formula for Doppler effect in moving medium?
  • · Replies 16 ·
Replies
16
Views
3K
Range of frequency heard by observer related to Doppler effect
  • · Replies 8 ·
Replies
8
Views
1K
High School Understanding the Doppler Effect at an Angle
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Deriving Doppler Effect Frequency w/ Stationary Person & Moving Source
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad How Does Acceleration Affect the Doppler Effect in Moving Sound Emitters?
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Explaining Doppler Effect Asymmetry
  • · Replies 14 ·
Replies
14
Views
3K
Speed of moth related to Doppler Effect
  • · Replies 8 ·
Replies
8
Views
1K
Undergrad Doppler effect in transparent media and the index of refraction
  • · Replies 11 ·
Replies
11
Views
1K