h a l f b a k e r yBunned. James Bunned.
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Suggested by an anno. by [bigsleep] on "Iridescent
Water".
Sound is simply compression waves travelling through a
medium. In the case of a gas, the degree of compression
is quite small, but not zero.
Small changes in air density cause light to be refracted -
this is commonly seen as heat-shimmer,
where the
density
changes happen as a result of heating.
Sound waves, therefore, ought to impose a sort of
refractive fingerprint on light. Normally, these effects
would be non-noticeable, because the degree of
compression is very small, and hence the deflections of
light would be minuscule.
However, what if you could generate sound in air with a
wavelength comparable to that of visible light (o.t.o.o.
1µm)? This should do something weird, akin to
iridescence. Iridescence happens when some periodic
structure (like ridges on a beetle's back) matches the
wavelength of a particular colour of light. A wavelength
of 1µm in air corresponds to 300mHz.
So, to wind up this rambling. Is there any way in which
a
gas could be made iridiscentish by the application of
ludicrously high-frequency sound?
Making sound waves visible with light
http://blog.modernm...ound-waves-picture/
Like this? [Alterother, Apr 23 2012]
Visible sound
Visible_20sound
We tried to find a good solution to this before [hippo, Apr 24 2012]
I really do like this picture
http://apod.nasa.gov/apod/ap010221.html
[hippo, Apr 24 2012]
The Prandtl-Glauert singularity
http://www.fluids.e.../conden/pg_sing.htm
[hippo, Apr 24 2012]
Discussion of maximum sound frequncy in air
http://www.physicsf...x.php/t-144509.html
[scad mientist, Apr 24 2012]
[link]
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Tesla or somebody frigged around with this. I read about it
in a book,
but I'll see if I can find anything linkable. |
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Yep, found it <seelink>. Not Tesla, though. |
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//Tesla or somebody// The problem with Tesla is
that he was half genius, half charlatan. Still, I'll be
interested in the link if you can find it. |
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I have also personally witnessed air distortion in front of
giant subwoofers at heavy metal concerts. There are
probably dozens of videos of that effect on the interwebs. |
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Well, I'm not sure if any of this is what you're getting at,
but it's what I have to contribute. |
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In fact, it's a lot more than I usually bring to the table... |
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300 MHz would be about 1 metre, rather than 1µm, if
you're talking about electromagnetic radiation, I'm
afraid. I think that's the point [bigsleep] was making,
rather discreetly. |
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Ah, a sound wavelength of 1µm = 340.29MHz at sea
level. |
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It's EM, but not as we see it, Jim. |
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You might pick it up on your iridescent UHF radio?
Tactical military comms are often in this sort of
range, I believe. |
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<hurriedly flips through a back-issue of Jane's> |
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You could add fine water droplets to the air. Then,
at certain angles, it might be possible to observe a
multi-hued iridescent effect, of light refraction. |
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<Bows. Wanders off, whistling Saltimbanco.> |
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//300 MHz would be about 1 metre, rather than
1µm, if you're talking about electromagnetic
radiation, I'm afraid.// |
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Indeed it would, but I am not talking at all about
electromagnetic radiation. |
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What I am saying is "create sound waves in air with
a wavelength comparable to that of visible light,
and see if the resulting refractive properties cause
some sort of iridescentoid interference". |
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If the air is at a normal temperature and pressure and very humid - i.e. 100% humidity - then, as the sound wave travels through the air the low-pressure parts of the wave should take the air pressure below a point where it's able to hold so much humidity and the water will condense out into clouds, the thickness of which will be proportional to the sound's amplitude.
Also, see links. This would be cool, just because you'd be able to shout "My God! It's a Prandtl-Glauert singularity!!!" |
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//the water will condense out into clouds, the
thickness of which will be proportional to the
sound's amplitude.// |
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By thickness, I presume you mean density? One
problem is that water droplets will need to be very
tiny (o.t.o.o. 1µm) in order to create the necessary
layered cloud structure for iridescence. |
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Yes, I mean density. Anyway it doesn't matter whether it shows iridescence - your audience will be too impressed with hearing you shout "My God! It's a Prandtl-Glauert singularity!!!" to notice. |
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//your audience will be too impressed// They
always are. |
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// Prandtl-Glauert singularity!// As long as you can
find me a large enough volume of perfect inviscid
gas, we're in business. |
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I'm not sure what "inviscid" means, but we could try locking
[8th of 7] in an airtight room and see what accumulates. |
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Inviscid = non viscous. Things like viscosity, and
departures from the perfect gas laws, are what
prevent Prandtl-Glauert singularities. |
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//_IM_pressed// - only if you're doing physics
experiments for the navy. |
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Non-viscous... I probably could have figured that out, but
I'm too busy with other words today. Deadline comin' up. |
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My plan probably won't work, then. There's not much about
our Borg that's inviscid. |
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Quoted from the link above: |
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// No. The highest (effective) frequency that can exist is linked to the mean free path. This at sea level is about 10^-7m. Hence the highest frequency (even loosly defined as sound) is 340e7 Hz. Even that is stretching a point. |
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[link removed]
gives a general account of attenuation. At 100kHz we have about 1800dB/km in fairly dry air. Attenuation on a simple model goes up as f^2. Range of 1000kHz therefore bein of the order of 10m (1.8dB/m). At 1MHz we have 10cm. At 10MHz 100 microns. In my book 100MHz+ can't really exist in air. |
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I can't independently vouch for everything said there, but it makes sense to me. |
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Thanks for the link, [scad]. The thread there says:
//At 100kHz we have about 1800dB/km in fairly dry
air. Attenuation on a simple model goes up asf^2.
Range of 1000kHz therefore bein of the order of
10m (1.8dB/m). At 1MHz we have 10cm. At 10MHz
100 microns. In my book 100MHz+ can't really exist
in air.// |
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So, it looks like 300MHz (corresponding to a 1µm
wavelength) is out, but a wavelength of a few
microns (<100MHz) should be producible over
small (millimetre) distances. A few-micron
spacing should still produce some iridescent
phenomena. |
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As would salting the air with very fine particulates of
glitter. |
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That's a good point - would this be an easier effect to create in a smoke-filled room? |
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I suspect it might be, [hippo]. The exact conditions
necessary to cause clean air to iridesce would
probably be in a very narrow band, difficult to
replicate. |
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//A wavelength of 1µm in air corresponds to 300mHz.//
300mHz in air is well over 1km. |
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//300mHz in air is well over 1km.// |
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Light at 300MHz / 300,000,000 Hz = .9993082 metres
in a vacuum or about .9990082 metres in air at STP |
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//300mHz in air is well over 1km.// |
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I fear you may have missed the point. |
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I am talking about _sound_ waves in air, _not_
_electromagnetic_ _waves_. Sound waves. Waves of
sound. |
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A wave of sound in air with a frequency of 0.3Hz would have a wavelength of well over one kilometre, assuming the speed of sound is around 340 metres per second.
I don't believe I have missed any points.
It is worth pointing out that I work on devices where frequencies are specified over more than 30 octaves. I have to know where my shift-key is, and what difference it makes. |
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Ah, I assumed the m was a typo. Sorry. |
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