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Connect two identical speakers in Parallel. Place them
adjacent to each other (rims touching, not facing each
other but facing same direction). Feed them output of
one of the
channels of a stereo amplifier. Now both speakers should
be
cancelling each other out due to destructive
interference.
( If
not, swap two connections of any one of the two
speakers).
This will form "dark spots" in the room where (and only
where) absolutely no sound from speakers will be
heard.
Arrange Halloween party in such a room.
random video of demo with two speakers
http://www.youtube....watch?v=_MM1l7AhMac
Even has a math derivation! [sqeaketh the wheel, Dec 18 2010]
Public Noise Cancelation
Public_20Noise_20Cancelation
[mouseposture, Dec 19 2010]
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Annotation:
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Baked ((wrong - see below)) Physics professors do this all the time in classes on acoustics. |
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[+] I think you'd get a wide middle dead spot if you connect them up out of phase. |
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//if you connect them up out of phase// |
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which I think will be achieved by swapping the two
connections of one of the two speakers. |
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Could be cool. Phase cancellation and augmentation as you walk through the peaks and valleys can give a weird "sound is changing in my head" effect that's kind of hard to describe. |
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You could also use this in conjuction with a sound absorbing "dead room" that feels like being in a dream or something. You hear your breath only and get to see how noisy a "quiet" room actually is by comparison. |
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You'd be rather limited in what sounds you could play on
these speakers, no? Just line spectra. But it could still be
pretty eerie. |
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At first I too thought it will work only with a fixed
sine wave. But Then again this may not be the case. |
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//Physics professors do this all the time... |
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[sqeaketh the wheel] do you have more details ? |
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//But Then again this may not be the case.// Explain that,
please, 'cause I thought the same (one or a very few sine
waves, anyway). |
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In case of a general purpose Noise Cancellation
device, I think tricky part is predicting what the next
signal is going to be and generate opposite of that. |
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But here, that isn't the case. Or, so I think. Also
source and anti-source are as close to each other as
possible. Which again takes away the need for simple
sine wave. |
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//Baked. Physics professors do this all the time in
classes on acoustics// |
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My physics teacher never did that. Has any one seen
a live demo of this ? |
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I just ran a speaker test program on my computer speakers. The polarity test featured a pure sine-sounding signal that was supposed to die out if the speakers were hooked up wrong. I don't think the noise-cancelling would work on anything else. |
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The storm sirens that are too close to my house give out a fairly pure tone. When my windows are open when they go off, the sound varies wildly as I run through the house to let the dog in before his ears implode. |
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Was same signal fed to both speakers; If it is a stereo
signal, then you can never be sure of it. |
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Also I think it is very essential that one of the
speaker connections must be swapped. This I think is
impossible in standard computer/laptop setup. |
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One will need to rip-open one of the speaker cables
and swap two connections. |
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//Physics professors do this all the time... |
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[+] Changed my vote. I was wrong about "all" professors, although it is well known for the case of two sine waves. (link) |
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This even works in a crowded lecture hall, where some students hear loud and others quiet. When the phase of one speaker is flipped (by switching the wires [VJW]), the opposite students hear loud or quiet. |
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For music or white noise, it doesn't really work dramatically, unless the two speakers are facing and near (< 1m) each other, and both speakers are fed by the same signal (not two different stereo channels [VJW]). Then, only certain frequencies will cancel, depending on the speakers' separation [bigsleep]. |
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Thanks for link, [sqeaketh the wheel]; |
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How ever I think he is forming a stationary wave in
the area between two speakers. Hence he is getting
fixed peaks and valleys of sound. |
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Yeah, though I walk through the peaks and valleys of sound, I shall fear no weird sounds... |
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I tried this a few years ago. In an ordinary room, the effect is fairly subtle. You certainly don't get any regions where no sound is heard at all, but you do get a //weird "sound is changing in my head"// effect. |
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If we try to understand noise cancellation in terms of
speaker cone movements then really waveform
should not matter, isn't it ? |
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We can say that when one speaker cone pushes
ahead the other one retracts back by same amount,
thus negating +ve pressure with -ve pressure. |
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So it should work for white noise, music or just pure
sine wave. |
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That is correct, as long as the detector is equally distant from the two sources. |
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In the real world, you don't hear silence because |
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1) Reflections from walls take various times to get to you |
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2) Speakers are not point sources (they are of significant size relative to audible wavelengths) |
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3) Ears are not point detectors; you (probably) have two of them, and they also have significant size. |
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Ear size should not matter because if this thing
works perfectly there should not any wave at all
anywhere in the room, unlike in the case of a
standing wave. |
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//there should not any wave at all anywhere in the room// But there will be, if it's done as you describe. You should actually try this. It's not difficult - just reverse the wires on one speaker and play a mono track, or use a sound editor to create a file where one channel is the exact inverse of the other. |
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The result will be different from what you describe, but interesting anyway. |
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The "dark spots" referred are not really valleys of a
stationary wave. I think they will be caused by
"imperfections" of experiment due to reflections
on the wall and other objects in the room etc. |
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As a result of this experiment, we are not
expecting a stationary wave to be formed. We are
expecting absolutely no waves formed at all. But
due to imperfections there will be dark spots. |
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We must remember that noise cancellation and
stationary wave formation are two different
things. I think demos which physics professors
show is about stationary waves. |
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[VJW] Ah, now I understand, I think. <link> |
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//there should not any wave at all anywhere in the
room//
//This will form "dark spots" in the room// |
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Can you clarify this apparent contradiction? Also, the first
statement is only true if the speed of sound is infinite, no? |
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//there should not any wave at all anywhere in
the room// as per theory! |
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//This will form "dark spots" in the room// due to
imperfections of the experiments such reflections
of walls, non-zero speaker size, etc. |
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We can NEVER achieve Noise Cancellation by
forming standing waves. Both are different things. |
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As per theory stationary waves are supposed have
peaks/valleys (AKA dark spots). ( this is what most
physics demos show) |
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Whereas if Noise meets anti noise both totally
cancel out each other. There should be no
vibrational energy in the medium ( not even dark
and non-dark areas). It should be all-dark in the
perfect world. |
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Ok here is the crucial difference between two: |
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Stationary waves are formed when two waves of
equal amplitude travelling in *opposite* direction
meet each other. |
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In case of "Noise" cancellation, two waves of equal
amplitude, opposite phase, travelling in *SAME*
direction superimpose each other. |
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//Stationary waves are formed when two waves of equal
amplitude travelling in *opposite* direction meet each
other
In case of "Noise" cancellation, two waves of equal
amplitude, opposite phase, travelling in *SAME* direction
superimpose each other.// |
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You are assuming a space of only one dimension, then
being inconsistent with that assumption by squeezing in
two speakers "side by side" on a second dimension. You
should have said "different direction" rather than
"opposite direction."
The waves can only travel in the same direction if the two
speakers occupy the same
point in space. |
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This is easy to accomplish: by linear superposition, you
can just use a *single* speaker, and disconnect it from the
amplifier so it makes no sound. In fact, you can
accomplish this by having no speaker at all. And, just as
you predict, there will be no wave, anywhere in the room. |
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Where the idea becomes nontrivial is in supposing that if
the speakers are sufficiently close on the second
dimension, you'll get a sufficiently
good approximation of that state of affairs. |
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The problem, so it seems to me, is this: |
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Consider what constitutes "sufficiently close." The
distance must be much smaller than the smallest
wavelength in your audio signal. Otherwise, you don't get
"noise cancellation" you get "standing waves." Now,
consider that your speakers aren't point sources, that their
size is, in fact, on the order of the *largest* wavelength in
your audio signal. |
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