Blocking sound has always meant blocking airflow. Walls, barriers, enclosures — sound attenuation and ventilation have been in opposition for as long as people have built rooms.
An acoustic metamaterial that is 70% open air achieves broadband noise suppression. Three subwavelength unit cells create a 2π phase gradient across the aperture. The outer two cells contain solid barriers that shift the acoustic phase. The central cell is completely open — physically, a hole in the wall. But the sound does not pass through.
The mechanism: incoming sound waves are converted into “spoof surface acoustic waves” — acoustic analogs of electromagnetic surface plasmons. The phase gradient forces the propagating wave to bind to the surface of the metamaterial rather than propagating through the opening. The sound becomes a surface-bound mode before reaching the hole. By the time it arrives at the open space, it can no longer propagate as a free wave.
The sound isn't blocked. It isn't absorbed. It is redirected into a mode that cannot continue. The distinction matters: absorption converts acoustic energy to heat, requiring mass. Blocking requires a solid barrier. Converting the wave to a non-propagating mode requires only the right pattern of phase shifts — which can be achieved with mostly empty space.
You don't have to stop something to silence it. You can change what it is, and the stopping takes care of itself.