Transducer research (1958 – 1964)
The laboratory put much work into developing transducers and hydrophones. Just after WW II, not much information could be found in public sources about these devices. Reliable theories were missing so most advancements were the result of numerous experiments. Moreover, the use of modern ceramic materials had to be studied because such materials had advantages over piezoelectric crystals and magnetostrictive metals.
Around 1960, the new design of a special passive sonar started using the principle that is known in radio technology as “Watson-Watt” to acoustically get directional information about a target. Despite the success of this research, industrial production did not start until many years later.
An example of the many developed hydrophones by TNO is the sonar interception hydrophone LWS20. If a ship is searching submarines by transmitting sonar signals, then those signals can be captured by a submarine before the ship can detect echoes. For this purpose, the submarine has a sonar interception receiver, a listening device that determines the direction and frequency of received sonar signals. Low-frequency signals come from long-range sonars, mid-frequency signals from attack sonars, and high-frequency signals come from target-seeking torpedoes. The sonar interception receiver must therefore be able to receive a wide frequency spectrum.
The transducer consists of a cube containing four hydrophones LWS20, each covering a 90o sector. Each hydrophone contains ten ZP84 elements (own development of the laboratory) arranged in a triangular pattern. The signals from the ten elements go to eight preamplifiers.
The frequency spectrum is divided into four parts. The single upper hydrophone element (1) covers the highest frequency band from 40 to 80 kHz. The three upper hydrophone elements (numbers 1, 2 and 3) jointly receive the frequency band from 20 to 40 kHz. The six elements (numbers 1 to 6) triangle received the frequency band from 10 to 20 kHz. All ten hydrophone elements (numbers 1 to 10) together process the lowest frequencies from 5 to 10 kHz.
The hydrophone consists entirely of titanium and can withstand water pressure at any depth a submarine can dive.
In 1958, work began on the construction of a low-frequency panoramic transducer with associated equipment. Such a transducer was not present in the Netherlands at the time. The large research and development project intended to gain experience with the fundamental problems related to such a design. The transducer, which was not intended as a pre-production model, was constructed from 216 hexagonal, mutually supporting elements. These were arranged in a cylindrical shape with 36 columns of six elements each.
In 1964, the panoramic transducer 216TP5R was completed. This panoramic transducer could transmit and receive sound signals over 360 degrees. The resonance frequency was 5 kHz. The total weight was 2,800 kg. The measurements performed with this transducer have contributed significantly to the knowledge needed to provide future users of such transducers with corroborating advice. The same applies to the electronic equipment needed to use such a transducer.
The operation of the 216TR5R was as follows: one-third of the columns, 12 columns corresponding to 120 degrees of the total circumference, were used for transmission and reception. By electronically switching the 12 columns, the sound beams could be adjusted all around in 36 directions. With five fixed delay lines it was ensured that, despite the curved front of the transducer, a flat wavefront of the sound was acquired. In the diagram, the dotted arc is 120 degrees. The five delay lines are D columns 2 and 11, D columns 4 and 9, D columns 6 and 7, D columns 5 and 8, and D columns 3 and 10. The lengths of the delay lines D indicate a relative measure of the delay.
The laboratory manufactured twelve transmitters each with a capacity of approximately 1 kW with the associated tuning coils. The experimental transducer 216TP5R was tested at the new measuring station VIANDA of the Navy in Hoek van Holland after initial measurements took place at the Roeleveense Plas in Nootdorp.
The transducer development should have led to an upgrade to the sonar system aboard of the Dutch Aircraft Carrier HNLMS Karel Doorman (R81). However, before the sonar could be installed, the Navy decided to operationally phase out the aircraft carrier.