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"On the surface, reducing the sound from an overflow is simply a matter of raising the water level in the overflow such that water doesn't fall as far and thus makes less noise. This can be done with a simple standing pipe, but this solution only works if the flow rate into the pipe is low relative to the full flow potential of the pipe. If the flow rate approaches a decent fraction of the pipe's potential, the top of the pipe first begins to gurgle and, as the flow is increased, form noisy vortices. Over the years a number of simple do-it-yourself solutions have been devised to address this problem. Two such popular solutions are the Stockman Standpipe and the Durso Standpipe. I have tried both of these solutions but have found neither as quiet or reliable as the HGB. Why? Here are the problems I encountered with those two solutions:
Stockman Standpipe (SS) Problems
Air injection achieved by mixing at the water's surface - this makes for quite a bit of noise inside the standpipe, some of which can be heard.
Back pressure is not controlled - in theory this shouldn't matter, but as a result, slower flows tend to be louder than the HGB in my experience.
Durso Standpipe (DS) Problems
All the problems above PLUS:
Air intake adjustments are sensitive, and if the air hole is too small or salt spray clogs the port, there is a loud flushing.
Too large to fit in many overflows"
"The water flowing past the end of the air tubing will draw air into the turbulent water flow inside the core pipe. This air is very important in order to prevent a strong siphon from forming which would otherwise induce vortices into the bell. Normally, air should be drawn below the water inlet holes as shown in step 4 above. If the back pressure adjustment was done properly, one should be running fairly quietly already, but for total silence, the air tube end may need to be slightly tweaked (usually by pushing the air tubing in further). Very slightly slide the air tubing up and down to hit the "sweet spot," the height at which all sound should stop or be minimized. Now cut the top end of the air tubing to the maximal height you would ever allow the water in your overflow. If for any reason the main holes are partially obstructed (say a large piece of bubble algae gets stuck in there), the water in the overflow will rise to the top of the air tubing, block the air flow into the tubing, and thus induce a strong siphon through the entire unit. This strong siphon action can be thought of as a backup measure just in case the holes are obstructed. The strong siphon will be noisy and will indicate there is an obstruction that needs to be cleaned out".