Little progress was made in sonar transducer development ..Lduring the period 1915-1940. The 1940 U.S. Navy sonar used a 24 kHz magnetostriction transducer comprising an array of nickel tubes driving a 1-foot-diameter steel plate mounted back to back with a Rochelle salt transducer inside a spherical housing. The latter was attached to a pipe that penetrated the ship’s hull. During operations, the pipe was manually rotated and a pulse of sound was initiated at various selected bearings. If a submarine was nearby, a· reflected echo would show up as a flash on a circular neon tube to indicate the range. The Rochelle salt transducer had a low Q and was less reliable than the magnetostriction unit, which was used as a substitute to maintain sonar operation during frequent periods of failure of the Rochelle salt tranducer.
In World War ll the Navy’s underwater Laboratory in New London, Connecticut, Harvard, NEL San Diego and Brush of Ohio undertook the development of magnetostriction transducers. An initial project was to protect individual slow moving ships from torpedo attack. The plan was to tow a streamer along each side of the ship with a small charge of TNT explosive at fiXed spacings along its length. A hydrophone was placed at each explosive location to pick up noise from the approaching torpedo and automatically fire the charge nearest the hydrophone over which the torpedo passed before reaching the ship. Using the only available piezoelectric material, a Rochelle salt hydrophone was designed to solve the problem.
A stream of new transducers were developed and put into production. It was soon apparent that Rochelle salt was a very unreliable material for sonar transducer applications. A search for something better indicated that ammonium dihydrogen phosphate (ADP) crystals had the desired stability. A transducer was developed using experimentally grown ADP crystals, and tests showed it to be far superior in power handling capacity and reliability.
Many tons of ADP crystal were used in the manufacture of tens of thousands of the many new transducers that were developed for use in new applications created by the rapidly advancing sonar system developments to meet the country’s urgent ASW efforts. Acoustic Mine Hydrophones:
One of the earliest applications of ADP was in a hydrophone design for use in acoustically activated mines. The hydrophone specifications included flat response in the low audio and subsonic frequency region; a low-frequency cutoff below 5 Hertz; ability to withstand water entry shock when airlaunched from 10,000 feet; and ability to withstand shock from neighboring mine explosions. Most important was a uniform sensitivity requirement within one-half decibel of a reference standard, which was easily met because ADP has zero aging characteristics and remains stable during many years of storage.
Acoustic Torpedo Transducers:
The earliest acoustically guided torpedoes made use of passive sonar. Four identical high-frequency directional hydrophones were located symmetrically on the nose of the torpedo; one pair was located in the horizontal plane with their axis inclined at equal angles to the left and right of the torpedo axis, and the second pair was similarly located in the vertical plane. At the cross-over point of the beam patterns along the axis of the torpedo, the sensitivities of the hydrophones are equal; therefore, if the acoustic noise generated by the submarine indicates the same output level on each of the four receiving channels, the axis of the torpedo is aligned with the target. H the target is not aligned with the torpedo axis, the relative levels of the detected signals in the hydrophones will change and the course correction is automatically adjusted until the signal levels in both pairs of hydrophone channels are equal. This simple acoustic homing system worked well until a countermeasure neutralized its effectiveness. The target submarine discharged an effervescent chemical and abruptly changed course where it remained undetected while the torpedo pursued the fizzing decoy.
However, the countermeasure was neutralized by converting the torpedo homing system to active sonar.
Passive Long-Range Submarine Sonar:
The high reliability and uniformity of ADP crystal made possible the development of a passive submarine sonar system for indicating the bearing of submerged or surface vessels at very long ranges. Several ADP crystal assemblies were precisely mounted inside a steel tube to form a high precision line hydrophone. A rubber boot and mounting flanges completed the hydrophone assemblies, after which they were vacuum filled with castor oil and mounted in parallel arrays.
The scanning sonar transducers developed during the late stages of World War II operated at relatively high frequencies, which meant that the transducer assembly was rather small, consisting of an array of ADP crystal staves attached to the perimeter of a tubular frame. A single rubber boot enclosed the assembled structure. With the progressive lowering of sonar frequencies, transducer dimensions became larger, and it became necessary to fabricate the transducer as a large structural array employing hundreds of modular transducer element assemblies. The use of a modular design in the transducer configuration imposed severe requirements in uniformity and reliability among the individual transducer element assemblies that were non-existent in the earlier high frequency sonar systems.
the past two or three decades, considerable effort has been expended in the development of high-power transducers for use in the low audible frequency region to meet the requirements of future ultra-long-range sonar systems not yet fully defined. Some examples:
Magnetostrictive Ring Transducer:
A large-diameter ring comprising a cemented multiple layer scroll of nickel strip (over which is wound a toroidal coil of insulated wire) will vibrate in the radial resonance mode when an AC current is superimposed on a DC biasing current supplied to the winding.
Ceramic Ring Transducer:
Large-diameter ceramic rings fabricated from wedge-shaped ceramic sections cemented together to form the ring have also been built for use as highpower low-frequency sound generators. However, they are subject to high risk of structural failure when operated at highpower levels.
