Non-symmetric nature of the Doppler Effect with sound

Click For Summary

Discussion Overview

The discussion revolves around the non-symmetric nature of the Doppler Effect for sound, specifically comparing scenarios where either the observer or the source is moving towards each other. Participants explore the implications of these movements on the observed frequency of sound waves, examining both theoretical and conceptual aspects of the Doppler Effect.

Discussion Character

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

Main Points Raised

  • One participant notes that moving towards a sound source and having the source move towards the observer yield different observed frequencies, questioning why this is the case.
  • Another participant uses an analogy involving wind-up toys to illustrate the concept of wavefronts and the role of the medium in the Doppler Effect.
  • Some participants challenge the analogy, suggesting that it may not accurately represent the behavior of sound waves, particularly regarding the speed of the source relative to the medium.
  • A participant emphasizes that the Doppler shift is dependent on the observer's rest frame and the medium's movement, leading to different calculations for observed frequencies in each scenario.
  • There is a discussion about the preferred inertial frame of reference for sound waves, which is the medium itself, and how this breaks the symmetry observed in other contexts, such as the relativistic Doppler Effect.
  • Participants provide calculations for the observed frequencies in both scenarios, with some discrepancies in their results, indicating uncertainty in the application of the Doppler formula.

Areas of Agreement / Disagreement

Participants generally agree that the Doppler Effect behaves differently depending on whether the source or observer is moving. However, there is no consensus on the best analogy to explain this phenomenon, and discrepancies in calculations suggest ongoing uncertainty regarding the correct application of the Doppler formula.

Contextual Notes

Some participants express confusion over the calculations and the implications of the medium's movement, indicating that assumptions about the frame of reference and the nature of sound waves may not be fully resolved.

sora
Messages
8
Reaction score
0
While studying the Doppler effect at school, it struck me as strange that the following two problems have different solutions:

a) you are moving at 40 m/s toward a source that is making a sound with a frequency of 1000 Hz. What frequency do you hear? (speed of sound= 340 m/s)
b) The source of a sound is moving toward you at 40 m/s, producing a sound with a frequency of 1000 Hz. What frequency do you hear? (speed of sound = 340m/s)

I thought that they would both have the same observed frequency, but they don't. Could someone explain why?
 
Physics news on Phys.org
I like to think of the traditional Doppler effect (i.e. waves propagating through a medium) with the following analogy. You know those little wind-up toys that walk in a straight line once you put them on the floor? Imagine a girl (call her "Sara") putting them on the floor at a constant rate, maybe once every t seconds.
  • The walking toys represent the wavefronts
  • Their speed represents the speed of sound
  • The floor represents the medium for the waves
  • 1/t is the original frequency
  • Sara represents the source
Naturally, the point of this analogy is to emphasize that once a wavefront is emitted, it doesn't matter what the source (or receiver) is doing; all that counts is the medium.

If you want to see, qualitatively, why they should be different, I think it's easier to show if the source and receiver are moving away from each other, rather than towards. In fact, let's say their relative speed is faster than the speed of the toys (speed of sound), and see what we get.

1) Moving source: The first toy is dropped, and it travels the initial distance and gets to the receiver. The next toy is dropped from a lot further back, so it has to travel the same distance plus a lot extra. The time between receiving toys is bigger than the time between dropping them, so the frequency is downshifted.

2) Moving receiver: The first toy heads towards the receiver, but the receiver is moving away even faster. The distance only ever increases! The receiver never receives.

Of course, an alternative is to crank the math and notice that you get different answers. But I think the starkness of the above scenario -- shifted frequency versus no waves at all -- makes it easier to grasp that yeah, they really should be different.
 
Chogg, maybe I'm misreading it, but wouldn't that be a bad analogy, because if the toy is to be analogous with sound, shouldn't it always have the same speed relative to the floor? (which would not be the case if Sara released the toy while moving relative to the floor)

Anyway, to the OP (maybe Chogg was saying the same thing after all, but I'd like to state it succintly): addmitedly, the two situations you sketch look symmetrical, but the symmetry-breaker is the fact that in one scenario the medium is moving, whilst in the other it isn't. (And sound moves at 300 m/s relative to the medium.)
 
mr. vodka said:
Chogg, maybe I'm misreading it, but wouldn't that be a bad analogy, because if the toy is to be analogous with sound, shouldn't it always have the same speed relative to the floor? (which would not be the case if Sara released the toy while moving relative to the floor)

Anyway, to the OP (maybe Chogg was saying the same thing after all, but I'd like to state it succintly): addmitedly, the two situations you sketch look symmetrical, but the symmetry-breaker is the fact that in one scenario the medium is moving, whilst in the other it isn't. (And sound moves at 300 m/s relative to the medium.)

