I am doing some scientific experiments and am having some difficulties
getting rid of high frequency vibrations.

Ultrasound (40 kHz) receiver sensor:
http://www.mouser.com/catalog/specsheets/062498.pdf

In the experimental setup, we are placing a PCB with this sensor on a
Lego Train cart.  We need to be able to detect US (ultrasound) pulses
while the train is running, but unfortunately vibrations from the
train are tripping the sensor which rests on it.

We have done this experiment before under the following conditions:

Train on floor - NO GO
Train on floor, and piece of cheap foam between train and sensor - GOOD
Train on table, with foam - NO GO

We need it to work on the table.  We have tried several memory cell
foams of various densities, none of which have work.  My suspicion is
that we need some kind of gel, but I have no reason to believe that
would work, just intuition.

We need this ASAP for a paper.  Thus, if you can answer within the
next 30-60 minutes that would be ideal, since I could then order the
appropriate materials overnight.

PLEASE only respond if you know what you are talking about. 
Unfortunately my background is not in this area, so I'm sure I've only
given you a small fraction of the ideal amount of information.  Feel
free to ask specific questions.

Thanks

I suspect that the table is generating interference by resonating. You
need to change the base frequency of vibration of the table, thereby
changing the harmonics. Try stiffly bolting the heaviest lead weight
you can find to the table, then using the same foam that worked on the
floor. This will increase the inertia of the table and dampen the
vibrations at both the underlying frequency and the harmonics.

Before you order any hardware, try it with clamps and a very heavy object.

If the table is a composite or segmented construction, this will not
work. Change the table for one that is solid and allows bolting.
Remove any composite structures (eg. drawers).

Depending upon the exact construction of the table, you may need more
than one heavy object. It may be best to use an object in which the
area of stiff contact with the table is maximized. The bolts or clamps
must be very tight.

Alan Kali

P.S.

Alternately, you could build an optical table like this
(http://www.3dimagery.com/table.html).

Holography enthusiasts use them to dampen vibrations that would
otherwise ruin the images. Working in the basement helps also.

Alan kali