Why are SSBNs so Quiet?

[Neutrino 123]

SSBN are reportedly very quiet, but why is this so? It seems to me in a basic analysis that they really shouldn’t be much quieter then SSNs of similar technology (though all sources say they are, so I would of course expect this to be true).

SSBNs have similar propulsion to SSNs, but they have a lower maximum speed. Approximately speaking, this implies similar machinery to the SSNs (this seems to be the case on cutaways I’ve seen, too…), but since the SSBNs are larger, they need more power to get the same speed, and thus, the machinery would generate more noise for the same effect. Presumably, this would be somewhat mitigated by additional quieting equipment. However, the storage space for the large missiles should limit the amount of space available for other things such as this, especially since the SSBNs usually have a larger crew and a fairly good torpedo armament, too.

In addition, the water noise flow that accounts for a portion of the BB noise (I am not sure how much, but this is something I would like to know…) would be significantly greater in the SSBNs.

…so, when considering the above, why are the SSBNs much quieter? Can they really fit enough quieting equipment to do this? If this is efficient, why not add similar equipment to the SSNs?