The problem
The new entrance hall shares a wall with the museum’s plant room. The mechanical contractor could get the breakout noise down to 46 dB(A) at the ticket desk, and no further without moving the chillers. It was tonal — a 63 Hz hum with a 250 Hz whine on top — and tonal noise at 46 dB is more intrusive than broadband noise at 55.
What we changed
A weir wall is a broadband noise source with a spectrum you can tune by changing what the water lands on. We built an 11 m machined weir plate with a 26 mm head over the crest — enough to hold an unbroken nappe across the full span, which is the whole trick; below about 18 mm the sheet tears into ribbons and starts to sound like a leak.
The receiving pool is the tuning fork. A hard granite invert gave us a bright, hissy 4 kHz peak that read as hostile in a room full of children. We changed it to a 220 mm deep pool with a submerged pebble bed, which pushed energy down into the 500 Hz–2 kHz band, exactly where the chillers live. Measured masking at the ticket desk is 52 dB(A) with the plant running, and the plant is no longer identifiable by ear.
The thing that nearly went wrong
The steel plate was specified as a single 11 m fabrication. It arrived with 3 mm of camber over its length, which on a weir crest is not a tolerance, it is a hole. We ground the crest flat on site over four nights against a laser datum. Every weir wall we have built since is specified with the crest machined after welding, and we say so on the drawing in capitals.
