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Comparing Encapsulation Methods Of Piezoelectric Transducers

By Adam Lipper • Joe Borden

The full range of output source levels measured during controlled-environment testing.

As oceanographic systems evolve, there is a constant evaluation of production methods, driven by the demand for high-quality reliable products that perform well in the harsh ocean environment. In underwater acoustic communications this holds true, especially as telemetry schemes continue to evolve. New developments necessitate the evaluation of technology, both old and new. As manufacturers continue to produce new systems, periodic evaluation of methods improves performance and the value of these systems for the end user.

In the piezoelectric transducer field, the comparison of technology is very important to both reliability and performance, not to mention manufacturability. One aspect of piezoelectric transducers is the encapsulation of the ceramic element, the core of the transducer. Different methods of encapsulation can affect transmit and receive capabilities of the transducer element, with particular advantages and disadvantages. Results of deepwater, open-ocean testing for the two most prevalent methods are presented here in order to discuss these advantages and disadvantages.

Encapsulation Methods
The encapsulation methods to be compared are potting in polyurethane and mounting in an oil-filled boot. Both methods offer advantages but neither can completely replace the other in all applications. The requirements of differing electronics systems and mechanical designs necessitate both methods in equipment operations. It is not unusual to find both methods in the current acoustic communications product offerings of many manufacturers.

Assemblies encapsulated in polyurethane are typically extremely reliable, which is desirable for both manufacturers and users. Transducers potted in polyurethane, called potted transducers, tend to be lower in cost and simpler to assemble but are often not economically repairable because of the harsh chemicals needed to remove the polyurethane. Potted transducers are also much simpler designs when compared to oil-filled boot transducers. Potted transducers tend to have lower acoustic-output levels and lower receive sensitivity when compared to transducers mounting the ceramic element in an oil-filled boot.

Hydroacoustic attenuation, or the absorption and dispersal of acoustic energy as sound waves travel through the water, is very prominent in deepwater environments. For this case, deepwater environments are defined as seafloor depths of 3,500 meters or more. Given this attenuation, the operation of some acoustic systems can be challenging in these deepwater environments because of the loss of acoustic energy.

This is acoustic energy that these high-technology systems require to process data, commands and operation requests. In these deepwater environments, the oil-filled transducer and its higher output capability, and increased receive sensitivity, are advantageous.

In the case of legacy equipment designs, the adoption of technology that can improve performance can be impractical because of program requirements or other factors. This enhanced technology may not be required in all cases to allow for successful operation of this legacy equipment. For instance, in shallower-water environments less than 3,500 meters depth, the attenuation levels may be the same, but operational ranges are greatly reduced. Therefore, the loss of signal due to attenuation can have a much smaller effect on the acoustic communications system.

Other environmental factors, such as multiple-path sound reflection, or multipath, which is the reflection of acoustic energy off features such as the seafloor, sea surface, geological structures or ships, can interfere with acoustic communications signals. Typically in shallower water environments, multipath is the major cause of acoustic interference, so using a transducer with a lower acoustic output can be desirable even if the equipment is capable of tolerating some amount of multipath due to software coding.

With knowledge in hand, manufacturers assist users in the selection of the best technology for their needs. In some cases, the easier-to-build and lower-cost potted transducer is ideal, while in other cases the increased output of the oil-filled transducer is the better option. These two offerings allow both users and manufacturers the ability to adapt products to fall within constraints of program budgets and requirements. No single transducer offering is ideal for all customer applications or budgets. To continue this article please click here.

Adam Lipper is an application engineer at Teledyne Benthos with almost nine years of experience. He has deployed moorings with customers, partners and third-party users, which gives him broad knowledge of the application of acoustic equipment in real-world scenarios. His primary technology focus is with acoustic telemetry modems and acoustic release products.

Joe Borden is the product line manager for acoustics and positioning at Teledyne Benthos. He leads the development and application of products for acoustic data telemetry and subsea positioning in commercial, scientific and government markets.

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