Soft mounts helps reduce noise mainly by reducing the vibrations transfered to the fuselage. Depending on the engine, the mount geometry and the dampening material used, a nose ring might be needed to avoid excessive engine vibrations at idle. It's very much a trial and error affair to determine the optimum set up. From my own experience with Dubro, Sullivan, Weston and Hyde mounts high speed engine vibrations is not a problem as long as the spinner and propeller is properly balanced.

What can you expect in terms of noise reduction:
Again this depends on a lot of factors but as a general rule one can expect a 1 - 2 db(A) noise reduction. Not very much one might argue, but one also have to take into account that the noise usually is perceived to be less harsh.

If noise reduction is the ultimate goal, consider doing the following things (in order of importance):
Invest in a good silencer or a muffled pipe
Keep the engine rpm down by selecting a proper propeller (high pitch)
Soft mount the engine
Enclose the engine in a cowl

For competition use the Hyde mount is probably the way to go but for general use (in sports and scale aircraft) I use Du-Bro soft mounts on everthing from my OS26-FS (totally unecessary but I do it anyway) up to my ST-3000 engine.

/Red B.