The specification for the Seoul courtyard said 40 dB(A) at the nearest openable window. We hit 36, with the bowl tip events peaking at 41 for under two seconds. On paper it was a pass before we had finished the commissioning sheet.
The clinical lead asked to sign it off from a bed instead, at three in the morning, in a room on the second floor. It is the right instinct: a two-second 41 dB event is nothing on a meter and can be the thing that wakes somebody who has been in that room for eight months.
It passed, but two things changed as a result. The tipping bowls were re-counterweighted so that no two are within four seconds of each other, because a coincidence of three tips is audible as an event in a way that three separate tips is not. And the programme now drops to six bowls between 23:00 and 06:00 rather than running all eighteen.
We have added a line to our commissioning protocol. Where an installation is overlooked by somewhere people sleep, the acoustic sign-off happens at night, from inside, with the client’s own people present. A meter on a tripod in an empty courtyard measures the wrong thing.