I'm a bit out of touch with air rifles but I used to be heavily involved with them, and the first thing I'd say is that the power isn't the aspect I'd be focusing on where noise is concerned.
Legal restrictions of air rifle power vary, and on that I can only go with what is considered high power here. 12 ft/lbs is the top limit on energy you can have here before you need an FAC. Although I've played with several air rifles much more powerful than this I think it is safe to assume that is commonly regarded as high power.
Here's the thing: The engagement range of the target species never changed much whether I was shooting 12 ft/lbs or 18/20 ft/lbs. The kill area didn't change size. I can't be certain of grouping my shots on a ping-pong ball size area with an air rifle beyond 50 yards under real world conditions. The way I see it, that is what I need to do to ensure a clean kill of the species I'm after. If I can't do that then I have no business shooting at animals at that range. 12 ft/lbs on the sweet spot at that range kills just as good as 18/20 /lbs. But hey, 50 yrds is fairly extraordinary with an air rifle. Much more common to me was to take rabbits and stuff at around 30 / 40 paces. On that, you can quite easily get an off the shelf air rifle that avoids all the power band crack and will do the job. The differences in sound these tools will produce can be great. Spring guns are noisy. It is in their nature. But there are more ways to build a sporting air rifle than a spring gun and many of these are very quiet indeed. A true poachers friend.
Frankly, I get quite sickened when I hear of extreme feats with high power air rifles. I've taken bets from people with great claims of prowess at 50yards that couldn't take all ten ping-pong balls I put up at 50 paces with ten shots. Good. It's nice to humiliate a 'tard in front of his friends. And it's even nicer to know that you may well have saved a bunch of wildlife from wounding by an idiot. I've even had a salesman in a gun shop trying to sell me off the shelf with the blessing this will kill a fox at 50 yards. I wanted to beat him with the blunt end of it. Then there's the 'tards that go on about the power of these new miracle air rifles as if I had no understanding of the antique shop air-canes, or ever killed a whole bunch of stuff in the wild, or competitively shot bell target. Aarrgghhh; keep it simple and honest. An off the shelf 12 ft/lbs air gun is plenty for taking the types of things you ought to be using it on [save crows, sh1t they're tough], out to about the range you are accurate with it. In sum, it's a very quiet cheap to run solution for getting small critters. Winding in another 10 ft/lbs is not going to magically make you more accurate, or change the size of what you are trying to hit, or even the species you are hitting. On that, it is easy with a air rifle to throttle it to produce extremely low noise levels without impediment to the size of your bag.
That said, if I could use one here without fuss I would also have a Ruger M77. I'd go the Tactical Operation suppressed version going by the name of Green Hornet. This to me is clear.
