I was wondering, can we use two sound sources so as to create a destructive interference at the position of a recorder at home? If possible, what is the easiest way?
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asked Oct 15, 2014 at 17:30
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1$\begingroup$ It is very easy if you have a stereo. Connect your computer output to the stereo system, aim the two speakers at each other. Use a program to generate a sine wave on both channels and there you have it... somewhere between the speakers there should be an easily observable destructive interference. $\endgroup$– CuriousOneCommented Oct 15, 2014 at 17:59
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$\begingroup$ Easily observable? I can calculate it, can't I? My confusion was about the fact that the diameter of the recorder is relatively big, while the destructive interference happens at a point. :/ So what will the recorder record? (No sound?) $\endgroup$– George SmyridisCommented Oct 15, 2014 at 18:15
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$\begingroup$ You will observe changes in intensity even if you might not get a perfect null. But even a finite size microphone, properly oriented, will see a "true null" at a certain point where one side of the membrane moves out (low pressure) while the other side moves in (higher pressure). $\endgroup$– FlorisCommented Oct 15, 2014 at 18:18
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$\begingroup$ Yes - Fragile, EL&P - Trilogy for nice bomb-blasts to try this out with (reverse the leads on one speaker). $\endgroup$– Carl WitthoftCommented Oct 15, 2014 at 18:25
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$\begingroup$ I am not sure what you are asking. Are you asking why the world is not ideal like simplifying calculations are? The answer of physics to that is the sweat of the experimental physicist who doesn't mind living in a non-ideal world. You can make the extinction as perfect as you like, even with a finite size microphone, you just have to work harder on it (and you won't learn much about nature while doing so). $\endgroup$– CuriousOneCommented Oct 15, 2014 at 18:29
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The only place where I have ever seen this done successfully, and actually heard the cancellation was in a room where there were no reflections from the walls or the floor (an anechoic chamber). In this case, the cancellation was so perfect that I couldn't hear anything. I took a couple steps laterally and then the sound returned. I was quite stunned. I think this experiment will be quite difficult to do at home unless you can come up with a way to prevent reflections from everywhere, including the ground! Of course, I would be foolish to suggest that it was impossible, but perhaps I am just not creative enough! Just for reference, an anechoic chamber looks like so:
