I have had a few standpipes, and when used singularly, they sounded like a toilet constantly running, or gurgling.
Using one of the multi drain configurations (herbie or bean animal) where one is valved and running at full volume, while another (or 2) is (are) taking up the slack is the only way to reduce 90% of the noise.
I have tried capping stand pipes of with a PVC cap resting on a strainer of sorts, and it did reduce the toilet sound slightly about 30-40%.


Using one of the multi drain configurations (herbie or bean animal) where one is valved and running at full volume, while another (or 2) is (are) taking up the slack is the only way to reduce 90% of the noise.
I have tried capping stand pipes of with a PVC cap resting on a strainer of sorts, and it did reduce the toilet sound slightly about 30-40%.

