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Waves and objects

  1. Jan 19, 2014 #1
    Hi,
    Quick question (sort of):
    Say that a car passes in the hood in a street near your house, it's speakers booming Mozart. At first, you only hear the muffled sounds, but as the car comes into a certain range not only you hear it clearly, but your window vibrates. Since this changes with distance, I guess this is related to Doppler effect (I'm wrong, aren't I?). Anyone care to explain? References are welcome!

    Thanks a lot!
     
  2. jcsd
  3. Jan 19, 2014 #2

    Simon Bridge

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    Doppler effect only matters when there is relative movement - i.e. the car or you would have to be moving.
    If the car were stationary, the vlume and clarity of the sound would still depend o how far away the car was.
    It affects the frequency you hear - the tone of the sound - not the volume.
    So it cannot be doppler effect.

    The overall effect is called "attenuation".
    Sound gets quieter over long distances because it uses up energy to push the air molecules around like that.
    Intervening objects also make the sound quieter and can mess with the quality as well - the objects absorb some of the energy of the sound, and reflect the sound as well. There will also be interference effects.

    Real life is generally pretty messy with lots of things going on at once.
     
  4. Jan 20, 2014 #3

    BruceW

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    yeah. and specifically about the window vibrating, this is due to resonance. i.e. in the music, there are some frequencies that are in just the right range to make the window resonate with the sound waves. The reason you only get this when the car comes close is because the sound wave needs to be strong enough when it reaches the window. So this comes back to what Simon said about attenuation.
     
  5. Jan 20, 2014 #4

    Simon Bridge

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    There's no need to invoke resonance to explain the rattling windows - though there will be a component contributing the the observed effects - need only a very inconsiderate driver cranking the volume to max. But it is correct that the bass beats will be better at rattling the windows than the higher notes.

    Shorter wavelengths carry more energy, but the longer wavelengths let the pane move farther between compressions and rarefactions.

    In resonance - the waves don't have to be all that strong: the next wavefront keeps the vibration going.

    OTOH: the window would have a lot of damping - vibrations die out quickly - so the effect of a weaker front may get damped out before the next one can reinforce it.
     
  6. Jan 21, 2014 #5

    sophiecentaur

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    It's largely to do with Matching the power of the sound inside to the outside. If the eejit inside is listing to equal sound levels at all frequencies, more LF sound will be radiated through the panels and window panes as the higher frequencies will not couple so well (plus they will be absorbed by the 'furnishings').
    Then ' the same eejit' will be wanting his money's worth out of his sound equipment so he will be boosting the LF way beyond reason so that his intestines are thumping along with the music. So that's two good reasons.

    Also, the loud LF will diffract round corners and other vehicles so it may well propagate to you much better than the HF.

    Consolation is that he is knackering his hearing by listening at such high levels. Kharma.
     
  7. Jan 21, 2014 #6

    BruceW

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    yeah if the music is on inside the car, then I'd guess the car would be vibrating as well as the windows, so there would be not so much rattling. When the sound comes from outside I would guess the rattling comes from the window vibrating, while the rest of the car remains still.

    I would have guessed the vibration to be almost solely due to resonance. But you guys seem to think different. hmm. I don't know. I'm not even sure roughly what the resonant frequency would be. I suppose if the window is not completely tightly fitted in the door frame, then low frequencies could cause the glass to move back and forth in the frame very slightly, and therefore vibrate. Maybe resonance is not so important.
     
  8. Jan 21, 2014 #7

    sophiecentaur

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    Resonances will occur but, if there is good coupling into the air, the resonances will be very broad. The cavity of the car is likely to behave like a helmhotz resonator (like the your home cinema sub woofer). This consists of a volume of air (the cab) and a restricted vent, which could be the air intake. This could well make an effective resonator - very broad band.
    I would imagine that the body panels are just as likely to flex and transmit LF sound as the windows. The flat area of the roof will flex when you slap it (gently!) so the sound inside can couple out via that route. Good soundproofing at LF requires Mass and Loss. In a car, there is not much surplus Mass and the damping gunge in the doors and panels etc. is not very massive or very thick. In fact, I heard somewhere that lightweight car doors are deliberately made to close with an expensive sounding 'thump' so that particular band of frequencies are actually 'encouraged'. There could possibly be some built in resonance for the doors so you could be correct to some extent in your idea. We are very easily suckered into liking new cars for all sorts of trivial reasons!!!
     
  9. Jan 21, 2014 #8

    BruceW

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    If only I had some (High amplitude) source of single-frequency sound, I could test this out. oh, and I'd need a car. haha.
     
  10. Jan 21, 2014 #9

    sophiecentaur

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    Don't you have a smart phone of some sort? There are Apps which will do all sorts - and an ordinary loudspeaker would do (borrow a s/woofer?). You could do it all in a garage so you'd have mains electricity to make things easier.

    Whoops - I missed the bit about not having a car. durrr
     
  11. Jan 21, 2014 #10

    Simon Bridge

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    Hmmm... need a loud sound system in my karma...
     
  12. Jan 21, 2014 #11
    If your going to boom classical music... then go for Handel.
     
  13. Jan 22, 2014 #12
    What causes the the windows to vibrate (what is the resonance, a match between the frequency to density/somethinelse?), assuming that it is in the range where no extinction occurs?
     
  14. Jan 22, 2014 #13

    sophiecentaur

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    A sheet of glass will move microscopically when any sound (pressure) wave hits it. You do not need resonance for vibrations in one medium to cause vibrations in another (the CIA, it's rumoured, detect speech in a room by bouncing a laser off the window but that doesn't have to involve any resonance. When you do get resonance, the power transferred can be greater (a match) and, sometimes, the 'booming' may be in a narrow band. But that could simply be the amplifier / speaker characteristic. Despite the hundreds + of pounds spent by 'these lads', there are some aspects of acoustics that they just don't consider.

    The impedance of air is pretty low compared with the impedance of the glass in its frame or body panels, so most of the power will be reflected. If there is a resonance, the impedance of the glass may be reduced and more power will be transmitted. Thin body panels will flex more and be less massive than the glass so there may be more sound transmitted that way (lower impedance path). Hence the dense loading / damping panels they put in.
    I think a sub woofer, mounted in the boot (trunk), with a good seal between the cabin and the back, could manage to do a fair job of increasing the LF sound pressure level in both. (An 'infinite baffle')
     
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