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Composite Transducers Enable Acoustic Instrumentation Innovation

By Eric Siegel • Mark Walsh • Atle Lohrmann

Illustration of a composite transducer.

The rapid improvements in high-speed digital components, substantially advanced by the medical ultrasound industry, are providing new opportunities in digital signal processing for acoustic instrumentation development. The improvements in digital signal processing mean that acoustic instrument manufacturers are investing a lot of effort in development to make products more competitive and offer customers improved measurement capabilities. Nortek Piezo Ltd. (Aberdeen, Scotland) has collaborated with many acoustic instrument manufacturers to design innovative composite transducers that provide increased bandwidth, more efficient energy transmission and greater receiver sensitivity. These improvements allow instrument manufacturers to implement more sophisticated broadband transmissions and reduce power output, while realizing gains in measurement range and resolution. The Signature55 acoustic Doppler dual current profiler with a 1,000-meter range is an example of how composite transducers can enable innovations in instrument development.

Piezo Composite Transducers
Acquiring the highest quality data from the front-end transducers is a prerequisite for improving signal processing. Piezo composite transducers offer many performance improvements over traditional single-element piezo transducers and are being more widely adopted to provide improved capabilities to underwater instrument manufacturers.

Composite Production. A piezo composite transducer is typically a combination of an active piezoelectric ceramic and a passive polymer to create a composite structure. Most piezo composite transducers start as a solid block or circular disk of piezoelectric ceramic, and then deep groves of the active material are removed through cutting processes to create a pattern of gaps that are filled with a passive polymer. The specific pattern of the active material, ratio between active and passive materials, and material properties of the passive fill matrix all impact the acoustic characteristics of the final piezo composite transducer assembly.

Improved Performance
Increased Sensitivity. Active acoustic instruments transmit acoustic energy and then listen for the return echo from the seafloor or objects and particles within the water column. These echoes are often very weak, therefore the measurement range is inherently reduced. Piezo composite construction greatly improves the sensitivity of the receiving capability of the transducer, improving the measurement range of acoustic products. Instinctively, cutting away portions of the active piezo ceramic material should reduce the receiver sensitivity. However, when the active material is removed in precise quantities and patterns, the receiver sensitivity can be improved. This is because the active material is formed into pillars, and each pillar is surrounded by the comparatively more flexible polymer, thereby reducing Poisson's ratio—allowing the pillar to more easily compress—which in turn offers increased sensitivity.

Increased Transmit Efficiency. Total measurement range of an underwater acoustic instrument is also controlled by the amount of energy that the transducers can transmit into the water. Composite transducers allow more efficient transmit energy into the water with less active piezo ceramic material compared to solid, single-element piezo transducers. Because a material's acoustic impedance is the square root of the product of its density and elastic stiffness, composite transducers have a lower acoustic impedance than solid ceramics. Therefore, the acoustic impedance is more closely matched to the impedance of water and the acoustic transmission efficiency is improved. An experienced composite design engineer must find the optimized balance between high electromechanical coupling and low acoustic impedance. The improved transmit efficiency allows instruments to realize gains in range or reductions in power.

Increased Bandwidth. Solid ceramic transmit devices are generally operated over a narrow band of acoustic frequencies with high projection sensitivity. Some transducer designs allow the band of operating frequencies to be increased, but typically at the expense of reduced gain (sensitivity). Piezo composite transducers are capable of operating over a much broader band of frequencies while maintaining good sensitivity, therefore delivering a higher gain-bandwidth product compared to conventional solid ceramic designs. This high gain-bandwidth product offers designers the ability to improve system performance (range, resolution, power consumption) through the material's ability to more effectively transmit and receive complex waveforms, such as chirps.

Optimized Beam Patterns. The manufacturing process for piezo composite transducers allows new opportunities to control and optimize acoustic beam patterns. Modifying the structure in this way dampens the transmission of lateral energies through the structure. This feature permits mechanical shading of apertures to tailor beam patterns for clearer images and reduced cross-talk within multielement arrays. Custom applications of electrically conducting patterns (electroding) can be applied to composite transducers to better manage sidelobe energy. No longer a solid block of ceramic, the composite structure may also be machined or molded into a curved array. The curved shape of the composite transducer may be used to focus acoustic energy into a very narrow beam angle from a small transducer, or diverge the energy into a wide beam angle from a larger transducer. Sophisticated composite designs can produce transducers with three-dimensional shapes and beam patterns and can even have in-situ, mechanically modified beam patterns using a flexible polymer within the composite matrix. To continue this article please click here.

Eric Siegel is managing director of Nortek Piezo Ltd. He enjoys collaborating with clients to develop new composite broadband transducers to drive innovation of acoustic measurement solutions. He holds an M.S. in physical oceanography and an M.B.A.

Mark Walsh is the chief technology officer of Nortek Piezo Ltd. and is focused on innovations in design, modeling and manufacturing processes for composite transducers. Walsh has lead transducer development and production projects in Europe for more than 20 years, and he holds a D. Eng. in electronics and an M.B.A.

Atle Lohrmann is a physical oceanographer and the managing director of Nortek AS. Lohrmann has been a key contributor to many of the innovations in the field of acoustic Doppler current profilers, velocimeters and wave gauges over the past 20 years.

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