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When discussing the Doppler effect, we use the word "apparent frequency". Does that mean that the frequency of sound is still that of the source and that it is some physiological phenomenon in the listener's ear that gives rise to Doppler effect? Is this different for the observer approaching the source and for the source approaching the observer?

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  • $\begingroup$ If the Doppler effect were physiological, then it likely wouldn't happen to all sounds for all people ;). But since it does, it follows that it is a physical phenomenon, no? $\endgroup$
    – Kyle Kanos
    Commented Mar 11, 2015 at 12:45

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The word "apparent" means "as observed at a particular point X". Different observers will observe different frequencies depending on their relative velocity to the source. This doesn't change the frequency of the sound that is generated; just the frequency of the sound that arrives at the ear of the observer.

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    $\begingroup$ This is the only answer (at this moment) that gets at what I think is the critical issue, which is that in physics the word "apparent" does not mean that something is a purely psychological effect, nor that it is not real. $\endgroup$
    – David Z
    Commented Mar 12, 2015 at 18:20
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Think of a car stopped with it's siren or horn or radio (playing the same note) turned on, it has a certain fixed frequency. Then the car moves towards you, it's siren stays exactly at the same frequency as when it was stopped , but this frequency is apparently higher only to you. This is because more waves are entering your ear per second than when the car was stopped. To the car driver, he/she does not notice any difference in frequency. So that's the source approaching the observer.

If the car stopped and you approach it, at high enough speeds, (in another car), you will still get the same effect, an apparent increase in frequency, but still only apparently higher to you, not the first car driver.

Last case, the car drives towards you and, at the same time , you drive away from the car. The apparent frequency you hear will depend on the difference in the two car,s speeds.

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The Doppler effect is a real physical effect that derives from the relative motion between a wave source and an observer.

Say I'm driving along, playing a recording in my car of a steady F# tone. Since I am in the car traveling along with the speakers, I hear the tone as F#. My friend is standing on the curb with his boom box playing the same tone, and hears F# from his speakers as well. This happens because there is no relative motion between me and my speakers, or between him and his speakers.

There is, however, relative motion between me and the boom box, and between my friend and my car; so as I drive toward him we each hear the other emitting a G tone.

That's the perspectival approach to it: the physical explanation of why that happens is that as the wave source (the speakers) move through a still medium (the air) they are "catching up" to the emitted waves ahead of them and "leaving behind" others. Let's say the tone is such a pitch that the wavelength is 1 meter when the speakers are stationary. Then we emit them from a vehicle travelling 0.5 m/s:

The waves moving in the direction of travel would normally have a wavelength of 1m, but because the emitter is "catching up" to them the wave crests are only 0.5m apart, so the frequency doubles. The waves traveling away from the back of the car are likewise "stretched out" to a wavelength of 1.5m, so the pitch lowers compared to the still tone.

This is a better generalization of the phenomenon since some waves (electromagnetic, for example) do not travel through a medium that can flow and thus independently alter the wavelength.

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It has nothing to do with physiology and everything to do with physics. If you were measuring the frequency with an entirely electro-mechanical system you would see the same effect. That was the whole point of Einstein's reasoning about light and relative motion that he used when developing the Special Theory of Relativity. The phrase "apparent frequency" is misleading. It should be something like "frequency measured in observing frame moving relative to the emitting frame" or just "measured frequency" once the conditions have been described.

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Imagine,you have some balls and you are throwing the balls towards your friend and your friend is catching them.

Suppose you throw 1 ball per second and then your friend will also receive 1 ball per second if you both are at rest with respect to each other.

Now if you have some non zero relative velocity with respect to your friend then you will still throw 1 ball per second but your friend will not recieve 1 ball per second,he will recieve X balls/sec and X depends on the relative velocity.

The similar is the situation with waves.here is the calculation,http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp.html

your friend really will not recieve 1 ball/sec.

AND ITS NOT AN ILLUSION.

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for source moving it is real frequency , it is actually true that air has frequency which is different than from source releasing it

but for receptor moving source at rest it is apparent air has same frequency as that of source but receptor detects it different

well it is one of short question in hcv

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  • $\begingroup$ You are absolutely right about the facts but write the same answer with an explanation if you can. Use mathjax feature to write readable equations. To learn more about mathjax, click here. $\endgroup$
    – Yashas
    Commented Nov 17, 2016 at 4:32

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