[BlueAbyss]

There is a gate valve between the lower 'main' drain that allows you to slow the flow... it will take some fiddling to get it to work right, but the basic principle is that head pressure built up in the overflow box by the water sitting above the main drain pipe will equalize the draining of the overflow, maintaining a certain water level above the main drain. The secondary or emergency drain is to alert you that there is an impedance in the flow through the main drain.

What makes a Herbie quiet is the lack of falling water, rather the lack of water mixing with air in a turbulent fashion. No air gets into the main drain pipe, so there is no noise. This system can reduce overflow noise also.