BRIEF: New Metamaterial Shepherds Sound Across the Water/Air Divide

(Inside Science) -- Normally, the underwater world is like an acoustic Las Vegas to land-based ears: what’s heard beneath the surface stays beneath the surface. Because the densities of air and water are so different -- and sound travels at such different speeds through each -- almost 100 percent of the sound waves that hit the water/air boundary bounce back. 

But now scientists have figured out a way to use a metamaterial -- an artificial material designed to exhibit properties not found in nature -- to shepherd sound across the divide.

The main component of the new material is a cylindrical cavity less than one-third of a centimeter across, which houses a five-part latex rubber membrane with a small weight at the center. When sound waves hit this structure, the membranes vibrate and create a secondary wave. After fine-tuning the mass of the center weight, the tension of the membranes, and the length of the cavity, the researchers created a structure that can generate a wave that effectively “cancels out” the sound waves that bounce off the water/air boundary. With the path thus cleared of reflecting waves, a significant part of the sound energy can now pass to the other side.

The researchers say the device could be easily optimized for different frequencies, and that multiple cylinder-shaped elements tuned to different frequencies could be combined to form an array for broadband sound transmission. They also note that changing the surface-area-to-volume ratio of the device could further increase energy transmission.

One likely application could be to improve underwater sound equipment like hydrophones, which are currently about 1,000 times less sensitive than air-based microphones. The metamaterial may enable engineers to use the superior air-based technology underwater. 

The findings are published in the journal Physical Review Letters.