[Work related to Broadside #5]
My basement floor is a bit of a rollercoaster; my mom’s floor (she doesn’t have a basement) is even worse. Not only is it a quarter of a mile from the LIRR tracks, which would be annoying; but in addition she lives on a very busy street, with tanker trucks going both directions even fairly late at night; the trucks are only a few yards away. I have started to build something that I hope will become a viable vibration-isolation (and damping) stack, figuring that if it will work here, it will work in my basement. Here’s what it looks like, as of the afternoon of Feb 13, 2015:
From the bottom, the layers are:
The piece of cardboard that you see in the background is standing in front of a radiator, helping to block some of the air currents. I found out, a long time ago, that air currents in the room can make it difficult or impossible to make transmission holograms, though it appears that they are less of an issue for Denisyuk-type holography, in which the plate and the object are often in direct contact.
When there isn’t any traffic outside, and when the wind isn’t gusting, this setup is reasonably stable. At some point I may post a video, perhaps showing what happens when a tanker truck goes by. It isn’t pretty...
This morning I was writing email to Cobra Wu, who has recently started to do holography, about ways of stabilizing holography platforms. It occurred to me that although I am not about to go out and buy a professional stabilized table, it might be possible to DIY one. That led me to think about air cushions as a nice easy passive starting point, and it occurred to me that nested stainless steel bowls would be very handy because they’d be self-centering. Some of them have a small rim that would deflect the effluent air out to the side, also handy. In order to build such a setup, however, I would need a bunch of hardware I don’t have yet, and it would take quite a bit of time, so I thought about alternatives.
A totally passive system would be quicker and simpler. People have used innertubes as supports, mostly with large heavy tables. For a small table I could use a single innertube intended for a child’s bike, but I worried about the wall of the tube conducting too much vibration. (Obviously, this is less likely to be an issue when there’s half a ton of granite sitting on top of a set of them.) That led me to the idea of balloons: the wall of a balloon is much thinner, so presumably much less vibration will be transmitted along it. The obvious issue is that a balloon is a nice omnidirectional wheel, so I went to a thrift store and bought four small bowls, low and wide. Then I went to a craft supply store and bought a bag of balloons. They are listed as 12", which is larger than I need, but they were available, and I figure that they’ll be stronger if they are somewhat underinflated.
I suspect that someone has used balloons for this purpose before, btw, but I am not finding mention of it in brief Websearches. I didn’t dig very deeply, though.
It is not easy to inflate four balloons to the same size. I ended up with two that were slightly larger and two that were slightly smaller, so I placed them diagonally opposite each other in an effort to create a reasonable balance, rather than putting the larger ones at one end (or side) and the smaller ones at the other, which would have tilted the top of the stack.
My first try at building a stack that included the balloons worked quite well. When I first set the second marble slab and then the chipboard down on them, the top layers wobbled crazily for about ten seconds, and I began to think I was in trouble, but as soon as they settled it became clear that there had been a huge improvement. I could pound on the piano bench, which is only about 3 feet away, and not see any movement at all in the fringe pattern.
I had positioned the balloons above the first marble slab and the cast iron griddle, because I was concerned about what might happen if I put too much weight on them. Later in the evening I decided to see what would happen if I moved them closer to the bottom of the stack. Somewhat to my surprise, that was not successful. After I tried two versions, both of which were more sensitive to traffic and exhibited some “fizz” when I pounded on the piano bench, I reverted to a configuration that is close to this afternoon’s original, though it now includes a cafeteria tray that I added for the unsuccessful attempts. Here is the current order, again from the bottom:
It looks about like this:
This evening’s version is at least as good as this afternoon’s, and perhaps even a bit better. The stack is still slightly responsive to traffic, but the motion is small and fairly slow, and it may not be too much of a problem. It is also, quite expectably, responsive to air currents; but the motion from those is also small, and quite slow. It will be an issue with transmission holography, for which it is necessary to block air currents anyway, but it may not be bad enough to interfere with Denisyuk holography.
The balloons are probably deflating, but the change is slow. They are tied off, so they will become unusable when they get too soft, but I think I remember seeing little removable plugs, and if I can find some I should be able to reinflate the next set at least a few times. (Not much luck finding them so far, though.) Small inflatable cushions might possibly do as an alternative, and I am on the lookout for those. I suspect that two of the u-shaped neck pillow type could be viable, and they don’t roll around, so I wouldn’t need the bowls. The walls, though, are much thicker than the walls of balloons, and it remains to be seen how much vibration they transmit. (Note, added later: I didn’t find any at local stores, so I have two on order from a vendor on eBay.) I may try small Mylar balloons, to see whether they will work they should take a lot longer than rubber balloons to deflate significantly. The trick, I think, would be to find any that are not already inflated too firmly for use in this setup. It is even possible that the preinflated bags that are sometimes used as packing material could be made to work, though most of them are pretty flimsy, and even if I can find some that aren’t likely to spring leaks I’m pretty sure it would take more than four to carry the weight. Still, they are worth considering.
Here is a video that I made today with my phone (mp4 format, about 21MB [somewhat reduced size and quality; the original was about 53 MB]):
At this point I think the problem is largely solved, at
least for the conditions I am obliged to contend with.
I may return to the possibility of air-cushion
stabilization later, when I have the time and energy; it
seems likely to be an amusing project. In the meanwhile,
I guess the take-home from this exercise is that
balloons and Sorbothane and heavy slabs are all
helpful. (If you aren’t up for spending money on
pieces of marble or cast iron, you can easily make slabs
by casting concrete into suitable trays; even old
broiler pans, which are routinely available at thrift
stores and are not usually expensive, should work.) If
you don’t get the results you want with one
configuration, take careful notes and try the components
in a different order. If you don’t get what you
want after a few such attempts, think about where the
failures were headed, and try something different
add a new layer, take a layer out, ...whatever.
