When a ringing bell is placed in a vacuum jar, the noise cannot be
heard outside the jar, since sound is not transmitted by a vacuum. I
would like to know whether the same method could be applied to
completely inhibit the noise of a motor, which would normally have a
noise level of aprox. 80 decibel in air . "completely" inhibiting
would mean to reduce the noise level close to the hearing limit of
aprox. 0 - 4 dB. The drive produced by the encapsulated motor could be
transfused to a gearing outside the vacuum box by contactless magnetic
forces, thus avoiding sound and vibration transmission through a
physical gearing connection. My question is whether this is possible
in principle and whether there are other important criteria to
consider.

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Request for Question Clarification by hedgie-ga on 22 Aug 2006 05:28 PDT

Are you assuming electric motor?

Is the question answered ? (If so you can expire it)

or do you want more info?

Unless the motor is floating there will be vibrations transmitted
through the mounting. If this is an internal combustion you would also
have to breach the vacuum for combustion air.

Most electric motors require air for cooling.

It is not very realistic, but I suppose that the motor could be
suspended inside the vacuum jar by magnetism, either of the motor's Fe
content or by fastening it to an iron plate.

The other problem of course is that it's pretty hard to hold a vacuum.
 Everything leaks over time.  As such, you'll need a vacuum pump
running, and there you have noise again.