All,

I have been thinking ( a dangerous thing, to be sure) about a seismometer 
that has no natural frequency. Perhaps (probably) this idea is not new, but 
never-the-less, here it is:

Imagine, if you will, a stack of, say 1" diameter aluminum tubes about 10 
inches long. There are 9 tubes, two on the bottom, three in the next layer 
up, and four in the top layer. These tubes are stacked like bowling pins 
with the head pin missing. Now imagine that, on the sides and top of this 
structure there are pieces of glass so that the sides are smooth, 
relatively accurate, surfaces. The top is not so important, and might be a 
piece of aluminum plate instead of glass. Now fill the tubes with something 
which has mass, perhaps a mixture of sand and epoxy (search the web for 
"Granitan" and you'll get the idea).

Now comes the hard part. Imagine this assembly of tubes supported on four 
circular slices of aluminum arranged two on each side, flat against the 
outward sloping surfaces of the glass plates. The outside flat surfaces of 
the aluminum disks. have countersunk holes that hold balls so that the 
disks can align themselves with the glass surfaces. The balls, in turn are 
located in stationary mounting plates that are attached to the non-moving 
base of the structure. The surface of the aluminum disks which face the 
glass is recessed about .001" leaving a smooth, lapped rim about 0.100" 
wide touching the glass. In the center of the recess is a small hole, say 
about .062" which is cross drilled to a small air fitting, I think the 
common size is 10-32 (Look in an aquarium shop for this.) All four aluminum 
disks are the same, and they are connected together with aquarium air hose. 
(The more flexible, the better.) This hose is also connected to an aquarium 
air pump. When the pump is turned on - Voila! The tube structure (The free 
mass) will rise slightly on an air film which will form between the 
aluminum disks and the glass, and the mass will float freely, with zero 
static coefficient of friction. In fact, it will slide off of the air 
bearings and fall on the floor if you are not careful.

Now imagine this free mass with a voice coil or other driver attached to it 
at one end, and the transducer of your choice attached to it at the other. 
Imagine also, a set of leveling screws between the base and good old mother 
earth.

There are enhancements possible, including an air receiver and filter to 
reduce air pressure pulsations and dirt (although the pressure pulsations 
are far above our frequency of interest, and the mass can't follow them 
anyway.), and a temperature control system. (Always include a temperature 
control system. Always.)

Now the caveats. I have not built the system just described, but I have 
built many air bearing devices, so I'm pretty sure of my technology here. 
The tube and glass structure is just my way of building an accurate 
prismatic shape in my garage. Other approaches are possible.

Please jump in and critique my ideas here. You can't hurt my feelings, and 
the device doesn't exist, so it can be changed with a thought.

I hereby declare the season open.

Dave...
  

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