Sonar transducers did not undergo any major progress during the period of 1915 - 1940. A 24 kHz magnetostriction transducer used by the 1940 U.S. Navy sonar included a range of nickel tubes driving a l-foot diameter steel plate that was placed back-to-back with a Rochelle salt transducer within spherical housing.
The latter was attached to a pipe that penetrated the ship's hull. During operation, the pipe was rotated manually and a sound pulse was initiated at different selected bearings. If a submarine was nearby, a reflected echo would appear as a flash on a circular neon tube to specify the range.
Although the Rochelle salt transducer possessed a lower Q, it was less reliable compared to the mangnetostriction unit; the latter was used as a substitute to maintain the operation of sonar during frequent periods of failure of the Rochelle salt transducer.
Decade of Progress
During early World War II, German submarines were successful in sinking U.S. supply ships while moving in slow convoys, which became intolerable. In order to prevent a national disaster, under the civilian direction of the National Defense Research Council, a high priority ASW effort was taken to enlist every available engineering and scientific resource in the nation.
The Navy's underwater Laboratory based in New London, Connecticut, was quickly expanded to handle the development of magnetostriction transducers. At Harvard University, an underwater lab was established to study magnetostrictive materials and assess their application in transducer design. In San Diego, a Navy laboratory was assigned to inspect the Rochelle salt transducer in order to enhance its reliability.
Submarine installation included passive sonar arrays inside the three fin-shaped sonar domes using length of the vessel as baseline for the complete receiving array.
Early model of sound pressure measurement hydrophones (A) used Rochelle salt. Later model (B) used ADP crystals.
During this time of increased activity, Frank Massa joined Brush Development Co. as director of electroacoustic research. At the same time a former associate at RCA-Victor was recruited into the naval service as commanding officer of the Naval Gun Factory, which went on become the Naval Ordnance Laboratory.
One of his initial projects was to guard separate slow moving ships from torpedo attack. The plan involved towing a streamer along both sides of the ship, using a small charge of TNT explosive at fixed spacings along it's length. He sought the help of Massa to develop a hydrophone to be positioned at each location of TNT to detect noise from the approaching torpedo and then automatically fire the charge nearest to the hydrophone over which the torpedo passed prior to reaching the ship.
To address this issue, a Rochelle salt hydrophone was designed using the only available piezoelectric material at Brush. After the success of the towed array, a contract was signed with the Navy to appoint the full-time services of Massa’s small engineering team, which launched Brush and Massa into the unknown field of underwater transducer design.
Under the services contract, a liaison officer from the Naval Gun Factory bypassed all of the time-intensive formalities. Usually, these were needed to prepare detailed engineering proposals for each new development to continually re-establish the well-proven capability to meet the procurement needs.
As an alternative, the transducer specification for a new ASW application was occasionally communicated through a telephone, and the development was started immediately without wasting the engineering time required to produce large paper proposals.
A stream of new transducers was developed under this efficient environment, and put into production by Massa, typically within a month or two of each fresh request. The development of a measurement hydrophone for accurate calibration of transducers was a critical factor that contributed to this rapid progress.
Rochelle salt transducers have inherent limitations, which were quickly resolved by replacing ammonium dihydrogen phosphate (ADF) crystals, resulting in a better Model M-115B measurement standard. The U.S. Navy laboratories and others have extensively adopted the M-l 15B, which was effectively used as a stable reference standard for many years.
The first ADP transducer was designed as a replacement for the 24 kHz Rochelle salt device. Crystal assembly shown in foreground is mounted inside housing; a sound-transparent rubber disk molded to metal rim seals the opening. Assembly was vacuum filled with degassed castor oil.
ADP Replaces Rochelle Salt
Soon, it became obvious that Rochelle salt was not a reliable material for use in sonar transducer applications. A search for a better solution indicated that ADP possessed the preferred stability.
Using experimentally grown ADP, a transducer was developed and tests showed it to be much superior in terms of reliability and power handling capacity. As a result, the Navy recognized that ADP provided more significant benefits over Rochelle salt and instantly released funds for Brush to build a facility to grow huge volumes of ADP.
Within nine months, Rochelle salt became almost obsolete. During the development of tens of thousands of new transducers, many tons of ADP crystals were employed. The new transducers were mainly manufactured for use in new applications, which were created by the rapidly advancing sonar system developments to fulfill the urgent ASW efforts of the country.
Earliest application of ADP crystals was in an acoustically activated mine hydrophone.
An expanded manufacturing facility, from a small number of personnel in early 1940 to several thousand by 1942, was required for the rapid advancement in ADP transducer developments.
During the 1930s, Massa acquired production engineering experience in the design and large-scale production of U.S. battleship communication equipment and sound motion picture equipment, which collectively proved valuable to produce various ADP transducer designs that were robust and had a low manufacturing cost.
Acoustic Mine Hydrophones
One of the earliest uses of ADP was in a hydrophone design for use in acoustically activated mines. Specifications of the hydrophone included a low-frequency cutoff of below 5 Hz; a flat response in the low audio and subsonic frequency region; the ability to endure water entry shock when air was launched from 10,000 ft; and the ability to tolerate shock from adjacent mine explosions.
A uniform sensitivity requirement within one-half decibel of a reference standard was most important, and this was easily met as ADP has zero aging characteristics and remains stable during many years of storage.
