Many people know about ultrasound because of its popularity in prenatal imaging - grainy, grey outlines of babies made using reflected sound waves. A new 'acoustic diode' could dramatically improve future ultrasound images by changing the way those sound waves are transmitted.

The theoretical framework for an acoustic diode achieves a one-way transmission of sound waves much the same as an electrical diode controls the one-way transmission of electrical impulses.

The one-way flow of sound would provide brighter and clearer ultrasound images by eliminating acoustic disturbances caused by sound waves going in two directions at the same time and interfering with each other, explained China's Nanjing University researcher Jian-chun Cheng.  "The propagation direction of the output wave would be controlled freely and precisely. These features are crucial for the medical ultrasound applications of the resulting devices." 

A schematic illustration of "acoustic diode" made of a zero refractive-index medium (ZIM) prism, which only allows the acoustic waves comes from the left ("positive incidence") to pass but blocks the waves from right ("negative incidence"). Credit: Urbana/ B. Liang

Sound waves easily flow in two directions. Yet in nature, total reflection of sound in one direction is known to occur at the air-water interface. This gave investigators the idea that an acoustical diode could be constructed by transmitting acoustic waves using an asymmetric prism to create total unidirectional reflection. 

The team developed its theoretical model based on a material not found in nature called a near-Zero Index Metamaterial (ZIM) and a prism to create high transmission efficacy acoustic waves that strike a reflective boundary from two opposite sides.

In theory, explained Cheng, "This would produce a unique tunneling effect and an unprecedented property that the output waveform is kept consistent with those of the waves traveling toward a boundary."