Sonography, or ultrasound imaging, is commonly used for diagnostic and therapeutic applications – the best-known example being photos and videos of developing fetuses that expectant parents excitedly wave around. Because ultrasound relies on sound waves being sent into the body and then reflected back to create the image, the interference creating by these waves meeting causes some degradation of image brightness and resolution. In order to enable stronger, sharper medical imaging, scientists at Nanjing University in China have developed an "acoustic rectifier" that forces sound waves to travel in only one direction.

The acoustic rectifier has two parts. The first is a liquid filled with microscopic bubbles: sound waves enter at one frequency but when they leave the liquid their frequency is doubled. The second part consists of a latticed layering of alternating thin slabs of glass and slabs of water which acts like a filter. Only the sound waves with the doubled frequency can pass through.

The researchers believe that as well as offering possibilities for making "one-way mirrors" for sound that protect ultrasound sources from disturbance, the design could serve as a barrier for blocking environmental noise.

The team's results are published in the journal Nature Materials.