Doppler's Effect

[Muzette]

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EK 1001 Physics #751
The source of a sound wave is stationary. The observer is moving toward the source. There is a steady win blowing from the source to the observer. how does the wind change the observed frequency?
ANS: The wind minimizes the Doppler Effect and decreases the frequency.

I thought that since the wind is blowing in the same direction that the sound is traveling from the source, that the wind would essentially 'carry' the sound and increase the Doppler effect. In the explanation it says that the velocity of the wind will be added to both Vobserver and V source in Doppler effect equation, making the ratio smaller than the original ratio without the wind. I have no idea why you would add the velocities to both Vobserver and Vsource. Please help!

 

[mehc012]

The Doppler equation doesn't really have a spot to put "velocity of the medium", so instead of using the reference frame that the observer and listener are both standing still and the air is moving, they are using the reference frame of the medium - the air.

 

[Shadowcaster]

EK 1001 Physics #751
The source of a sound wave is stationary. The observer is moving toward the source. There is a steady win blowing from the source to the observer. how does the wind change the observed frequency?
ANS: The wind minimizes the Doppler Effect and decreases the frequency.

I thought that since the wind is blowing in the same direction that the sound is traveling from the source, that the wind would essentially 'carry' the sound and increase the Doppler effect. In the explanation it says that the velocity of the wind will be added to both Vobserver and V source in Doppler effect equation, making the ratio smaller than the original ratio without the wind. I have no idea why you would add the velocities to both Vobserver and Vsource. Please help!

i think your study guide has an error if that is the answer. wind blowing from source to observer increases the speed of sound relative to the earth, and increases the frequency. imagine the wind was blowing at the speed of sound, away from the observer; there would be no sound observed. everything becomes much easier when you use extremes.

the percentage of the pitch shift might be less than without the wind, as observer speed increases, due to adding wind speed to Vo and Vs, but still, the observed frequency is going to be higher than it would be without wind.

Then again, I COULD BE WRONG. but I don't think I am.

 

[Temperature101]

That question on EK 1001 physics had me scratch my head as well. I hope and I think they are wrong.

 

[Qester]

Firstly I have not started my physics studying and I do not have any idea or notion of the Doppler effect. However from what I can derive from the information given in the question, I presume the Doppler effect is created by the observer moving in a direction with an incoming stimulus (sound). Let's assign a (+) value to the observer since he is moving closer to the origin. Now that the wind is moving away from the point of origin (-) it helps to counteract the doppler effect. If perhaps the wind and the person were moving at the exact same speed the frequency of the wave would be the exact same as if the person was not moving at all and there was no wind.

I think this since the answer states that the wind "minimizes" the effect. If the wind speed were less than the walking speed, there would still be an overall increase in frequency, just to a smaller degree.

Like I said though I actually have no idea about any of this and am just guessing.

 

[sazerac]

If ignorance were cheerios, this thread would be general mills.


Let's look at the windless case.

source /---\___/---\___/---\___/---\___ ear

Sound is produced at 1Hz, and there are four wavelengths of sound between the source and the ear. Let's say the ear quickly and suddenly went all the way to the source, in an instant. Whoosh, he heard four cycles in one instant, due to the doppler effect.


Now let's say the wind is blowing towards the source. I like this example because it is easy to think about. Those waves are going to pile up, but they are still produced a 1Hz and heard at 1Hz.

source /-\_/-\_/-\_/-\_/-\_/-\_/-\_/-\_ ear
<-------- WIND <--------

Notice that the SPEED of the wave is lowered by one half, the WAVLENGTH is lowered by one half, and the FREQUENCY of the sound is the same. v = f lambda. It all works out

Now let's say the ear again goes to the source in an instant. Whoosh, now the ear hears a much HIGHER frequency sound, a whole eight cycles in that instant.

What have we learned here? The doppler effect is INCREASED if the wind blows from the ear to the source.


Finally let's look at the question. The wind is blowing towards the ear.

source /-------\_______/-------\_______ ear
--------> WIND -------->

Those sounds are still produced at 1Hz. They are still heard at 1Hz. While they are traveling towards the ear the WAVELENGTH is doubled and the SPEED is doubled, but the FREQUENCY is the same 1Hz it always was. Think about this. The frequency can't change because you are only building waves at 1Hz. You aren't building them any faster or slower, and you can't create extra ones or destroy them per unit time. v = f lambda, again it all works out.

Now the ear once again runs at a million miles an hour to the source. Whoosh, now the frequency of sound heard picks up to a whopping two cycles per instant. Kind of lame, compared to the other scenarios, above.



CONCLUSION: If a steady wind is blowing from the source to the ear, and the ear is moving towards the source, the doppler effect will be minimized, and the observed frequency will be smaller than if there was no wind at all.

Sure this could be proven with Vo's and Vs's and some weird equations that are hard to memorize and easy to get upside down, but like most MCAT questions you can solve this one in under a minute if you just build up some simple yet extreme examples.

 

[Shadowcaster]

If ignorance were cheerios, this thread would be general mills.


Let's look at the windless case.

source /---\___/---\___/---\___/---\___ ear

Sound is produced at 1Hz, and there are four wavelengths of sound between the source and the ear. Let's say the ear quickly and suddenly went all the way to the source, in an instant. Whoosh, he heard four cycles in one instant, due to the doppler effect.


Now let's say the wind is blowing towards the source. I like this example because it is easy to think about. Those waves are going to pile up, but they are still produced a 1Hz and heard at 1Hz.

source /-\_/-\_/-\_/-\_/-\_/-\_/-\_/-\_ ear
<-------- WIND <--------

Notice that the SPEED of the wave is lowered by one half, the WAVLENGTH is lowered by one half, and the FREQUENCY of the sound is the same. v = f lambda. It all works out

Now let's say the ear again goes to the source in an instant. Whoosh, now the ear hears a much HIGHER frequency sound, a whole eight cycles in that instant.

What have we learned here? The doppler effect is INCREASED if the wind blows from the ear to the source.


Finally let's look at the question. The wind is blowing towards the ear.

source /-------\_______/-------\_______ ear
--------> WIND -------->

Those sounds are still produced at 1Hz. They are still heard at 1Hz. While they are traveling towards the ear the WAVELENGTH is doubled and the SPEED is doubled, but the FREQUENCY is the same 1Hz it always was. Think about this. The frequency can't change because you are only building waves at 1Hz. You aren't building them any faster or slower, and you can't create extra ones or destroy them per unit time. v = f lambda, again it all works out.

Now the ear once again runs at a million miles an hour to the source. Whoosh, now the frequency of sound heard picks up to a whopping two cycles per instant. Kind of lame, compared to the other scenarios, above.



CONCLUSION: If a steady wind is blowing from the source to the ear, and the ear is moving towards the source, the doppler effect will be minimized, and the observed frequency will be smaller than if there was no wind at all.

Sure this could be proven with Vo's and Vs's and some weird equations that are hard to memorize and easy to get upside down, but like most MCAT questions you can solve this one in under a minute if you just build up some simple yet extreme examples.

this man is a genius