Urethane transducer encapsulation versus oil filled boot encapsulation of piezoelectric transducers

With oceanographic equipment being deployed in deeper ocean environments and the requirement of more advanced acoustic systems there is a growing need for piezoelectric transducers with clean output signals. Because of this, manufacturers are driven toward polyurethane to encapsulate these transducers. There are many reasons to use polyurethane as an encapsulation material such as cost, manufacturability and reliability. However there are inherent issues when potting piezoelectrics in polyurethane. When potting in polyurethane there is a certain amount output lost due to the nature of the polyurethane. These losses can be seen on both transmit and receive signals of the transducers so in many cases alternate methods of encapsulation are desired. Therefore, in systems that require higher output an oil filled boot encapsulation method is recommended. When comparing a newly designed polyurethane transducer to a previously used oil filled boot transducer there can be noticeable advantages to the oil filled units. In some legacy electronic systems when an oil filled transducer was compared to a polyurethane transducer the signal differences were very small but with the new highly advanced systems the differences can be vast. In newer electronics systems that use what can be considered far superior transmit and receive circuitry the differences are amplified. In some cases it is highly recommended to use the oil filled transducer, such as when high output is desired for long range acoustic operations. Certain users of acoustic equipment that operate in deep water situations would greatly benefit from the higher output capabilities of an oil filled transducer when compared to a polyurethane transducer. Many manufacturers offer high output options of their acoustic equipment for use in deep water and higher efficiency lower output versions for use in shallow water. Shallow water systems that use the potentially more efficient polyurethane transducers benefit from the lower- output because in shallow water it is not typically desirable to have high output due to the potential for multipath signal reflection. Users in deep water typically have little reason worry about multipath reflection because of the rate of acoustic attenuation in the water column. Therefore, in deep water long-range applications do benefit from the higher output of the oil filled transducer and in shallow water, shorter range, applications the more efficient polyurethane transducer can be more desirable. The cost of the two encapsulation methods can also be part of the user´s decision making process. The oil filled boot encapsulation method requires more intricate mounting hardware because without it the piezoelectric ceramic transducer would be free floating in the boot, which could cause an undesired shift in output patterns. When using the polyurethane encapsulation method there is little need for mounting hardware because the polyurethane will harden and hold the transducer in place so the cost of polyurethane transducers can be much lower. The expense of the mounting hardware in the oil filled transducer can elevate the sell price and affect the user´s budget so there must be a balance of cost versus performance versus system requirements. This paper discusses the results of a test done off-shore in the waters surrounding the Big Island of Hawaii in September of 2011. The comparison of operation is outlined as seen when communicating from surface deck units to acoustic releases which utilize a Frequency Shift Keying (FSK) command scheme in a water depth of 4,400 meters. These releases operate in the range from 7.0 kHz to 14.0 kHz at ranges from 4,800 meters to over 10,000 meters. A comparison of functionality using the oil filled transducer and polyurethane transducer is made.