Over the past two evenings I rebuilt the stack in my basement, twice, with small Mylar party balloons that I got at a store that sells things for a dollar or less. (Not the usual such store, but it doesn’t matter. These balloons are variously available, and should be relatively cheap.) The balloons are prefilled, with something that is definitely not air: it is cold outside, and two of them shrank a bunch, just sitting in my car. They reinflated as soon as they got warm.
The first time I built the stack, I only had three of them, and it was okay but not great. I used four in the second version, which works better. Here is a set of photos, showing most of what’s in the stack:
(I say “most” because you can’t see the Sorbothane hemispheres that are between the first square stone tile and the red granite rectangle. Also, it is less than obvious that there is a second square stone tile sitting directly on top of the first one.) There is a third stone tile, further up (5th photo); the white rectangles are chipboard.
I don’t know how much good the other layers of padding actually do, but I doubt that they are hurting anything.
Here’s a look at the Michelson interferometer that is currently on the stack:
The green glow on the viewing screen, a little to the right, is an artifact coming from the laser.
This stack is definitely stable enough for reflection holography, as I hope to get a chance to demonstrate at some point in the not-too-distant future.
[NOTE, added later: That happened on Feb 23. You can see the results on this page.]
It may be stable enough for transmission holography when
the furnace isn’t running, the laundry isn’t
going, and there’s no traffic, but it will be a
bit longer before I get to try that, because I need to
make sure I can stabilize the laser first.
The cushions arrived today. I filled them, not very full, and reconstructed the stack, with the cushions near (but not at) the bottom. (In this photo, which was taken later, they are filled considerably more.)
I also put 5 quilted placemats between the top marble slab and the chipboard. That configuration was not as stable as the best one with the [regular] balloons, even though they were further up.
I inflated the cushions more and rebuilt the stack, this time with the cushions sitting directly on the quilt; the tray, inverted, on top of them; and the placemats between the top marble slab and the griddle. That was better, but not enough better, so I built it yet again. That brought it closer to being as stable as it was with the balloons, though still not as good. (I must note that this stack has fewer layers than the one in my basement, and is not as heavy. I hope to add at least one more dense layer to it soon, to see if that improves it.)
At that point, the order was:
I tried several more arrangements, even putting the balloons back in up toward the top. None of them was significantly better. I looked for some large-bubble packing material to try between layers, but didn’t find any. (Several layers of small-bubble material did little or nothing.)
Unless I find that adding a reasonable amount of mass makes a large difference, I am going to conclude that this type of inflatable cushion is not a particularly good option under my conditions.
This afternoon I happened to pass a local Mason Supply, so I stopped and inquired about offcuts and chipped things. They were kind enough to give me a piece of concrete that is 2"x12"x16". It nearly doubles the amount of mass in the stack, but it made almost no difference to the behavior. I switched from the inflatable cushions to 4 small Mylar balloons, and the behavior improved, but the stack still responds to traffic. This strongly suggests that the floor of my basement is less bouncy than the floor at my mom’s.
On a whim (admittedly after I tried several other configurations without notable improvement), I have made a moderately large change.
I removed the cast iron griddle and the 4 Sorbothane hemispheres; put the two pieces of memory foam on the quilt and set the concrete slab on them; put the Mylar balloons on the concrete; added a large chipboard shelf, and put the balloons in bowls on it. The two marble slabs are on the balloons (with the Sorbothane pads between them), and the chipboard shelf with the Michelson is on top of the upper marble slab. This configuration seems to be slightly better than the best version from last weekend, with one possible exception: although it is good at blocking vertical motion, it continues to oscillate for much too long when the motion is from side to side. Fortunately, nearly all of the motion caused by traffic is vertical. (Still, I think about filling the bowls partway with sand to help stop the balloons from rolling around quite so much when there is some side-to-side stuff happening.)
Why did so many variants fail so miserably, even when some of them had the regular balloons in them, and a few even had both kinds? Why have none of the stacks with the new concrete slab in them worked except this one, which positions the slab nearly at the bottom rather than using it to add mass above the Mylar (or regular) balloons? It seems possible, and even likely, that the added mass caused the stack to have a resonant frequency that was too close to one of the frequencies or to a range of frequencies at which the house bounces. That would enhance the response of the stack (at that frequency or in that range) to traffic, which is reasonably compatible with what I was seeing.
My conclusion, however, is hindsight. As to the whim, my
guess is that I made the correlation between the
addition of the concrete slab and the wobbles outside of
consciousness. When I get an urge to do something
strange for example, putting the concrete slab
below the balloons, when I got it specifically with the
idea of putting it above them I say that
“Ogg made me do it.” I have learned, over
the years, to have considerable respect for Ogg. The
conscious mind is much overrated.
Did I mention taking careful notes? It’s
important. By the time I wrote this paragraph, a few
hours after I revised the stack at my mom’s on Feb
17 and got lousy results, I was already unable to
remember all the details of the configurations I had
tried. That should tell you something. (I’m lousy
at taking notes; that’s part of why I take lots of
pictures, and also why I try to record a lot of the
failures on my Web pages, as well as the successes
it helps me go back later and rethink things on
an informed basis. Understanding a good solid failure
can give you a lot of insight into something
you’re trying to accomplish. Also, having a good
record of what didn’t work helps prevent you from
doing it again the same way, without tweaking it to get
more information from it.)
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Last modified: Sat Feb 28 17:05:23 EST 2015