Acoustic Diode Will Take Ultrasound Into The 21st Century
    By News Staff | November 4th 2013 12:29 AM | 2 comments | Print | E-mail | Track Comments

    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."

    Citation: Xin-Ye Zou, Bin Liang, Ying Yuan, Xue-Feng Zhu and Jian-Chun Cheng, 'Controllable acoustic rectification in one-dimensional piezoelectric composite plates', J. Appl. Phys. 114, 164504 (2013);


    A better name for this device is an acoustic duplexer. A "diode" would rectify sound waves, creating molecule motion in only one direction. While novel, it means the object would be displaced since regular sound waves move in both directions and average out, causing zero net displacement of the media carrying them.

    Duplexers (and Diplexers) are microwave devices which act differently for forward and reflected waves. Duplexers are commonly used to allow a transmitter and receiver to share a common antenna without allowing the transmitter to be seen by the receiver.

    Best to get the terminology correct.

    This is about the spectra for Lamb/plate wave modes in a one-dimensional piezoelectric composite plate - that is a diode and they put "acoustic diode" in quotes. I don't think they are incorrect.