From what I've read these are very quite.
The way they work is you use 2 drains. That would be both in a OF box. The larger one is your primary drain. You run this up a few inches below where you want to run your water line. Place a ball valve, or better yet a gate valve on this drain. Run the smaller one up higher than your running level. This is the level INSIDE the OF we're talking about here. This is an emergency drain.
By closing the BV off you restrict the flow in the pipe, causing the water level to rise in the OF. Once it gets to a certain point it will stop sucking air and only drain water into the line. This will handle MUCH MORE FLOW than a standpipe or drain with air in it. You can see that the full diameter of the pipe is now filled with water ONLY. In addition, the water level inside the OF box is higher, this causes the water to be under pressure pushing it through the line. Now you can see where there is an inherent risk doing this. What Herbie did was to add this additional drain which acts as an emergency line. If the PD plugs then the ED will kick in. By setting this lower than the teeth in the OF it too would eventually become under pressure and possibly be able to handle the full flow in itself. But then you will hacve a waterfall noise in the OF box. Since a complete blockage is exrtemly rare, setting it higher is an option.
Leaving the ED above the water level in the sump causes it to make noise when it does drain, which will alert you to a problem. But couild be placed at or below the water level in the sump.
Appaerntly, the drains themselves are very quite now as theres no air sloshing around. But the water rushing into the sump becomes a noise issue. I can however see it being as noisy as a standard drain with air sloshing around. as it's just flowing in. I dunno though!
As Whitebird asked, has anybody got any experience with these?