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Fifteen minutes into the DJ's hour-long set, the fans blowing across the dancefloor reached their first maximum. Hair streamed from right to left, and as the music approached its crescendo, empty plastic drink glasses started to topple from their precarious perches and roll across the hard wooden floor.
The crowd were going wild, punching the air and yelling into the howling wind; and right as the music reached its peak, the gale started to ease off, leaving the eager dancers looking around and grinning as the atmosphere relaxed.
Over the next fifteen minutes, gradually at first but then more rapidly, the power to the fans was reduced, until at the midpoint of the show the air was as still as it had been when the DJ first took to the decks, and the only signs of the earlier tempest were the dishevelled hair of the partygoers and the piles of plastic drinks containers at the left hand side of the dancefloor.
The show was far from over, though. The giant fans had started turning slowly in the other direction, and now they ramped up, just as they had earlier, to the crowd's great delight; and this time, when they reached their second peak, the plastic glasses were sent back across the dancefloor the other way, and the crowd faced to the left this time.
The air became still once more just as the DJ finished his final tune, and as the crowd became still, he switched on the microphone and said "That was the lowest bass frequency you'll ever experience in a one hour set. Thank you, ladies and gentlemen!"
I Can Name That Tune In One Note!
http://www.youtube....watch?v=2vYlHTGsmso
...or not... [Grogster, Nov 12 2011]
Rotary Woofer
http://en.wikipedia.../wiki/Rotary_woofer
DC response using audio-pitched fan blades [csea, Nov 12 2011]
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If a tree falls in the forest and no one is able to hear
it, does it make a sound? |
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I thought this was going to be an idea about overfishing a certain species of fish almost, but not quite, to extinction. |
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Didn't Hiawatha have a throwdown with the Ultimate
Bass? |
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I mean my, my, my, my your vibe's pelican fly
I mean, it's so live and I’m loving your jive
You’re acoustic-er than the guy without a sine on his mind, oh... |
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Not exactly widely known to exist, but baked in the form of the Rotary Woofer. [link] |
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I would argue that the blowing of air in one direction then the other through a room does not constitute a low frequency sound. A closer analogue would be to make the room airtight, and slowly vary the pressure over the course of the set using a reversible pump. |
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Perhaps best of all would be if one wall acted as a well-sealed piston, and went through one complete oscillation, forcing the revellers to resort to riverdancing in sardine formation against the opposite wall at TDC and sprawling on the floor gasping for breath at BDC. |
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A propagating sound wave has both pressure maxima and minima, and velocity maxima and minima, which are of course 90 degrees out of phase with each other. |
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If you stood in a sufficiently large space and actually experienced a 1/3600 Hz sound wave of sufficient amplitude, you'd feel both effects, so I suppose you could argue that the show should incorporate both the fans and the piston. (This presents a practical problem, though, since with only one of the two you can have the show start and end with "normal" room conditions.) |
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A vaguely related but more abstract question: if you record a sound using a microphone and then calculate its FFT, what is the physical meaning of the real and imaginary components of the DC bin? Which is pressure and which is velocity? (I imagine it depends what kind of microphone you used.) |
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//both pressure maxima and minima, and velocity maxima and minima// Correct, but a rather large pressure change corresponds to a relatively small amount of flow; to be precise, just enough flow to increase the density by the required amount. |
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//I imagine it depends what kind of microphone you used.// In a discrete Fourier transform the imaginary part corresponds to the relative phase of the particular frequency bin; I don't think it's correct to think of it as representing either the pressure or the velocity as such. |
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There are nearly pure velocity microphones (such as ribbon microphones), and nearly pure pressure microphones (such as condenser microphones) and in-between ones (such as dynamic microphones); in each case, since the microphone records a 1-dimensional signal (which is pure pressure, pure velocity, or some function of both), the pressure and velocity components are not represented separately in the sample, so neither are they represented separately in the FFT transform. |
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If you recorded the same signal with a pressure- and a velocity-sensitive microphone, you would get (I think) closely similar real components but different imaginary components in the FFT. |
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//the relative phase of the particular frequency bin// But the DC bin has both real and imaginary parts too; how can DC have a relative phase? That implies it's a complex quantity so it has to correspond to 2 physical variables, namely pressure and velocity. But which is real, and which is imaginary? |
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OK, you've got me there; I'm not sure what the imaginary component of bin 0 represents, though I intend to find out. But it can't correspond to one of two physical values, since your data only contain the equivalent of one. |
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Hmm... if you had a microphone that recorded both pressure and velocity, and combined them into a single complex number, would the FFT be able to differentiate between frequencies approaching the microphone from the left vs the right, separating them into the negative and positive frequency bins? |
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If so, could you make a stereo microphone by putting it in a suitable case? |
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Maybe this is worth posting as another idea. |
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I have a friend who teaches a degree course in this stuff, and understands it much better than I do. I'll see what he thinks about it. |
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Combining pressure- and velocity-microphones to produce directional information is a good idea - it's also baked (although certainly not WKTE). |
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It is possible to carry out an FFT transform on multidimensional data (such as the 2 dimensions, pressure and velocity), but using real vectors, rather than complex numbers, as input. |
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Ah, I knew there was a catch: the fans would have to be able to project the sound at the speed of sound in air. |
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Thanks for that very interesting link, [csea]. More interesting links off the linked page. From the wikipedia talk page, //the age of the universe to date wound be 2.3 aHz// |
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