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The downfall of education in Australia

  1. Jun 15, 2004 #1
    I'm not really debunking anything, but I didn't know where else to put this. One of the chaps I hang out with sometimes used to lecture computer science at a Melbourne university. He's done some physics of course - you must in such degrees. So we're all talking crap one day, and this fellow pipes up with the idea that playing a stream of sound backwards in opposition to the stream itself (ie. if you play Stairway backwards from point A while it is played normally from Point B), you cancel out the sound, so there is silence. I can only assume the person has absolutely no idea what destructive intereference is all about. A truly whacky idea, and this was from a university lecturer. Amazing but true!
     
  2. jcsd
  3. Jun 15, 2004 #2

    Ivan Seeking

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    Since you took the time to tell the story I want to explain your objection a little more.

    Firstly, it is possible to destructively interfere two sound waves at a point or along a mathematical surface, but the energy can not be cancelled. If the sound interferes destructively at one point, it must [by the demand for conservation of energy] likewise interfere constructively to the same degree at another point or over some volume [edit: or we at least need to sum the energy over all space in which case it all still adds]. Another way to say this is that both the inverted and primary waves each come with only positive energy.

    Next, simply playing the stream backwards does not invert the signal. A one-to-one mapping of the incoming wave is needed; with a rarefied volume of air meeting a compressed volume of exactly equal and opposite magnitude. At some random point, if we take a freeze frame of the sound wave in the stream and look at the wave we see some value that we might call positive - lets say this is a push out for the speaker. Displacement at a point in the stream is still positive regardless of the direction from which it is approached. To create an inverted wave would require that all positive displacements are made negative – the speaker is pulled in instead of pushed out. This must track exactly throughout the entire stream. Then, it is only possible to hear this at a specific point or along some line.

    If you have a large room handy, one way to get this effect [a little] is to simply reverse the polarity of the speakers. Then, by placing the speakers next to each other, if you stand in front of them at a distance of about six feet, a significant reduction in the bass can ususally be detected by ear. If you walk around the room you should be able to detect hot and cold spots. Moving the speakers around can create different regions in space in which the low frequencies tend to cancel. The same effect can be found without even reversing the wires. Note also that high frequencies are very difficult to cancel at a specific point. This requires very precise positioning of all elements in addition to damping of ambient sound. I imagine the dynamic response of the speaker begins to kill the effect at high frequencies. The phase opposition is not likely preserved.

    I once played with some high frequency speakers and had some really interesting interference results from the 20KHz range. Being just at the upper frequency limit and the minimum amplitude limit for hearing, and with a wavelength of about 1/2 inch, as the 20KHz wave interferes with itself around the room, just a slight movement of one's head makes the sound audible or not. I could hear it as I sat, but if I turned my head only slightly the sound was completely inaudible.
     
    Last edited: Jun 15, 2004
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