Acoustic lens generates tunable 'sound bullets' for ultrasound applications

Jan 17, 2014 by Lisa Zyga feature
(a) Illustration of a sound bullet, where the surfaces are contours of constant pressure. The positive (red) and negative (blue) regions of the sound bullet are compact in 3D. (b) Experimental set-up of the acoustic lens, which is composed of 13 chains arranged in a square lattice. The hydrophone is used to measure sound under water. (c) Cross section of one of the chains and its casing. Credit: Donahue, et al. ©2014 AIP Publishing LLC

( —Scientists have developed an acoustic lens that produces pressure pulses that are so intense they're called "sound bullets." Although they are too high-pitched to be audible to the human ear, the sound bullets could have a variety of uses such as in medical ultrasound, underwater mapping, and other high-intensity acoustic applications.

An that could generate was first demonstrated in 2009 by Professor Chiara Daraio and postdoctoral researcher Alessandro Spadoni at the California Institute of Technology in Pasadena, California. In that study, the researchers developed a 1D array of stainless steel spheres that struck each other similar to the way in which the metal balls in a Newton's Cradle toy strike each other. An impact at one end of the chain of spheres generates whose speed and focal points can be controlled by controlling the properties of the device.

Now in a recent paper published in Applied Physics Letters, Daraio and a new team of researchers have expanded this 1D acoustic lens into a 2D version consisting of 13 vertical chains of 30 stainless steel spheres arranged in a square lattice. In addition, they experimentally demonstrated the ability to create sound bullets in water, which moves the technology a step closer to biomedical and naval applications. A 2D acoustic lens has two main advantages over the 1D version: the ability to control the focus in three dimensions and the potential for larger pressure gains due to the more compact arrangement.

"This work was started to move a step closer to applications," Daraio told "A 2D array of 'acoustic sources' (i.e., chains of particles) allow us to focus the 'sound bullets' in 3D, creating a more compact and controllable acoustic signal. This focused pressure field can then be moved (or even scanned) in a 3D volume. This is a very desirable feature in acoustic imaging and surgery, for example. Most importantly, we demonstrated the ability to produce sound bullets in water, which was something we had predicted earlier with numerical simulations, but that was never validated experimentally. Given that most acoustic imaging methods are used in a water setting (think sonars, or ultrasonic images of the human body), this is a big step forward towards a practical implementation."

The 2D acoustic lens functions similarly to the 1D version. In each chain, the 30 spheres are held in place within a hollow tube. Above the vertical chains are other elements, including compression screws and springs at the top, weights underneath them, followed by pulsers and piezoactuators that are just above the chains of spheres. The pulsers emit rapid, high-voltage pulses that exert downward pressure on the top stainless steel sphere, resulting in a solitary wave that propagates through the chain. The researchers calculated that the waves attain speeds of up to 643 m/s.

The researchers showed how, by changing the placement of the chains and the wavelength of the solitary waves, they could control the shape and size of the sound bullets. The focal point of the sound bullets could be controlled in three dimensions by changing the pre-compression applied to the individual chains, or by triggering signals in each chain with a slight time delay. Controlling these properties of the sound bullets is essential for applications, such as for high-intensity forced ultrasound for therapeutic applications in which unhealthy tissue is targeted, while damage to healthy tissue is prevented.

"The next step we would like to achieve is the ability to 'image' objects under water using sound bullets," Daraio said. "This will require smaller improvements on the experimental setup and a bit more work on the data acquisition/image reconstruction analysis. We are also working on miniaturizing the systems, in order to obtain higher focal resolution, while maintaining a good signal amplitude (which is expected to improve the signal-to-noise ratio in imaging applications, for example). Finally, we are testing the acoustic lenses in solid materials (for example, composite plates used for aeronautical applications) to use the sound bullets to detect defects and analyze the properties of adjacent materials."

Explore further: Used MRI magnets get a second chance at life in high-energy physics experiments

More information: Carly M. Donahue, et al. "Experimental realization of a nonlinear acoustic lens with a tunable focus." Applied Physics Letters 104, 014103 (2014). DOI: 10.1063/1.4857635

Related Stories

Sound bullets in water

Nov 19, 2012

Sound waves are commonly used in applications ranging from ultrasound imaging to hyperthermia therapy, in which high temperatures are induced, for example, in tumors to destroy them. In 2010, researchers at Caltech led by ...

Caltech researchers create 'sound bullets'

Apr 21, 2010

Taking inspiration from a popular executive toy ("Newton's cradle"), researchers at the California Institute of Technology have built a device -- called a nonlinear acoustic lens -- that produces highly focused, high-amp ...

Designing an acoustic diode

Nov 01, 2013

Most people know about ultrasound through its role in prenatal imaging: those grainy, grey outlines of junior constructed from reflected sound waves. A new technology called an "acoustic diode," envisioned ...

One-way transmission system for sound waves

Jul 26, 2011

While many hotel rooms, recording studios, and even some homes are built with materials to help absorb or reflect sound, mechanisms to truly control the direction of sound waves are still in their infancy. ...

Recommended for you

SLAC gears up for dark matter hunt with LUX-ZEPLIN

May 21, 2015

Researchers have come a step closer to building one of the world's best dark matter detectors: The U.S. Department of Energy (DOE) recently signed off on the conceptual design of the proposed LUX-ZEPLIN (LZ) ...

First images of LHC collisions at 13 TeV

May 21, 2015

Last night, protons collided in the Large Hadron Collider (LHC) at the record-breaking energy of 13 TeV for the first time. These test collisions were to set up systems that protect the machine and detectors ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jan 24, 2014
Btw... detecting defects in the kind of ceramics used by space planes/shuttles would allow load-bearing heat resistant ceramic components. Or maybe ceramic components in engines with very high combustion temperatures.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.