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B Why do concerts not impact the way we see light?

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  1. Mar 13, 2017 #1
    If concert environments are saturated with sound, why don't we witness a change in light colours (frequency shift) or reflection or any other effect witnessed in laboratory experiments documenting acoustic-optic effects?
     
  2. jcsd
  3. Mar 13, 2017 #2

    Drakkith

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    Those acousto-optical effects require that both the light and the sound be traveling through specific mediums such as ultrasonic gratings. Light and sound do not normally interact significantly in the air at a concert.
     
  4. Mar 13, 2017 #3
    That's what I originally thought. However, an IEEE paper and other papers did experiments on the interaction in air. So this further confused me. - For those who can't access the paper in IEEE - It is a paper which reports the measurements of Sound Pressure via the acoustic-optic effect in air with a laser. And the interaction is confirmed.
     
  5. Mar 13, 2017 #4

    Drakkith

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    I can't read the full text, but I think the authors just measured the amount of acousto-optical effect the air has. I expect it is very low.
     
  6. Mar 13, 2017 #5
    The conclusion of the paper says: "Sound pressure was able to be measured by a laser doppler velocimeter through the optical phase modulation due to the acoustic-optic effect of air"

    So it's not so insignificant as to not cause any phase change.

    But perhaps not strong to influence the frequency.
     
  7. Mar 13, 2017 #6

    anorlunda

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    Neat stuff, but I think the answer is simple. Laser measurements can be much more sensitive than human sensory organs.
     
    Last edited by a moderator: May 8, 2017
  8. Mar 13, 2017 #7

    sophiecentaur

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    If you want to see an opto acoustic effect, stand at some distance from a crt computer monitor (several metres) and whistle or hum. It can often produce the impression of moving bars on the display. The effect needs a scanned display, in my experience and doesn't seem to work up close (for me). It's not Physics, tho'. It's in your head.
     
  9. Mar 13, 2017 #8

    anorlunda

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    That sounds cool. Do you mean that it makes your eyes bounce up and down?

    Like everything else human, I believe that human abilities vary widely between individuals. I've heard that artists can be trained to resolve more colors than most people. I'm not surprised that some people can see stroboscopic effects that others can't see.

    It is also a good age test. If you know what the horizontal hold and vertical hold knobs on a TV were for, you are an old fart. :woot:
     
  10. Mar 13, 2017 #9

    A.T.

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    I think so. I noticed it when eating crunchy stuff in front of some displays. Or when when using an electric tooth brush.
     
    Last edited: Mar 13, 2017
  11. Mar 13, 2017 #10

    A.T.

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    You need higher intensity to see those with the naked eye:

    http://images.slideplayer.com/21/6283886/slides/slide_27.jpg

    Click on image below for full scale:

    HMnow.jpg


    More discussion:
    http://physics.stackexchange.com/qu...adient-in-a-shock-wave-of-supersonic-aircraft
     
  12. Mar 13, 2017 #11
    The paper mentions that they use "a strong ultrasonic field" to measure the effect.
    The measured value of the acoustic pressure was 610 Pa.
    For a sound in air this corresponds to an intensity of about 460 W/m^2 or over 145 decibels.
    This level is unlikely to be encountered in a concert hall, even if it's a hard rock band.
    But even at this level you need a sensitive instrument to measure the Doppler shift. They are talking about velocities produced by the sound in the order of mm/s.
     
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