Water leaves a level crest as a coherent nappe and falls clear. Done properly it is the calmest large-scale effect available and the most useful acoustically, because it produces genuinely broadband noise that masks plant and traffic without sounding like either.
Everything depends on two numbers. The head over the crest must stay above about 18 mm or the sheet tears into ribbons; below 12 mm it stops being an effect and starts looking like a defect. And the crest must be flat to better than 1 mm over its whole length — which means machined after welding, never before, because a fabricated plate will camber as it cools and a 3 mm camber is a hole in the middle of your curtain.
The plenum behind the crest does the invisible work: it has to arrive at the weir with no lateral velocity at all, which usually means a two-chamber box with a perforated baffle. Feed a weir directly from a pipe and you will see the pipe’s position in the sheet from twenty metres away, permanently.
Tune the sound by changing the receiving surface, not the flow. Hard stone gives a bright 4 kHz hiss; 200 mm of water over a pebble bed pulls it down into the 500 Hz–2 kHz band where most mechanical plant sits.