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'Doppler Effect' advanced Sound Wave question

  1. May 5, 2012 #1
    When a car is approaching it has a higher frequency and shorter wavelength. When it has passed sound waves are a lower frequency due to a longer wavelength.

    Question: Does the higher frequency contain more energy when compared to the low frequency? IF IT DOES NOT, then could it possibly be viewed as since the waves are more compressed when approaching you, these compressions allow for more waves to be created per second? (in a sense, adding more 'sound energy 'because of the 'density of the waves' increases)
    Last edited: May 5, 2012
  2. jcsd
  3. May 5, 2012 #2


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    you mean shorter wavelength there :)

    I would suggest that the amount of energy hasnt changed since the freq and energy of the noise emitted from the car hasnt changed. It only appears to change from your stationary point of view (your frame of reference).

  4. May 5, 2012 #3


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    Hi Rorkster2!

    You seem to have a good grasp of what the Doppler effect is and how it works. I am sure you simply made a "typo" above...you know when the sound source is approaching the frequency is higher and the wavelength shorter. OK

    The higher frequency may contain more "sound energy density", because that is measured in Watts-seconds per cubic meter. Notice that is not the same as "sound energy". "Sound intensity" is measured in Watts per meter squared and "sound power" is measured in Watts. After that we have the dB scale which is often used as well.

    In dealing with this topic it's imperative to use the technical terms correctly so there is no misunderstanding. I suggest you read up on these definitions and then, when you've decided exactly what you'd like to know, come right back here and post your query.

  5. May 5, 2012 #4
    Before I posted this I thought how I'd be likely to make that typo but wouldn't because I'd double check. So much for that. Thanks Bob that sound density equation in the link answered my question.
  6. May 5, 2012 #5


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    In the case of sound propagation the velocity of the emitter relative to the medium (the air) makes a difference.

    In this particular case you, the receiver, are stationary with respect to the air, and the car has a velocity relative to the air. That means that the sound ahead of the car is shorter wavelength than the sound behind the car.

    Conversely, if you are moving relative to the air, and the car is stationary relative to the air, then you get a doppler effect too, but now the sound wavelength around the car is symmetric. Now it's your velocity relative to the air that creates an apparent shortening of the wavelength.
    Last edited: May 5, 2012
  7. May 5, 2012 #6


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    For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects must be analyzed separately. http://en.wikipedia.org/wiki/Doppler_effect

    If you are moving relative to the air, and the car is stationary relative to the air, then by definition you are moving relative to the car. Then of course there will be a real Doppler shift in the received frequency. It is not your velocity relative to the air, it is simply your velocity relative to the car that causes the effect.
  8. May 5, 2012 #7


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    Here's another case: a sound emitter and a sound receiver are co-moving, with uniform velocity, moving with a particular velocity relative to the air.

    Then for the receiver the two frequency shift effects cancel out. Still, the sound around the emitter is not symmetric. In the air ahead of the emitter the wavelength is shorter, behind it longer. That is, distributed in the volume of air surrounding the cars there is a physical frequency shift effect.

    There is danger of babylonian confusion here. Are we to understand the expression 'Doppler effect' as 'a shift of locally measured frequency'? Or as 'a physical difference as evaluated over an extended volume of space'?

    Also, the expression Doppler effect is used both in the context of sound propagation, and in the context of propagation of electromagnetic waves. As we know, in sound propagation the velocity relative to the medium matters, but for electromagnetic wave Doppler effects only the relative velocity of emitter and receiver enters the equations.

    I used the expression 'Doppler effect' in a way that is natural for the case of sound propagation. In the case of sound propagation there is the actual physics of the sound in the air.

    Once again comparing: as we know, in the case of electromagnetic waves there is no such thing as distinction between physical Doppler shift and apparent Doppler shift. There's just that: a frequency shift.
    Conversely, in the case of sound propagation velocity relative to the medium is a factor, and that is what I referred to in distinguishing between physical effect and apparent effect.
  9. May 5, 2012 #8
    This has all been helpful and as I said a while ago I'm pretty sure I have my answer from the wiki page about sound density. JUST TO DOUBLE CHECK am I correct in saying, as a truck approaches someone who is standing still will receive higher frequency waves that are a slightly more energized compared to when the truck passes the observer who then hears lower frequency sound waves that are less dense, and therefor slightly less energized?
  10. May 5, 2012 #9


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    I'm curious why you choose to express it in terms of more/less energized.

    What is your focus? Is your focus the propagating sound in itself? Or is your focus on what the observer can tell by ear? It makes a difference. For instance in the case I described earlier, two trucks co-moving. Then the receiving observer does not hear a shifted frequency, because the frequency shift effects that are physically occurring cancel each other.
    To avoid that kind of ambiguity I don't phrase my answer in terms of what an individual observer can tell by ear.

    For the case of sound, when a truck is moving relative to the air the frequency shift effect happens at the point of emission. The sound itself around the moving truck is not symmetric.