Acoustic Torpedo Transducers
Passive sonar was used by the earliest acoustically guided torpedoes. Four identical high-frequency directional hydrophones were symmetrically placed on the torpedo nose - one pair was placed in the horizontal plane with their axes inclined at equal angles to either side (right and left) of the torpedo axis, while the second pair was similarly placed in the vertical plane.
The hydrophones sensitivities are equal at the cross-over point of the beam patterns along the torpedo axis. As a result, if the acoustic noise created by the submarine indicates the same level output on all of the four receiving channels, the torpedo axis is aligned with the target.
If the target is not aligned with the axis of the torpedo, then the relative signal levels detected in the hydrophones will change and the course correction is adjusted automatically until the levels of the signals in both pairs of hydrophone channels are equal.
The simple acoustic homing system functioned well until a countermeasure neutralized the effectiveness. An effervescent chemical was discharged by the target submarine, which suddenly changed course where it continued to remain undetected while the torpedo followed the fizzing decoy.
However, the countermeasure was neutralized by converting the torpedo homing system into an active sonar. A small transducer mounted on the torpedo nose transmitted periodic tone bursts, and the echoes of the target were received by the torpedo hydrophones and employed to make the course correction in the same way as in the original passive system.
The crystal area has a diamond-shaped pattern that reduces secondary lobes and produces a narrower beam in the vertical plane than in the horizontal plane, which is preferred for this type of application.
Ultrasonic ADP transducer for acoustically guided torpedoes uses low-cost shading for secondary lobe reduction.
The application of identical rectangular crystal plates organized in staggered rows (as indicated) achieved a much lower cost when compared to the mathematical shading methods, where each element in the array must have different sensitivities.
Passive Long-Range Submarine Sonar.
Active sonar cannot be used by a submarine during combat, because it would be suicidal to expose its position. The high uniformity and reliability of the ADP crystal made it possible to develop a passive submarine sonar system to indicate the bearing of submerged or surface vessels at very long ranges.
A number of ADP crystal assemblies were precisely placed within a steel tube to form a high precision line hydrophone. The hydrophone assemblies were completed with mounting flanges and a rubber boot, following which they were vacuum filled with castor oil and placed in parallel arrays.
In order to leverage the extreme stability and uniformity of the ADP crystal, the mechanical structures were developed to accommodate the parallelism of the acoustic axes of the hydrophones within 0.006”, or the highest detection range would be limited by the phase shift of the signal received at the high frequency end of the audible range because of misalignment.
Polarized Ceramics Developments
At the end of World War II, a range of ADP crystals operating at 18 kHz were used by the standard U.S. Navy scanning sonar transducer. With a better understanding of sound propagation in the sea, it became obvious that the range of detection would be improved by lower frequencies.
However ADP crystals have practical size limitation, which prevented their use at lower frequencies. During the early 1950s, barium titanate and magnetostriction ceramic designs were developed for low-frequency use.
Both of these materials had certain limitations prevented them from obtaining uniform impedance properties among the individual stave assemblies, which considerably deteriorated the beam pattern during the sonar operation.
Ultimately, the more stable lead zirconate titanate was used as a substitute for barium titanate and the scanning frequency of the sonar was progressively reduced from 19 to 5 kHz. This became the standard AN/SQS-23 sonar in extensive use for many years across the U.S. fleet.
Developed for the SQS-23 sonar, the magnetostriction transducer used several tons of nickel, which was not only expensive but also consumed significant amounts of an important material. A lead zirconate titanate ceramic design eventually replaced the magnetostriction design.
During the first decade of the application of the SQS-23 sonar, several sonar system manufacturers were developing ceramic transducers, each employing a different design. However, all of the designs developed serious issues in the fleet, requring regular replacements of transducers due to many electrical and mechanical failures.
The required performance specifications could not be met due to variations in the piezoelectric characteristics among the ceramic elements.
The Navy accepted this inability to control the reliability and uniformity of the ceramic transducer as being “beyond the state of the art.’’ In addition, deterioration in the transducer’s mechanical integrity immediately after installation increased the Navy’s concern to either find a replacement design or an acceptable solution to these problems.
Massa technician installs one of 432TR-208A elements in the Navy's SQS-23 scanning sonar transducer system.
During the mid-1950s, a proprietary ceramic transducer design was developed by Massa Products Corp for use on the SQS-23 sonar. The transducer was able to satisfy all of the “beyond the state-of-the-art” specifications, and its weight and price were approximately one-half that of the non-conforming previous designs.
More Reliability, Lower Cost
Transducer designs can be considerably improved by simply using the specialized skills of production engineers, who have extensive experience in the development of competitive commercial electromechanical products.
Better transducer designs can be realized if a greater effort is made to enlist these production engineering talents - beyond the capabilities or interest of physicists and other scientific workers. Also, if redundant structural components are eliminated, it would lead to a lower cost and higher reliability. Using production control methods can help to acquire a high degree of uniformity during large-scale production of transducers.
During the late stages of World War II, the scanning sonar transducers that were developed worked at relatively high frequencies, meaning the transducer assembly was quite small and included a range of ADP crystal staves joined to tubular frame perimeter. The assembled structure was enclosed by a single rubber boot.
Due to the progressive lowering of sonar frequencies, transducer dimensions became larger, making it essential to develop transducers as a large structural array using hundreds of modular transducer element assemblies.
The application of a modular design in the transducer configuration imposed stringent requirements in reliability and uniformity among the separate transducer element assemblies, which were not available in the previous high frequency sonar systems.
This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.
For more information on this source, please visit Massa Products Corp.