It's crazy, but I had a dream last night about how to make my downspout on my indoor unit quieter. So, I got up this morning and tried it out, and after a little tinkering, it worked!
OK, so the image on top is what it looks like in the water. Don't worry about the darker sponge-like filter things wrapped around the pipe. I tried some holes near the top that I don't think do anything, and since I didn't have any spare pipe, I put that filter around it, and it keeps the water in. I'll replace with a fresh piece when I get a chance to prove that it's not actually doing anything in the process, because there's a chance that it's letting in air through the holes but not letting water out.
The top light blue filter, though, does serve a purpose. It seals up the two pipes in a way that doesn't let water come out, but does let air in.
In general, my silencer is just a length of pipe (pictured 2nd), that goes from the bottom of the fish tank up to just past the bottom of the downspout. It's 2" pipe over 1" pipe, so it's a loose fit. That lets the air in. At the bottom of the pipe, there's a notch to let water/air out, as well as a few holes drilled all the way through. These are always submerged.
When it's going full blast in siphon-mode, this thing is like a mega-aerator! There are bubbles everywhere! It really stirs up the sediment, which gets the fish all excited, because there are likely some floating edibles in the water.
It's much quieter than just letting the water from the downspout hit the top of the fish tank, and as the siphon gets going, I believe that water fills the inside of the 2" pipe such that it meets the water coming down, and then it gets very quiet, as all air and water is then directed out through the bottom of the 2" pipe.
Here's a video from close range. It's hard to tell that it's quieter, of course, 'cause it's so close to the drain, but just trust me, it is! It does show the incredible aeration. And, near the end, you can hear the phone ringing (hence the end of the video) - just for reference, that phone is 2 rooms away.
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