    (We have no devices that harvest energy from sound, so it feels strange to me to talk about the energy of sound when discussing Doppler effects. By comparison, we do harvest energy from light, and some devices have a frequency threshold. For light it's natural to talk about the frequency related energy, but for sound it isn't.)
  11. May 6, 2012 #10

    My reasons are complicated, but in a nutshell I'm trying to figure out if, due to the dopplar effect, objects can emit different energy levels relative to where the observer is. First I was wondering if stars that appear red due to redshift have lower measurable light wave levels then blue stars moving away.

    Although we have no way of 'harrnessing' these energies because of how small they are, I think that these may be questions science failed to take into account when looking at some large scale problems where these tiny differences can add up to a documentable effect in ways that may have been attributed to something else
  12. May 6, 2012 #11


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    I don't think you can mean energy, unless you're thinking of energy per cycle. You probably mean power, energy per unit time.
    It will be the same energy per cycle, but since the cycles are faster it will be higher power.
    E.g. suppose the source emits at a 1m wavelength (about 300Hz), it's now 100m from you, approaching at 10m/s. There are 100 wavelengths between you. In the next 10 seconds, all of those will reach you, plus all the extra waves it emits in 10 sec, about 3000. That's a total of 3100 waves, each with the same energy, compared with only 3000 in 10s if the source were a fixed distance away.
  13. May 6, 2012 #12


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    Well, propagation of sound and propagation of light are different cases. Statements that are valid for sound are generally not valid for light, and vice versa.

    So for the case of shift of starlight you cannot use anything that has been written in this thread; the cases are very different.
  14. May 6, 2012 #13
    Light is both a partical and a wave. Because of the big bang the universe exploded out in all directions and because of inflation most matter is accelerating its speed instead of slowing downg. This gives vast majority of galaxy's we see a red color because of redshift. The light waves are made longer and as a star is approaching us it has a blueshift caused by a higher 'frequency' wavelength.

    The cause and effect of both are on par. No?
  15. May 6, 2012 #14


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    Rorkster2, Finally, after all the efforts of other members to discover what you were asking about, you have posted your "complicated reasons" and now you allude mysteriously to things "science failed to take into account..." without saying exactly what. Why all the secrecy? Why not just propose your idea up front?

    As others here have already told you, trying to compare the Doppler effect in acoustics to electromagnetic radiation does NOT work. EM radiation travels in a vacuum and sound needs a medium to propagate.

    Furthermore, you have ignored the suggestion in post number three: "In dealing with this topic it's imperative to use the technical terms correctly so there is no misunderstanding. I suggest you read up on these definitions and then, when you've decided exactly what you'd like to know, come right back here and post your query."

    Look up, please, at the terms you have just used: objects can emit different energy levels What does that mean? What are the units? And: light wave levels. What does that mean? You cannot expect a meaningful answer if you don't communicate your meaning accurately.

    Finally, will you please explain what you mean by "some large scale problems where these tiny differences can add up to a documentable effect in ways that may have been attributed to something else"? If you decide to continue to be evasive I will not participate here any more, and I would not be surprised if others refused also.
  16. May 6, 2012 #15
    That's a lot to say to someone who said he's found what he came here for. I'm not saying their the same, just talking about conceptual similarities in two given phenonomons. I'm merely working on an understanding and am not one to post about idea fragments and the thoughts behind them. Maybe I did come off a little vague but so are my thoughts, as are you're thoughts (Bobbywhy), or even yours (the other readers)
  17. May 6, 2012 #16


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    This is not good.

    You're mixing things up really badly. By now it's clear that your understanding is very vague.

    In cosmology the theory of 'cosmic inflation' is about something at the very beginning that lasted only a very short time. In the theory of inflation there is no perspective on the cause of cosmic inflation. That theory just says, that if there was cosmic inflation then you expect the universe to look so-and-so.

    Then there is the cosmic expansion that has been going on after the stage of cosmic inflation (assuming this thing called 'cosmic inflation' has occurred). Astronomers have inferred from measurements that at some stage the cosmic expansion has started to accelerate (rather than deceleration throughout the history of the universe, which is what you would expect.)

    Cosmic expansion can be described with the equations of general relativity. These equations allow a form that describes accelerating cosmic expansion, but you need an entity to cause that acceleration. The properties of this entity are inferred by working out what factor needs to be added in the equations to end up with accelerating cosmic expansion. This additional factor is referred to as 'Dark Energy'.

    The only reason I wrote here about cosmic inflation and cosmic expansion is to point out that the two are distinct.

    What we can tell from your quote is that you got them mixed up, and you got several other things mixed up too.
  18. May 6, 2012 #17
    Yes, thank you I did say the wrong one however I stand by that my understanding is clear and my comparison of similarities is just. My uncompleted ideas are also not wrong for the same reason why all unformed ideas ar right or wrong.
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