when you say the medium is moving, do you mean the medium is moving in the observer's frame of reference?
 
uhu

(it won't allow me to post a three-letter response so i had to add this in)
 
mr. vodka said:
uhu

(it won't allow me to post a three-letter response so i had to add this in)

:-p

thought so.. so the doppler shift is basically the change in the measurement made in the measurement-taker's rest frame?

if you think about the person ("me") who hears the sound, it clears it up especially if i consider "my" rest frame in each scenario

a) I am stationary (this is my rest frame after all! :-p ), and the air is rushing past me at 40m/s and it is carrying a ripple moving at 380m/s relative to me (340m/s relative to the source + the source's motion relative to me) oscillating at 1000Hz.

b) I am stationary, the air is stationary and there are waves coming towards me which have a velocity of 340m/s relative to me (speed of sound doesn't depend on the source, only on the air which the vibrations are propagating through) and which have been doppler-shifted so that their frequency is now 1133Hz

is this right?
 
mr. vodka said:
Chogg, maybe I'm misreading it, but wouldn't that be a bad analogy, because if the toy is to be analogous with sound, shouldn't it always have the same speed relative to the floor? (which would not be the case if Sara released the toy while moving relative to the floor)

I think you missed the following line (emphasis in the original):

chogg said:
Naturally, the point of this analogy is to emphasize that once a wavefront is emitted, it doesn't matter what the source (or receiver) is doing; all that counts is the medium.

If the toys walk by themselves, then the speed of the source is completely irrelevant, right? Would this illustration help?
 
Last edited by a moderator:
When I said, "emphasis in original", I didn't realize that the entire blockquote is in italics. :P
 
RK1992 said:
:-p

thought so.. so the doppler shift is basically the change in the measurement made in the measurement-taker's rest frame?

if you think about the person ("me") who hears the sound, it clears it up especially if i consider "my" rest frame in each scenario

a) I am stationary (this is my rest frame after all! :-p ), and the air is rushing past me at 40m/s and it is carrying a ripple moving at 380m/s relative to me (340m/s relative to the source + the source's motion relative to me) oscillating at 1000Hz.

b) I am stationary, the air is stationary and there are waves coming towards me which have a velocity of 340m/s relative to me (speed of sound doesn't depend on the source, only on the air which the vibrations are propagating through) and which have been doppler-shifted so that their frequency is now 1133Hz

is this right?

Part b is right, but for part a i got 1117Hz. Shouldn't the frequency be greater in part a since the sound wave is moving at a greater speed relative to you (when you factor in the relative speed of the medium)?
 
  • #10
sora said:
Part b is right, but for part a i got 1117Hz. Shouldn't the frequency be greater in part a since the sound wave is moving at a greater speed relative to you (when you factor in the relative speed of the medium)?

yeah, i don't know why i didnt do that.. i get 1117.6Hz
 
  • #11
sora said:
I thought that they would both have the same observed frequency, but they don't. Could someone explain why?

For sound waves, there is a preferred inertial frame of reference - that of the medium, because sound waves always have a definite speed in that frame (e.g. 340 m/s in air). This is the reason why symmetry gets broken. Notice that in the formula for the Doppler Shift, the velocities of the source and the receiver you should plug in are the ones relative to the medium. In general, if you use another frame of reference, you will get wrong answers.

If there is no preferred inertial frame of reference, then the symmetry will not be broken - as is the case of the Relativistic Doppler Effect.
 
  • #12
Thank you, it makes a lot more sense now.
 

Similar threads

Undergrad Deriving Doppler Effect Frequency w/ Stationary Person & Moving Source
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Sound Wave Doppler effect question
  • · Replies 13 ·
Replies
13
Views
3K
Undergrad Find period of circular-orbiting source based on max observed freq
  • · Replies 1 ·
Replies
1
Views
2K
High School What sound frequencies can pass through a hole in a wall?
  • · Replies 31 ·
2
Replies
31
Views
5K
Speed of moth related to Doppler Effect
  • · Replies 8 ·
Replies
8
Views
1K
Graduate Doppler effect with moving medium but no relative motion
  • · Replies 10 ·
Replies
10
Views
4K
Undergrad Deriving the doppler shift equation for sound
  • · Replies 1 ·
Replies
1
Views
4K
Doppler effect and beat frequency
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Asymmetry in the Doppler Effect for sound
  • · Replies 8 ·
Replies
8
Views
6K
High School Why is the constant speed of light so unique?
  • · Replies 19 ·
Replies
19
Views
2K