Types and designs of underwater acoustic transducers(1)

Sound waves are the only carriers that humans have mastered to transmit information and energy over long distances in the vast sea. people use electromagnetic waves to develop radars. Similarly, people use sound waves as information carriers to develop underwater acoustic transducer. Electronic equipment for positioning, identification and communication sonar. In the face of the vast ocean, Sonar shoulders are an important mission:it is to reach out to all corners of the vast sea, to identify the various things, to tell people the true face of the underwater world, to help people explore the mysteries of the ocean. To become underwater communication navigation,they are in the fields of aquaculture, fishery, marine resource development, marine geological and geomorphological exploration, military weapons, etc. The reason why sound waves become the best underwater information carrier is that the sound waves in the water medium have the smallest attenuation coefficient compared with other physical fields such as electromagnetic waves, and long-distance propagation can be obtained. This advantage makes the sonar which observes the underwater from the initial use of ultrasonic waves. The goal begins and continues to develop. At present, the working frequency range of sonar has been extended to a wide range. The active sonar is from tens of hertz to several tens of megahertz. The low frequency of passive sonar has been extended to the infrasound range. In such a wide frequency band, according to regulations. The signal form excites an important device that generates sound waves and senses and receives sound waves in the water without distortion. This is called a sonar transducer or a sonar array. These devices are the front-end equipment of the sonar system. They are also the "window" for the sonar system to interact with and exchange information with the water medium. They are the the sonar system, so the sonar transducer or sonar array is vividly referred to as the "eyes and ears" of the sonar system. With the continuous expansion of the application field of sonar technology, the improvement of military confrontation and operational needs have new principles, new technologies, and new sonar equipment have emerged in an endless stream. The development of new sonar technology has driven the rapid development of underwater ultrasonic transducer technology. The same technological breakthroughs in the field of transducers and the development of new materials, new mechanisms, and new structural transducers have also made the sonar system a new look. Here is a brief overview of the development of transducer technology,it is including the new material hydroacoustic transducer, new structure and new mechanism hydroacoustic transducer, new hydrophone,broadband transducer technology, etc.

 New material underwater acoustic transducer:

The ADCP piezoelectric transducers is a device that implements energy conversion in a sonar system.there is a special material with the ability to convert energy. This material is called functional material. The functional materials used to make the transducer mainly include piezoelectric materials (such as piezoelectric crystals, piezoelectric ceramics, piezoelectric polymers, etc.) and magnetostrictive materials (such as nickel, cobalt, nickel-iron alloy, ferrite, rare earth ferroalloy Etc.), they use the piezoelectric effect and the magnetostrictive effect to realize the mutual conversion between electric field energy, magnetic field energy and mechanical energy. The breakthrough in the transducer technology is fundamentally determined by technological breakthroughs in functional materials. In recent years, the technical achievements in various fields of functional materials have also brought to the development of transducer technology. In 1963, Dr. Clark discovered that the rare earth materials of the lanthanide series have amazing magnetostrictive properties, but they have not been put into practical use because the curie point is lower than room temperature. it was found that rare earth elements and iron composed of binary, ternary or quaternary alloys also have supermagnetostrictive properties at room temperature. The most representative earth alloy is Terfenol (components Tb, Dy, Fe).

It has become a new functional material that has received much attention since the 1980s. ferroelectric single crystal bismuth magnesium silicate magnate-lead titanate (PMN-PT) and lead bismuth citrate-lead titanate (PZN-PT), is a new type of composite perovskite crystal material, which is also a sudden rise. A new class of functional materials with promising applications. Prior to this, nickel was commonly used in depth finder transducer materials. In 1917, the French scientist Lang Zhiwan used quartz crystal to make a sonar transducer, setting a precedent for the application of piezoelectric materials on sonar in 1940s , BaTiO with strong piezoelectric properties.Piezo ceramics were successfully developed and used extensively in sonar systems during the Second World War; PZT piezoelectric ceramics developed in the 1950s compensated for Ba-TiO, ceramics with their wide operating temperature range and excellent electromechanical conversion efficiency. The shortcomings of the rare earth alloy material, which was once the preferred material for hydroacoustic transducers,they are greater at low temperatures than at room temperature, such as Tb and Dy0 at 77 K. The magnetostrictive strain of the material has a maximum value of 0.65%, while the Tefenol-D has a magnetostrictive strain of 0.25% at room temperature.

About ultrasonic hydroacoustic transducer,The rare earth alloy rod material is placed in the cold air chamber, and is circulated and cooled by the cooling tower of the refrigerator. The cold gas chamber is provided with a DC bias magnetic field and an excitation magnetic field by the coil of the superconducting material, and the magnetostrictive rod is excited to generate the stretching vibration and passes through the machine. The transition is transmitted to the piston radiating surface, and the piston radiating surface pushes the water medium to generate pressure wave radiation. The vacuum chamber is designed in the structure, the purpose is to isolate the heat conduction. The outer wall of the vacuum chamber is a shaped pressure-resistant cover, which can withstand the pressure of 10 atmospheres. The main technical parameters are as follows: resonant frequency 430Hz, maximum sound source level 181.4dB, efficiency is about 25%. This type of transducer is complicated in its fabrication process. In recent years, people are still willing to use Terfenol-D materials that work at room temperature, discarding some magnetostrictive strains and replacing them with new structures to achieve radiation performance. 

The following is a brief introduction to the research progress of several structural magnetostrictive materials for underwater acoustic transducers. The longitudinal transducer has a simple structure, and the magnetostrictive rod is combined with the front radiation head and the tail mass to form a one-dimensional vibration system. The front radiation head is generally a lightweight material, and the tail mass is generally a dense material to achieve a radiation surface and  larger vibration displacement. Two kinds of longitudinal transducers developed with Terfenol-D materials are introduced. One is a general longitudinal transducer with a resonant frequency of 1200 Hz, a sound power of 3 kW and a transducer weight of 60 kg. The other is the two ends of the rare earth rod. They are designed as flared double-end radiating longitudinal transducers with a resonant frequency of 400 Hz, a sound power of 1.5 kW and a transducer weight of 100 kg.About the circular
Ultrasonic depth sensor transducer,it consists of a number of rare earth rods enclosing a regular polygon, and a series of circular surfaces are excited by the transition piece for radial vibration to achieve high-power acoustic radiation. Which developed a series of rare earth low frequency high power toroidal transducers, with a resonant frequency of 200 Hz (inner diameter 0.56 m, outer diameter 0.94 m, height 0.37 m, sound source level 193 dB, weight) 410kg) and a transducer with a resonant frequency of 30Hz (2m diameter, height 1.1in, sound source level 195dB, weight 5t).