Pyro Board
April 18, 2014 12:22 PM   Subscribe

Pyro Board. Or flammable sound waves and music. Danish Fysikshow demonstrates a 2-D Rubens' tube (wiki, demo).
posted by severiina (14 comments total) 18 users marked this as a favorite
Hmmm, kinda cool, but I don't think the guy has his physics right. I'd be interested in hearing from someone else who is more of an expert than me. But, the Ruben's Tube works because it's a long tube - a medium that allows for the creation of a standing wave when you reach a resonance pattern based on the length of the tube. So you play the speaker through the tube and vary the wave patterns through the gas, finding standing waves at certain frequencies. I don't really see how the board does the same thing at all? It's certainly much less clear visually. A wave propagates...but the board doesn't really have that capacity(?).

Also, the narrator guy (and bless him, because I think he's just stoned and thinks it's really cool) can almost certainly not hear nodes and anti-nodes.
posted by Lutoslawski at 12:51 PM on April 18, 2014

I think maybe it's something like this, but, you know, with fire.

But the speaker is side-mounted, so maybe not?
posted by Sys Rq at 1:05 PM on April 18, 2014

This demonstration was invented by my friend Harold Daw in 1979. He described it in two papers in the American Journal of Physics in August 1987 and in October 1988. The first paper has construction details; the second describes the normal mode patterns. There are 3 of these tables in our physics demo room, a square one, a circular one, and a triangular one.

Harold also invented the air table, a physics demo that eventually morphed into the air hockey table. He loved building big demonstrations, and he was really good at it. He passed away last month.
posted by Killick at 1:08 PM on April 18, 2014 [19 favorites]

Lutoslawski: a wave can exist in more than one dimension, it's just that the rules are much more complex and the shapes and behaviors are much less clear.

One may not be able to hear a node or anti-node precisely (at least not with both ears at once), but one way to know you have a standing wave (in the room you are in, not an object in that room) is when you move and the amplitude changes in a drastic and space-determined manner.
posted by idiopath at 1:16 PM on April 18, 2014 [1 favorite]

This is excellent in all of the ways.

Killick: I'd love to hear more about Harold Daw!
posted by Freen at 1:20 PM on April 18, 2014

Right, waves propagate out in 3D from the source, but I guess when I think of standing waves I think of a wave that must be contained in some kind of tube or column, in order to bounce back and forth and create the standing phenomenon. So, if you were to have a standing wave in a room but not an object, wouldn't the room have to be perfectly square in order to create a perfectly 3 dimensional standing wave? And is that what the pyro board is attempting to create? It almost seems like it would need to be way larger in order to clearly show this sort of phenomenon.

This has always been a sort of tricky concept to me actually.
posted by Lutoslawski at 1:22 PM on April 18, 2014

3d standing waves are actually easiest to make in a sphere. There are likely 3d standing waves possible in any enclosing shape (but I don't have a proof for that handy).
posted by idiopath at 1:27 PM on April 18, 2014

Sys Rq, it is very much the same idea as the Chladni plate demonstration you link to, except the boundary condition for the Chladni plate is no vibration at the center support and the boundary condition for the flame table is no vibration (position node, pressure antinode) at the sides of the table. For the square table, the patterns are combinations of cosine functions, one in each direction; for the cylindrical table the patterns are Bessel functions.

Lutoslawski, I wouldn't be surprised if the narrator heard nodes and antinodes if he moved his head horizontally. The angle between nodal lines is a function of the spacing of the holes and of the wavelength of the sound. If you twirl a tuning fork near your ear you can hear the variation from node to antinode and back as the fork rotates, and the frequencies and distances are about the same for this demonstration.
posted by Killick at 1:27 PM on April 18, 2014 [2 favorites]

3d standing waves are actually easiest to make in a sphere

Oh right, duh, that makes sense.
posted by Lutoslawski at 1:28 PM on April 18, 2014

A dozen different nightclub designers in Las Vegas all just tripped over their checkbooks, simultaneously.
posted by JoeZydeco at 1:36 PM on April 18, 2014 [3 favorites]

Freen, Harold Daw taught in the physics department at New Mexico State from sometime in the late ‘50s until 1990. He first published about the air table in 1963, and the Ealing Corporation manufactured it as a physics demonstration for many years. (The first patent for air hockey is by the Brunswick Corporation filed in 1971 as far as I can tell, and I don’t know whether Harold ever attempted to patent the air table.) He showed me his first prototype, about 6 inches on a side that lifted a puck when you blew through a tube. In our department we also have various versions of air bearings for bowling balls and bocce balls, and some of them have a metal rod sticking out of them – when the ball is spun rod shows the precession pattern. He built a gyroscope out of a car wheel and tire mounted so that as it spins it can also rotate about both a vertical and a horizontal axis. When the tire is spun up (using a hand drill, hard on the wrists) you can sit opposite the wheel and ride along as the whole thing precesses. He also bought a cylindrical cattle watering tank, about 8 feet in diameter, that he mounted on a surplus WWII searchlight frame – the whole tank can spin at up to 20rpm. Four people can sit in it, and if you toss a ball around you can see the Coriolis effects. We also have a smoke ring generator that he built that will make a beautiful 1.5 foot diameter smoke ring and send it across a lecture hall – kind of like this one.

Harold was one of a generation of physicists in our department who could dream up some physical device and then go out and build it, usually from scavenged parts. They knew how to machine and weld, how to grind lenses, and build whatever electronics they needed. A lot of what they started didn’t pan out, but a lot of it did, and the sense of fun and wonder they had about physics is embodied in the stuff they made. Harold loved physics and he loved doing demonstrations, and even well into his 80s he would pop into my office to describe what he was building or to give me advice on something I was putting together.
posted by Killick at 2:43 PM on April 18, 2014 [14 favorites]

Killick: "This demonstration was invented by my friend Harold Daw in 1979. He described it in two papers in the American Journal of Physics in August 1987 and in October 1988. The first paper has construction details; the second describes the normal mode patterns."

These are the two papers.
posted by James Scott-Brown at 4:50 PM on April 18, 2014 [2 favorites]

> 3d standing waves are actually easiest to make in a sphere.

So how many years until this is proven in the form of a pyrosphere disco ball at Burning Man?
posted by polymath at 11:37 PM on April 18, 2014

You know, I would have finished that video if I could have actually heard anything past a certain point.

Just a thought, maybe mic some people and retouch the sound in post? I mean, they can watermark it, why not make it audible?
posted by Samizdata at 12:42 PM on April 20, 2014

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