h a l f b a k e r yNot so much a thought experiment as a single neuron misfire.
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Yes, the idea is to play the noise "inverted", but it wouldn't be enough. The cylinders and nozzles could maybe be replaced by pistons. The force applied to the surface wouldn't be greater overall than the fan in a conventional hovercraft, and if the nozzles were very small the damage could be eliminated. |
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I think that hissing air would be easily as loud as the fan motor. Have you ever seen an air sled [nineteenthly]? Link |
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Hissing air is closer to white noise than hovercraft-fan type noise, and therefore easier to cancel out. So far as ants are concerned, in the technocratic utopia through which i envisage these vehicles travelling, we all live in ant-proof domed cities anyway. |
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at the risk of making an annotation purely in order to squeeze in an obscure reference; To what capacity might this hovercraft be able to transport say, eels? |
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Put the air nozzles around the skirt, with the nozzles pointing in and under the edge. The airflow would push in and under, inducing flow and entraining more air. The noise would be going under the skirt, and could be absorbed by material on the bottom of the craft. (High-pitched noises tend to dissipate more quickly with distance, BTW.) |
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If operated over water, the mist kicked up would be going under the skirt instead of coming out. Any eels around would be blown under as well. Soon, your hovercraft is full of eels. |
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Whatever you do, you've got to move a
huge volume of air around, and it will
be the same huge volume
(averaged over a longish period of time)
whether you move it as needed (ie a
conventional hovercraft) or via two sets
of alternately-used storage cylinders.
So, on average, your 'peristaltic pumps'
need to move just as much air as the
conventional fan system. So, the use of
two sets of pumps and storage tanks is
irrelevant - you might as well just use
your peristaltic pumps directly to
generate the air cushion.
Now, is
there any reason to believe that
peristaltic pumps (which are not well
suited to pumping gases) would work
any better than fans or be any quieter?
I'm not at all convinced that
your initial premise, about the fans per
se being noisy, is correct. I would have
thought that most of the noise comes
from (a) the engines driving the fans
[and what's going to drive your
peristaltic pumps?] and (b) the whooshy
air-flow around the skirt (which will be
the same either
way).
Regarding the eel issue,
I think the answer to that question is
self-evident when one considers the
tangent component to the air-flow.
And bugger the ants. |
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<definition> Whooshy: adj., whoosh-
like. </definition> |
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[zen_tom], the answer to your question would have to be in Hungarian and i don't know how to do the accents on the Halfbakery.
The idea of the nozzles pointing inward is interesting, but wouldn't that make the source of the noise closer to the edge and therefore louder? I don't know how much difference that would make.
I've rejected the idea of peristaltic pumps now. I don't know why i thought they were a good idea in the first place. I now think pistons would work better. I do think the noise is mainly from the fan and any propellers involved whooshing through the air. Presumably, the smaller the fan, or the nozzle for that matter, the more high-pitched the noise would be, so i'm currently thinking along the lines of a large number of tiny fans spinning so fast the noise would be ultrasonic and therefore inaudible to humans. The trouble with this is that they would then presumably be extremely noisy to any animal that could hear sound that high-pitched, and also i imagine there could be pretty nasty health effects from sufficiently intense ultrasound on humans.
Electrically-powered toy hovercraft are still very noisy for their size, so i don't think the noise does primarily come from the engine. If it does, clearly a quieter engine is needed.
I originally thought the kind of technology that makes computer fans quieter would work, but the trouble is they aren't designed for lift. |
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I think the answer might be to go in the
opposite direction. If the real source of
the noise *is* the fans (which I'm not
sure of), then the answer would be to
have a greater area of fans so that there
were no local areas of especially high
velocity. Maybe if you had the whole
deck of the hovercraft covered with
modest computer-type fans? |
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I think the fans would need to be bigger for them to provide enough thrust if they were running slowly. Computer-type fans wouldn't work if they were exactly the same design of fans as in a computer because they are not meant to provide lift but to focus the air flow onto a small area. There might be a way of doing it with small fans somewhat different than computer ones, but i think they would have to rotate fast. It's more difficult for a smaller fan to move the air because it's less massive, so it would have to move faster. The smaller a fan is, the faster it can move because the lower mass means it won't be torn apart by momentum.
Very small electrically-powered hovercraft are still very noisy for their size, and the noise doesn't come from an engine. Whether it comes from the engine in a large hovercraft or not, it would still be very noisy even if electrically powered. Therefore, the noise may come from the moving parts before the fan, from the fan itself, from any propellers there may be or from the movement of the air against the surface over which the hovercraft moves.
