Quieting a room with metamaterial wall panels

The electronics in noise-cancelling headphones generate sound waves that are 180° out of phase with the ambient sound and thus damp environmental noise. Now, using the same basic idea, Guancong Ma and colleagues at the Hong Kong University of Science and Technology and the Langevin Institute in Paris have designed and built a surface that dampens a noise source to create quiet zones within a room.

The sound one hears at any location in a room is a complicated sum of waves scattered off the ceiling, walls, and other objects. Changing how those waves interfere with each other offers a way to control a reverberating environment. Ma and colleagues developed an array of membranes, each connected to an electromagnet, that resonate when excited by a sound wave. Switching the polarity of a voltage across the electromagnet either pins or releases the central part of the membrane, which causes a shift in the resonance to 850 Hz or 450 Hz, respectively. When transmitted through the membrane, an incident sound wave with a frequency above or below the membrane’s resonant frequency will shift its phase. For 600 Hz incident waves, the phase difference after passing through both activated and deactivated membranes adds up to nearly 180°, and the waves destructively interfere.

The team built an array of 360 membranes like the one in the photo and positioned it in the middle of a room. A small microphone measured the sound amplitude and served as a feedback to determine which membranes should be switched to each resonance to yield the smallest amplitude. The researchers demonstrated that the surface could minimize the amplitude of 600 Hz sound waves to create quiet zones throughout the room.

The surface offers a way to dampen the noise from a single-frequency source, like a generator or motor. A different feedback system will also make it possible to tune broadband frequencies. (G. Ma et al., Proc. Nat. Acad. Sci. USA, in press, doi:10.1073/pnas.1801175115 .)