A team of mechanical engineers from Boston University has developed a device to block up to 94% of the incoming sound waves, while maintaining breathability. This is a 3D printed metamaterial structure. Its internal structure has a complex structure and is aimed at reflecting the acoustic vibrations of the medium. It looks like a thick ring that is placed near the source of noise in the path of the sound waves.
Sound is generated not by the movement of air itself, but by small harmonic vibrations in it, which can be amplified or damped using simple mechanical devices. To do this, you need to correctly calculate at what angle the wave will meet the obstacle and where it will then be reflected. The metamaterial of the Boston engineers has many cells that reflect the waves back, almost in antiphase, so that vibrations do not propagate outside the device.
The design of the muffler ring has an internal helical shape, so the sound waves are forced to move in a spiral, from the outer boundary to the center of the ring, constantly reflecting back. As a result, the air stream that has reached the exit practically does not vibrate and leaves the ring with a quiet, even hum. It remains only to select the dimensions of the ring to the sound source so that the waves cannot bend around the muffler at the edges, and reliable noise protection is ready.
Practical experiments have fully confirmed the forecasts, the effectiveness of the muffler is excellent. And what is important - it works in a passive mode, without energy consumption and without moving parts subject to wear. The shape of the ring is also not a limitation, the muffler can be made in the form of a hexagon or in a more complex configuration. Plus it is a lightweight product - such mufflers can be installed both on jet engines of aircraft and on compact quadcopters.