Water does not support ultra sonic sound. That is why powerful sonar use around whales destroys their communication ability. Sure, air makes a wonderful cushion because it is compressible. Water is not compressible, so the sound vibrations are conducted.
What exactly are you looking to vibrate? That would help determine the best solution as to a shaker. There are shakers like Paul mentioned, a dynamic shaker. There are all sorts of vibration applications. There is much more to dynamic shakers than the cone of a speaker unless you can visualize a really heavy speaker. If we put a large dynamic vibe in an oven we get shake and bake. 
Ron
It probably has better bearings if it's designed for that purpose. But I made a rock-polishing vibrator by taking a cheap AC fan motor and mounting a small offset weight (a few oz.) to the shaft. You can hear the shaking through the whole house!
Water does not support ultra sonic sound. That is why powerful sonar use around whales destroys their communication ability. Sure, air makes a wonderful cushion because it is compressible. Water is not compressible, so the sound vibrations are conducted
that makes sense - i had the concept of sonar wrong
a wooden floor or wall - say for a spooky halloween house (concept)
by dynamic shaker do you mean a shaker where the frequency/speed can be changed or am i missing something?
pneumatic is very impressive but i want something purely mechanical/electrical
it is a surface that i want to vibrate, not the contents of a drum and i want control over the pulses both duty cycle and intensity
It probably has better bearings if it's designed for that purpose. But I made a rock-polishing vibrator by taking a cheap AC fan motor and mounting a small offset weight (a few oz.) to the shaft. You can hear the shaking through the whole house!
what would the weights be shaped like?
did you source the weights or repurpose something that can be added to the shaft?
i imagine a rock polisher would put quite a bit of pressure on the motor, what's the durability like?
Well alrighty then going back to Paul's link. They give you some data to work from.
The motor is an Asynchronous AC Motor where the motor speed is based on the line frequency. In the link 3600 RPM based on 60 Hz line frequency. The 50 Hz versions would be 3000 RPM. That is not subject to change easily. The motors come in several power ratings as mentioned 30 W/40 W/60 W/90 W/100 W which will run with the Vibrating Force: 20 kg/25 kg/38 kg/48 kg/50 kg. The units also feature an Adjustable Eccentric Block. You can adjust the angle of the eccentricity block, and then the excitation force will be changed. The weights are shown in Paul's link.
So with that in mind if we want to shake a wall or a wood floor there are considerations as to weight and area. Never gave it much thought but shaking / vibrating a heavy wall or floor will present some challenges. My experience ended with shaking a special motor frame to get a two part epoxy into the nooks and crannies. We just made a holding fixture for the motor stator sections. Walls and floors I never tried. 
Yes, large dynamic shakers are controlled including type of vibration like sine or random. Typically used for test and acceptance of certain products. Big expensive stuff like this stuff.
Ron
Thanks for the detailed reply Ron,
Yes, the motor is AC which poses problems as i want to control the speed with an MCU but a link provided above to an AC Dimmer Module using "Phase Angle Control" seems like i would at least have some control over the motor in question, but i cannot seem to source this here without waiting weeks.
I realise the weight and area of the surface in question would be a huge factor in the wattage and vibration force needed to get the result but we are not talking too much weight and if so i could always go over what i needed in choosing and drop the speed/intensity.
That's really interesting to use vibration to get epoxy into all the gaps, i think some people use these vibration motors when casting concrete products.
Thanks for the info, i might just try and source the motor and an AC dimmer board and see if i can get the results that i want
Wow, i didn't realise there was a whole industry attached to shaking stuff but it makes sense, so many applications and ways of getting it right!
You can try the AC dimmer. They are not expensive fortunately. Just make sure the AC dimmer is of a high enough power rating. Yes, vibration motors like this have a variety of uses including concrete work and tamping before a concrete pour with a limestone base. Thinking about it I use much smaller versions in vibratory cleaners for cleaning brass with media. Best of luck getting this to work out for you.
Ron
Shape? I just cut a chunk of aluminum off a bar, drilled a hole for the shaft, drilled and threaded another hole for a set screw and that was it. This was not something carefully designed!
I don't use it any more, but the motor is fine. I ran it for a few days at a time.
I don't see how phase angle control would change the frequency...
You would need a frequency converter. And I would not recommend to build one yourself for mains voltage...
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