“A team of Boston University researchers recently stuck a loudspeaker into one end of a PVC pipe. They cranked it up loud. What did they hear? Nothing.

How was this possible? Did they block the other end of the pipe with noise canceling foams or a chunk of concrete? No, nothing of the sort. The pipe was actually left open save for a small, 3D-printed ring placed around the rim. That ring cut 94% of the sound blasting from the speaker, enough to make it inaudible to the human ear…”

Source: http://www.impactlab.net/2019/03/17/scientists-have-discovered-a-shape-that-blocks-all-sound-even-your-co-workers/