The rapid improvements in high-speed digi-tal components, substantially advanced by the

medical ultrasound industry, are providing new op-portunities 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. (Ab-erdeen, Scotland) has collaborated with many acoustic in-strument manufacturers to design innovative composite transducers that provide increased bandwidth, more efficient energy transmis-sion and greater receiver sensitivity. These im-provements 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 devel-opment.

Piezo Composite TransducersAcquiring the highest quality data from the

front-end transducers is a prerequisite for im-proving signal processing. Piezo composite transducers offer many performance improve-ments over traditional single-element piezo transducers and are being more widely adopted to provide improved capa-bilities 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 circu-lar 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 poly-mer. The specific pattern of the active material, ratio be-tween active and passive materials, and material properties

of the passive fill matrix all impact the acoustic characteris-tics of the final piezo composite transducer assembly.

Improved PerformanceIncreased Sensitivity. Active acoustic instruments trans-

mit 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 receiv-ing capability of the transducer, improving the measurement range of acoustic products. Instinctively, cutting away por-

Composite Transducers Enable Acoustic Instrumentation InnovationImproving Bandwidth, Sensitivity to Foster New Product DevelopmentBy Eric Siegel • Mark Walsh • Atle Lohrmann

(Top) Illustration of a composite transducer. (Bottom) The Signature55 was first deployed offshore Toulon, France.

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Optimized Beam Patterns. The manufacturing process for piezo composite transducers allows new opportunities to control and optimize acoustic beam patterns. Modify-ing 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 pat-terns (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 compos-ite 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 us-ing a flexible polymer within the composite matrix.

Application of Composite Transducers: The 1,000-Meter Range AD2CP

Piezo composite transducers are used in many underwa-ter acoustic instruments to improve range, sensitivity, reso-lution and measurement capability. A recent innovation in technology achieved through composite transducers is the

development of the Signature55 AD2CP (acoustic Doppler dual current profiler) by Nortek AS (Rud, Norway).

Measurements of ocean cur-rent velocities to 1,000 meters depth is a Holy Grail for many offshore oil and gas operators and ocean circulation research-ers. Historically, the only acous-tic current profiler that could achieve the 1,000-meter range was a very large and power-hungry 38-kilohertz system. This made the system prohibitively

expensive and greatly limited applications predominately to offshore oil platforms. The large size and high power re-quirements did not allow for deployment on a surface or subsurface buoy for operational or research observations.

Nortek AS collaborated with Nortek Piezo Ltd. to de-velop a smaller and highly efficient piezo composite trans-ducer that would provide enough bandwidth to give high-accuracy velocity measurements and achieve the goal of profiling over a 1,000-meter range.

The result of the collaboration is the Nortek Signature55 AD2CP that employs three 25-centimeter-diameter com-posite transducers, each having a nominal center frequency of 75 kilohertz and a useful bandwidth from 52 to 95 kilo-hertz.

The composite transducers provide several capabilities that make the Signature55 an innovative instrument. The broadband transducers allow the Signature55 to operate in a narrowband mode centered around 55 kilohertz. This low frequency, coupled with the highly efficient transmit and re-ceive capabilities of the composite transducers, allow the Signature55 to achieve a typical current velocity profiling

tions 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 ma-terial is formed into pillars, and each pillar is surrounded by the comparatively more flexible polymer, thereby reduc-ing Poisson’s ratio—allowing the pillar to more easily com-press—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 transduc-ers allow more efficient trans-mit energy into the water with less active piezo ceramic mate-rial 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, compos-ite transducers have a lower acoustic impedance than solid ceramics. Therefore, the acous-tic 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 cou-pling and low acoustic impedance. The improved transmit efficiency allows instruments to realize gains in range or re-ductions in power.

Increased Bandwidth. Solid ceramic transmit devices are generally operated over a narrow band of acoustic frequencies with high projection sensitivity. Some trans-ducer 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 abil-ity to more effectively transmit and receive complex wave-forms, such as chirps.

A plot of the Signature55 testing.

“Measurements of ocean current velocities to 1,000 meters depth

is a Holy Grail for many offshore oil and gas operators

and ocean circulation researchers.”

range of 1,000 meters (maximum range depends on acous-tic scattering conditions in the water). Alternatively, by us-ing the 50 percent bandwidth of the composite transducers, the Signature55 can be operated toward the high-frequency end of the bandwidth to provide increased vertical resolu-tion and accuracy over a shorter profiling range, say 400 to 500 meters. This allows the same instrument to be used in a variety of applications from long-range, open-ocean mea-surements to projects on the shallower continental slopes. The high-efficiency transducers allow the Signature55 to use very little power, permitting smaller battery packs and au-tonomous deployment on surface and subsurface buoys. Fi-nally, the composite construction of the transducers means that the AD2CP is lighter than similar instruments of the same frequency range.

The Signature55 is an evolution of the AD2CP hardware that was developed between 2011 and 2013. The Signa-ture55 was first deployed offshore Toulon, France, in ap-proximately 1,200 meters water depth in June 2013. Initial tests deployed the instrument pointing downward next to a Teledyne RD Instruments (Poway, California) 75-kilohertz Long Ranger ADCP. The two instruments were configured with similar settings for range and resolution, and controlled to interleave acoustic pings in order to reduce cross inter-ference. The results document that when the Signature55 is operated in broadband mode, it gets approximately the same profiling range as the Teledyne 75-kilohertz ADCP, but the Signature55 uses about 50 percent less power.

When the Signature55 uses the same output power as the Teledyne 75-kilohertz ADCP and operates in the nar-rowband, lower-frequency mode, it consistently achieves 200 meters more range than the Teledyne ADCP. Therefore, it can typically measure over a range of 800 to 1,000 me-ters. Autonomously deployed on a subsurface buoy, looking upward from a depth of 1,200 meters offshore France, the Signature55 observed energetic current profiles over a range of 1,000 meters, with a vertical resolution of 20 meters.

Future GainsSignificant advances in active and passive phase materi-

als, particularly single-crystal piezoelectric materials, offer the potential to achieve even greater gain-bandwidth prod-ucts. Coupled with the novel electrical matching of transduc-ers to electronics, acoustic instrument manufacturers have a strengthening technical platform from which to develop new

“Significant advances in active and passive phase materials,

particularly single-crystal piezoelectric materials, offer the potential to achieve even greater

gain-bandwidth products.”

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acoustic-based products and improve the performance of existing products. n

Eric Siegel is managing director of Nortek Piezo Ltd. He enjoys collaborating with clients to develop new composite broadband transducers to drive innova-tion of acoustic measurement solutions. He holds an M.S. in physical oceanog-raphy 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.