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Thread: visible sound waves

  1. #1 visible sound waves 
    Moderator Moderator AlexP's Avatar
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    if a speaker were placed inside a chamber filled with a colored gas and it was used to play music, would the sound waves be able to be seen? the chamber can be as large as it needs to be (i think sound waves are pretty large...)


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  3. #2  
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    Yes. If you have some way to make sound slower, the amplitude is high enough, and the wavelength is long enough.


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    Forum Professor leohopkins's Avatar
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    I am going to say NO.

    But simply because you would have say Red gas moving against, erm, red gas. You wouldn't see it !!

    Why not put your speakers next to a tub of water and turn up the volume and bass. Then you can see soundwaves move through the water. (well they move through all of the water, you your would see the top of the water move.
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    Why not just look at the speaker cone moving about?
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  6. #5  
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    Quote Originally Posted by leohopkins
    I am going to say NO.

    But simply because you would have say Red gas moving against, erm, red gas. You wouldn't see it !!
    Suppose you have a transparent Red medium in a long glass tube, and generate pressure oscillation at one end of the tube. Then you will see sharp Red and more transparent Red stripes running from one end of the tube to the other. Because sound wave is density wave.

    Quote Originally Posted by leohopkins
    Why not put your speakers next to a tub of water and turn up the volume and bass. Then you can see soundwaves move through the water. (well they move through all of the water, you your would see the top of the water move.
    "Top of water move" is not a sound wave, I think it is called a surface wave.
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  7. #6  
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    Quote Originally Posted by leohopkins
    I am going to say NO.

    But simply because you would have say Red gas moving against, erm, red gas. You wouldn't see it !!

    Why not put your speakers next to a tub of water and turn up the volume and bass. Then you can see soundwaves move through the water. (well they move through all of the water, you your would see the top of the water move.
    you wouldn't see darker red against lighter red when some of the red gas was pushed into a higher concentration?
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  8. #7  
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    Making pressure waves visible is a subject that falls into the category of "flow visualization". There are a number of techniques to make pressure variations (indirectly) visible, but as far as I know these mostly apply to relatively strong waves, i.e. compression shocks, not sound waves.

    Sound waves, although detectable by our ears and audio instruments, are actually waves of very (almost infinitesimally) small amplitude. Pressure itself cannot be made visible, so you have to rely on density. The light refraction coefficient depends on density, and viz techniques like the "Schlieren" method make use of that property. However, the problem with sound waves is, as I said, the pressure amplitude is actually very small, and the density variations is typically much smaller than the pressure variation.

    In addition, a sound wave moves at the speed of sound (duh!), and even if you could make it visible, it would be very hard to follow at 340 meters per second in air (you'd have to cool down the air significantly to reduce the speed of sound which is proportional to the square root of temperature). Your best shot would be to create a standing wave and make it very strong, like the "compression shock" you can observe standing over an airplane wing, when the lighting is right. I have also seen photos of pressure waves eminating from a small hole in a wall, where a bullet just flies by. But again, this could not be seen with the naked eye in real-time, because it happens too fast.
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    You can visualise how little the air pressure varies for sound by thinking of a speaker playing music in the open air, suppose the sound travels 40 metres and no more, and at that distance you place a stake in the ground.

    Now look at the speaker and you will see the cone moves by only a few mm in each direction from rest. What you now have is some 40,000mm of air compressed/ rarified by about 3mm ie about 0.013 pecent. THink about the change in pressure ratio and you'll soon see why we mostly rely on computer modelling for such questions.
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