A mathematically designed meta material can "make" silence without interrupting airflow, according to work by researchers at Boston University. The meta material might be used to silence the irritating drone of quadcopters, the even more disturbing noise of airplane yet turbines, MRI machines, and urban noise pollution.
"Today's sound barriers are literally thick heavy walls," explains Reza Ghaffarivardavagh. Materials built as sound baffles, can help reduce some noise pollution they are not suitable where airflow is needed. You cannot simple baffle a jet engine's exhaust vent. Now, writing in the journal Physical Review the team has turned to 3D printing and no little acoustical mathematics to design and print an open, ring-shaped structure, that allows air to flow through it, but baffles sound.
Sound is carried as vibration through the air, so if you want to muffle a sound but allow air to flow you have to find some to inhibit the standing wave vibrations without interrupting the fluid flow. The team calculated that a meta material could be made that would reflect sound waves back to their source but not impede the bulk flow of air.
As proof of principle, the team attached a loudspeaker to their meta material baffle and pumped up the volume of an irritatingly high-volume note. Standing in the lab, nobody could hear the noise, but they could see the speaker cone vibrating. The effect worked exactly as it had in the sound pressure modeling software on the lab computer. The meta material works with 94% efficiency at cutting the noise, which is sufficient to make a loud noise from the speaker inaudible to a person with normal hearing.
The team is now looking at designing a lightweight baffle that could be fitted to a quadcopter and silence the fans without interfering with flight. The same approach could be used on fans and air-conditioning units, and perhaps with more development on jet aircraft. The same structures might be incorporated into air-permeable walls to act as sound insulation in buildings and along busy roads. The team suggests that although their proof of principle device is doughnut shaped, it could be a cube or a hexagon with a hole and so be used as a building block for a variety of structural components. Patients and radiographers in the MRI scanning room are well aware of just how loud the MRI machine is. A sound baffling structure lining the MRI cylinder could dramatically reduce the noise experienced by the patient.
The possibilities are almost limitless, the acoustic model could be used to design a 3D printable meta material to block any noise.