The following is taking from the 'Outline Answers' of Part (A) of the 2009 Noise and Vibration Control Engineering paper from an examination as part of the Diploma in Acoustics and Noise Control which is accredited by the Institute of Acoustics.
"Inertia blocks are massive supports used for isolating foundations beneath vibration-sources and/or vibration-sensitive machines. Inertia blocks increase the isolated mass and therefore decrease the natural frequency of the vibrating system; particularly useful for dealing with very low frequency vibration problems, e.g. isolating a recording studio from an underground railway."
It is generally considered that the equipment should be rigidly mounted to the inertia block, a view also taken by Bies and Hansen (2009). The inertia block should also be made of rigid material such as concrete.
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"inertia"
A motor when running has inertia in the rotating parts, which is a reserve of kinetic energy. The kinetic energy is found as the moment of inertia times the square of the angular velocity.
define moment of inertia§ I is the moment of inertia of the mass about the center of rotation. The moment of inertia is the measure of resistance to torque applied on a spinning object (i.e. the higher the moment of inertia, the slower it will spin after being applied a given force).
The difference is in the design of the head. Also due to the head design, the physical size of the motor is larger even if CID (displacement) is identical. The best visual example I can think of is Ford 351 windsor which was a small block: http://www.mustangsteve.comW351_65.jpg and a Ford 351 Cleveland which was a big block: http://thunder-performance.com/images/Tony\'s%20351%20Cleveland%20(1).JPG Generally you can tell the difference by looking at the valve covers. if they are long, narrow and rectangular, it's a small block. If they are fat squat and irregular shaped, you're looking at a big block. There are other differences too, for example bore and stroke. A small block generally has a shorter stroke and is thought to rev higher, whereas a big block has a longer stroke with more torque. There are other disputed difference between a small and a big block. And for the most part, you can simply tell by the displacement. Generally, anything larger than 366 CID is a big block.