Sounding out a sharper ultrasound echo.

Like a hose nozzle that squirts a central jet of water accompanied by a smaller, diverging ring of spray, conventional generators of ultrasound usually produce multiple beams that fan out from their source. The presence of side beams represents one of the factors that at present limit the definition of images produced by bouncing high-frequency sound waves off a surface and detecting the echoes.

Now Mack A. Breazeale and Dehua Huang of the University of Mississippi in Oxford have developed improved ultrasound generators, or transducers, that emit single beams at frequencies of either 375 or 332 kilohertz. Moreover, in each case, the beam's intensity peaks in the middle and tails off toward the edges, exhibiting the bell-shaped curve of what researchers call a Gaussian, or normal, distribution. Breazeale described the improved transducer design at an Acoustical Society of America meeting held this week in New Orleans.

Any kind of acoustic imaging--sonar, medical ultrasound imaging, acoustical microscopy, ultrasonic nondestructive evaluation techniques--would benefit from the availability of well-defined sound beams, he notes.

An ultrasound transducer typically consists of a circular plate fabricated from a quartz crystal. Applying an alternating electric field via metal electrodes deposited on the plate's two surfaces causes the quartz to expand and contract, making the plate get thicker and thinner. Immersed in air or water, such an oscillating plate generates high-frequency sound waves.

The trick is to shape the plate and electrodes to generate a single, Gaussian beam rather than multiple beams. "What I want to do is to have the center vibrating with large amplitude and the edges with zero amplitude," Breazeale says. "The idea actually is obvious. The trouble is trying to make it work."

In a project that has taken more than a decade, Breazeale and his collaborators have gradually lowered the frequency at which their custom-designed transducers generate single, Gaussian beams of sound waves, starting at 4 megahertz. Many ultrasound applications, however, require beams of much lower frequencies. For example, sonar usually operates at frequencies ranging from 5 to 50 kilohertz.

"We have developed a technique over the past couple of years that seems to work, so we've gotten down to around 375 kilohertz," Breazeale says. "We're going in the right direction. We're shooting for as low a frequency as we can practically arrive at."

The researchers are now working on a Gaussian transducer made from a quartz plate 6 inches in diameter, which would operate at frequencies closer to 250 kilohertz.