During the mid-1950s, a ceramic transducer design was used on the SQS-23 sonar.
Megawalt Low-Frequency Electromagnetic Transducer Array:
Electromagnetic transducers that operate by electromagnetic forces have shown the most success. The forces are generated in an air gap between the magnetic laminations attached to the vibratile structure and the magnetic laminations attached to the massive inertial non-radiating structure. The world’s largest transducer array — a 300,000-pound array, 1500 square feet in area — was designed and manufactured by Massa for the Office of Naval Research as a replacement for a magnetostriction scroll array that failed while operating at high-power levels. The large array operated at a measured 60% efficiency and could be driven with up to 1 megawatt of audio power in the 400 to 500 Hz frequency band. Polarized Ceramics Developments: At the end of World War ll, the standard U.S. Navy scanning sonar transducer used an array of ADP crystals operating at 18 kHz. With increased knowledge of sound propagation in the sea, it became evident that lower frequencies would improve the range of detection. The practical size limitation of ADP crystals prevented their use at lower frequencies; therefore, magnetostriction and barium titanate ceramic designs were developed during the early 1950s for low-frequency use. Both these materials had limitations that prevented achieving uniform impedance characteristics among the separate stave assemblies, which in tum deteriorated the beam pattern during the operation of the sonar. Barium titanate was eventually replaced by more stable lead zirconate titanate and the scanning sonar frequency was progressively lowered from 19 to 5 kHz, which became the standard AN/SQS-23 sonar in widespread use for several decades throughout the U.S. fleet
The magnetostriction transducer designed for the SQS-23 sonar used several tons of nickel, which was expensive and consumed enormous quantities of what was considered a critical material. The magnetostriction design was eventually replaced by a lead zirconate titanate ceramic design. All designs, however, developed serious problems in the fleet and required frequent transducer replacements due to various mechanical and electrical failures.
The inability to control the uniformity and reliability of the ceramic transducer was accepted by the Navy until a replacement was found.
High-power Electromagnetic Transducer for Very Great Depths: A spherical low-frequency transducer element assembly approximately 2 feet in diameter and driven electromagnetically, push-pull, from opposite sides of a circular disk that lies in the equatorial plane of the sphere. The spherical shell oscillates as a dipole. When rho-c loaded, it delivers from 2 to 4 kilowatts of sound with a Q between 3 and 4 depending on the frequency range of operation and the thickness of the spherical housing, which in tum depends on whether the depth of operation is 20,000 or 2,000 feet. An underwater hom was designed for increasing the radiation resistance on one face of the spherical dipole by more than an order of magnitude over the loading on the rear face of the sphere, thus eliminating the need for a pressure-release baffie.
Electromagndic Transducer for High-power ww-frequency Armys:
A very recent development is an inertial mass-loaded electromagnetically driven vibrating piston. The vibrating piston is a rigid 17-inch-diameter plate, spring-mounted to the inertial mass portion of the vibrating structure. The heavy copper coils and E-laminations which comprise part of the magnetic circuit are bonded to the massive steel plate and contnbute to the total inertial mass. The lighter !-laminations that complete the magnetic circuit are bonded to the inner surface of the VIbratile plate. A circumferential row of springs separate the VIbrating plate from the inertial mass and the spring stiffness determines the resonance frequency of the transducer. The transducer design achieves a low Q of less than unity for the rho-c loaded array. Each transducer element can deliver 2-3 kilowatts of sound at approximately 50% efficiency over an octave bandwidth within the lowfrequency region below 1 kHz.
Lightweight Sonar Systems.
The continued lowering of sonar frequencies for achieving greater range has resulted in very massive sonar transducer arrays weighing many tons. A conventional scanning sonar transducer for operating in the 3-4 kHz region weighs approximately 20,000 pounds. A novel modular high-power transducer weighs only 10 pounds and generates 3 kilowatts of sound in the 3 kHz region when used in an array for a radically new sonar system.
Two conically tapered pistons are driven at opposite ends of a lead zirconate titanate ceramic stack. Both pistons move in phase and a stationary node is established at the center of the stack, equivalent to an inertial loading for each piston of infinite mass but zero weight. The tapered pistons form a radial underwater hom at the junction of each pair of modules when a m:mber of modules are aligned along the vertical axis of a tubular housing. The increased resistance loading due to the circular hom structure results in a Q of 2 which achieves 1/2 octave broadband response. A line array 9 inches in diameter by 50 inches long weighs 125 pounds and generates 20 kilowatts of sound in an omnidirectional horizontal beam with a vertical beam angle of 25°.
In the mid-50s Massa developed the PRS-208 transducer of the SQS-23 sonar system.
An engineering prototype of a portable long-range sonar system includes the new lightweight transducer mounted axially within a 9-inch-diameter housing. The unit also contains the power supply and a very high precision nine-element line hydrophone receiving array that achieves a target bearing accuracy of 7,0. The complete underwater portion of the Model M-1002long-range 3kHz portable sonar system weighs 250 pounds. The new portable sonar system has a range of 20,000 meters and a bearing accuracy of 1P. Preliminary tests conducted in Massachusetts Bay have located targets at ranges of 5 to 10 miles.