It could be that a very light but strong fan could be used, so that it could be made bigger but slower to avoid being torn apart, which suggests the use of either some synthetic material, perhaps polycarbonate, or titanium. Considering their proximity to seawater, it seems likely that many hovercraft fans are already made of titanium. |
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//Hissing air is closer to white noise than hovercraft-fan type noise// This is not true. |
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This idea is based on a misunderstanding of aerodynamic noise. Turbulence = noise. |
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//the answer to your question would have to be in Hungarian and i don't know how to do the accents on the Halfbakery// |
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Ceate your annotation in a text editor with the accents and copy/paste it here. A neat trick on many platforms is to dial up the ASCII characters by holding down ALT and typing the ASCII value on the numeric keypad, eg ó is ALT-162. Jó szerencse. |
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I have been working with computers and software for ALL of my adult life, and I never realized that I could hold down the ALT key and type a number to get the ASCII character. How did I miss that one? |
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I don't know how I missed this, but would like to clear up a couple of misconceptions: |
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>Hissing air is closer to white noise than hovercraft-fan type noise, and therefore easier to cancel out.
--[nineteenthly] |
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Agreed with the first half (see below), but noise cencellation relies on waveform inversion and phase cancellation, and is entirely impractical with random noise. |
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> //Hissing air is closer to white noise than hovercraft-fan type noise// This is not true.
--[zigness] |
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Hissing air, by definition, is fairly random, and causes high frequency noise, similar to white noise at first approximation. Hovercraft-fan noise is likely dominated by periodic engine noise, and therefore not random, and not even close to "white." |
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//entirely impractical with random noise// Agreed (sort of). Read Cryptonomicon for a practical use of noise cancellation; two parties each have a recording of noise, one inverted relative to the other. These are used for encrypting telephone conversations, like an acoustic one-time pad, using noise cancellation. Read the book for a full description. Of course, this works because the noise is pseudo-random to the people who have the recordings. I did a simulation of this, by digitally mixing noise with a recording of my voice so that I couldn't hear the words, re-sampling using a shunt from the oudio out to oudio in ports of my soundcard, then adding the inverse of the noise to the result. After a bit of tweaking (finely adjusting the volume and time relationship of the mix) I could hear the words again. I tried the same thing but re-recording via a speaker and microphone but could not reduce the noise enough to hear the words. With a small, high quality speaker in an echo controlled chamber this would be possible. You simply can't apply active noise cancellation to prevent a dirty great diffuse sound source like a hovercraft from radiating sound energy, at least for wavelengths that aren't long relative to the hovercraft. |
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<Hand-waving Conjecture> The point is, for cancellation of noise, you need at least a point source or a point sink, and preferably both. My sound-card shunt made an aproximation of both. For good noise cancellation of hovercraft air hissing noise you could enclose the entire craft and a bit of the sea in a bottle-like sound-proof chamber and mount the microphone and speaker at the neck (a point source), or fit everyone * with noise cancelling headphones (a point sink). A cheaper method in each case would be a giant stopper or a pair of earmuffs. Or a sailing ship. They make really nice sounds.</HwC> |
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To enlarge a little on [csea]'s comments, noise cancellation relies on information. If you have a periodic sound, you just need to be able to pump out that sound phase shifted by half the period to cancel, at least to the extent that the sound has half-period translational symmetry. Incidentally, there's a theory that our hearing systems do the opposite - they work out how much of a phase shift is needed to cancel the time delay between sound's arriving at each ear and make the combined signal from the two ears as large as possible. This provides directional information.
If the sound is non-periodic, phase shifting won't work. You need to record the sound and play back its inverse. This is how active noise cancelling headphones work. |
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Actually, [nineteenthly] may simply be saying that people find white noise less intrusive than some other sounds, such as engine or fan noise. Which would make my rant redundant. |
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(edit) * I don't yet have a practical method of keeping the noise cancelling headphones on the eels. |
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How about trying to make an engine which makes predictable noises rather than hissing or roaring? For instance, if there were nozzles of the right shape, they could be designed to make a sine wave whistle, which could maybe be predictably cancelled out by either speakers or another set of nozzles making the same sound but out of phase with the first set. Basically, make the whole engine melodious and the problem is either soluble by active noise cancellation or doesn't matter anyway because the engine is making music instead of noise.
Maybe there could be hover-ice cream vans. |
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