EK Phys 1001 - Doppler Effect

[raab32]

---

Members don't see this ad.

I need some help with questions 749 and 750 in EK Physics 1001. Probably very easy conceptual questions, but I'm a little burnt out from studying other stuff so I was hoping you guys could help me out. Understanding 749 will probably help me get 750, so I'll post that for you:

If the source of a sound wave and the observer are stationary, and there is a steady wind blowing from the observer to the source, how will the Doppler effect change the observed frequency?

A. The observed frequency will be greater.
B. The observed frequency will be smaller.
C. There will be no Doppler effect.
D. The change in freq. cannot be predicted.




The correct answer is C. Why is this the case? If wind is blowing from the observer to the source, isn't this working against the net velocity of the sound waves? I know there's something I'm missing. Thanks in advance for your help.

 

[Omni]

Here's my qualitative reasoning:
If wind affected sound, a car honking would have two different sounds and on a windy day vs a regular day.

And remember, sound isn't a particle. Depending on the strength of the sound wave, it will vibrate anything in its vector path to move across.

It's not like one of those river problems, where the river is going 4 m/s north to south, and a boat, that wants to move directly across from east to west. If he leaves his point at 3/ms, where will he end up?
lol, that doesn't happen with sound, because sound isn't a particle.
(that'd be funny though...you're standing at point A of a right trianlgle, and you yell something across to someone at point B, but the wind carries your sound away to a person at point C)

I'm pretty sure wind doesn't affect sound noticeably, however I'd like to know someone else's thought on this.
(BTW, what does EK say about that?)

 

[raab32]

Hahaha great point, that's a pretty dumb oversight on my part. I hate missing the big picture like that, thanks for setting me straight there.

I suppose my confusion arose from ek's explanation, which is as follows:
"C is correct. The wind is equivalent to giving both the source and the receiver a velocity equal in magnitude and in the opposite direction to the wind. Their relative velocity remains the same. In this case, the relative velocity is zero, and there is no Doppler effect."

But yeah, I get that now. Thanks for your help!

 

[Omni]

From my experience, EK does give pretty vague and really pathetic responses. That one being one of them. With EK's logic, I still don't understand how wind will affect the wavelength and cancel itself out...

 

[Isoprop]

Omni's explanation is not quite on mark.

Doppler shift is the change in frequency between a wave source and an observer due to changes in relative velocity. There is no change in relative velocity between observe and source, thus no doppler shift.

Here's another way to think about it:

Instead of the wind moving, think about the objects moving relative to the air. If the wind is traveling 5 m/s, you can think of both objects moving at 5 m/s in the same direction with the air standing still. If source and observer are traveling in the same direction at the same speed, the change in frequency is zero.

BTW, the speed of the medium DOES affect the velocity of waves. It's just the relative velocity of the object and source in this case doesn't change.

It's hard to explain without drawing pictures. Does this make sense?

 

[Omni]

Yeah, that does.
So lets say the wind was blowing perpendicularly, would that affect the frequency?

 

[Isoprop]

it shouldn't. there is no difference in velocity.

this is like driving a car and honking the horn. you hear the same frequency no matter what speed you drive at because you and the horn are traveling at the same speed. it doesn't matter what direction the air is moving.

 

[Inkoate]

I'm resurrecting this thread just to see if I'm right:

Since sound waves move through the air by longitudinal waves (parallel to displacement), they compress the molecules of air, producing areas of lower pressure and higher pressure within the wave.

If wind is blowing from observer to source, the wave is essentially shortening, ie the wavelength become smaller. However, no matter what, frequency never changes within the same medium. Therefore, the velocity of the wave will be slightly slower, but the frequency will remain the same. There is no Doppler Effect.

 

[[email protected]]

I'm resurrecting this thread just to see if I'm right:

Since sound waves move through the air by longitudinal waves (parallel to displacement), they compress the molecules of air, producing areas of lower pressure and higher pressure within the wave.

If wind is blowing from observer to source, the wave is essentially shortening, ie the wavelength become smaller. However, no matter what, frequency never changes within the same medium. Therefore, the velocity of the wave will be slightly slower, but the frequency will remain the same. There is no Doppler Effect.

Did anyone respond to this? I am having similar issues with these problems #749-#751 in EK 1001. Were you able to figure them out?