From what I know, the first boomers were SSNs with additional hull sections, they were as noisy as first SSNs or even more. But later they become specialised constructions. SSN have to be fast, somewhat maneuverable, have large torpedo armament, many sensors, it needs ALL those things. When 688 class was constructed, high speed had same, or maybe even higher priority as stealth. We can guess that if the requirement was not 35 but only 25 or 30kts, they could be even quieter.
Now they are in 2/3 filled with reactor and machinery, rest with armament, and people are packed in corners that left ;-)
SSBNs don't need speed and some other things, they need only one - quietness. They needs it to be able perform their task, and their task is to be undeteced. So they are build with this requirement in mind, specialised in that. Don't need very powerfull propulsion, they are larger but don't need to be fast. The shape is much more hydrodynamic than in first constructions.
And... in fact they are not that quiet :-). Most of them isn't. From all boomers, Russian ones are more noisy than same generation SSNs. Both Typhoon and Delta IV are more noisy than Akulas. Don't know about British boomers, if they are better than Trafalgars. The one and most famous quiet boomer is Ohio, and it was probably only one similar in quietness to best SSNs, at least untill le Thriomphant appeared, it is supposedly very quiet too. And it have to be, it's his main and only task. With improvements in sonars, noisy SSBN would simply not be able to perform his task good enaugh, or at all...
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