Underwater acoustics (1946 – 1957)
During the Second World War, underwater acoustics found application in the search for submarines and surface vessels on a fairly large scale. The devices used for this purpose were first called ASDIC after the Anti-Submarine Detection Investigation Committee. Later name was sonar (Sound Navigation and Ranging). The systems could be of an “active” or “passive” type. Active sonar is based on the reception of the echo of an object after the transmission of a signal (“ping”). The passive method uses the sound produced by an object such as ship propulsion.
In 1946, a reluctant start was made in the Netherlands with new fundamental research in the field of underwater acoustics. The first research concerned noise absorption in liquids. At the same time, work started on the development of a streamlined sonar dome that generated less cavitation, or in other words sound. The latter work was in collaboration with the Dutch Shipbuilding Testing Facility, the later MARIN. Moreover, the Royal Netherlands Navy commissioned in 1947 the investigation of an extensive sonar installation that was present in a retarded German warship and the repair of an American sonar system of the Hr.Ms. Queen Wilhelmina patrol vessel (ex USS PC 468).
At the end of 1947, the Royal Netherlands Navy expressed the opinion that, given the future importance of sonar, the formation of ASDIC expertise and experience within the Netherlands was a necessity. In 1948, the laboratory, therefore, was tasked to develop a sonar for the (then) new-build ship program of the Navy. Neither the Brits nor the Americans were willing to provide any information about sonar technology. Because France was internationally in a similar position as the Netherlands, a good research cooperation started between both nations. This concerned the joint development of transducers and hydrophones (= underwater microphones) as well as the exchange of experience with equipment under development. France also provided sonar test facilities.
The most important, discussed below, sonar developments in the period 1946 – 1957 were:
- Measurement posts and the Hr. Ms. Paets van Troostwijk (1947 – 1977)
- Active sonar developments: ADI, CWE-1 and CWE-1 (1950 – )
- Passive sonar developments: PAI and OLA (1951 – 1957)
- Research on transducers and hydrophones
- Measuring station Roeleveense Plas (Nootdorp) (1953 – )
Measurement posts for sonar and sailing sonar laboratory Hr. Ms. Paets van Troostwijk
In order to be able to carry out experiments with underwater sound, a raft was composed of pontoons mid-1948. During the occupation years, these pontoons served as armoured doors to close off a German bunker for Schnellboote (SBB) in the Waalhaven in Rotterdam. This raft was stationed in the same Waalhaven and placed under the supervision of the anti-submarine warfare school Hr. Ms. Zeearend. This enabled the laboratory to carry out measurements with the own-developed transducers. In addition to this test possibility, a second opportunity arose soon.
The Royal Netherlands Navy was in the possession of an approximately fifty-year-old Norwegian whaler used by the German Kriegsmarine during WW II. Initially, it was thought that this ship, given its high age, could be better served for demolition. However, it turned out that the whaler was upgraded by the German Kriegsmarine to make it suitable as a test or training ship under the name “Süd III”. It was equipped with a lot of ASDIC equipment of the German Kriegsmarine. It was therefore obvious to use the ship as a sailing test laboratory for ASDIC research and experiments.
In 1948, the damage to the existing onboard ASDIC-equipment was repaired by the laboratory. A number of improvements were implemented such as indication and reverberation controlled gain.
From 1950 on, the ship served many years, initially as “HE-2”, later under the name Hr.Ms. Paets van Troostwijck. In 1950, the first ASDIC tests were carried out in the Nieuwe Waterweg, followed by a trip to Toulon on the Mediterranean. In 1961, it became clear that the old “Paets van Troostwijck” would not be able to fulfil its services as a sailing testing platform anymore for a long time.
When finally it became apparent that the ship’s demolition was unavoidable, the Royal Netherlands Navy provided a permanent test rig in Hoek van Holland as a replacement to TNO. This consists of a scaffolding with a hoisting mechanism and a shelter for the equipment. This jetty is connected via a fixed walkway to an onshore laboratory. Naturally, the necessary provisions were also made for moving and hoisting heavy loads. Since 1966 this installation has been used for experiments with sonar equipment by TNO.
Active sonar research (1950 – )
Around 1950, as a result of the aforementioned marine commission, the laboratory model of an Anti-Dive Boat Installation (ADI) was completed. Piezo-electric crystals, made from Seignette salt (potassium-sodium tartrate), were stuck to a thick steel plate, serving as counter mass. These crystals resonated at a frequency of 25 kHz. The frequency of this active sonar was adjustable between 17 and 35 kHz with a transmission power of 250 W. This “searchlight sonar” was for that time ultramodern and contained everything one could wish for in such a device. The arrangement in diagonally placed squares makes it possible to create a directional determination in the horizontal plane between the left and the right quadrant, while the upper and the lower quadrant provide the directional determination in the vertical plane. This arrangement allowed the determination of the direction of the incoming sound.
In addition to the electronic part, which was of a completely new design, the transducer was also an own development. The latter contained as active elements Seignette salt crystals manufactured by a specialised Dutch industry. For underwater use, the crystals were enclosed in a watertight housing filled with castor oil and with a rubber ‘sound window’ at the front. The electronic equipment included a part that could eliminate the movement of the own ship as part of the speed determination of the target, a so-called own Doppler nullifier.
In parallel, research was carried out into other piezoelectric materials such as barium lead titanate. This research took place in collaboration with the Central Laboratory of the Dutch PTT .
In the night of November 7 – 8, 1952, the Panamanian steam-powered freighter Faustus ran ashore north of the Noorderpier near Hook of Holland. Later, the wreck broke through the Noorderpier and sank across the Nieuwe Waterweg shipping channel. The wreck blocked the access to the port of Rotterdam. The experimental ADI on the Paets van Troostwijk and an experimental active sonar for the detection of sea mines was used to precisely determine the wreck location. With buoys, half of the navigation channel could be made operational again within 24 hours.
On August 23, 1954, the KLM DC-6 aircraft ‘De Bontekoe’ crashed in The North Sea between Egmond and Bergen. The ADI on the Paets van Troostwijk was used to locate fragments of the aircraft. A personal report of this search mission can be found on Our Fleet (in Dutch).
The laboratory model of the was successfully tested on board Hr.Ms. Paets van Troostwijck on the North Sea, off the French coast near Brest, and in the Mediterranean Sea. The decision was made that the Dutch industry would take over the serial production of the sonar device under the technical responsibility of the laboratory. The production prototype (under the name DATO: Detection Device Against Submarines) appeared on board Hr.Ms. Marnix and worked to full satisfaction. This was followed by serial production of the PAE-1 by the Van der Heem company. These installations have served for a long time on ships of the Royal Netherlands Navy. This sonar also attracted international interest. This is evident from sales to the German, Swedish and several other foreign navies. This success was not only related to the design but also to some special features. For example, there was a particular indication on an electron beam tube of the Doppler effect, that is the frequency shift that occurs as a result of the movement of a possible target. In addition, the device directly provided electronic information for the onboard fire control and weapon systems.
The active sonar was continuously improved and expanded in the laboratory. This way the WARO was created as a warning version of DATO. The WARO was produced by Van der Heem as the CWE-1. The successor CWE-10 was equipped with a newly developed, magnetostrictive transducer and a more powerful transmitter. This larger transmitter was driven by the existing CWE-1 transmitter and was therefore placed as a module between the original transmitter and the transducer. With that transmitter, the energy output increased from 250 W to 10 kW.
In addition to the active sonar, experiments were also carried out with a Corrective Attack Plot device (“ACP”) that gave, under favourable conditions, an indication of the location of a target if the length thereof was large in comparison with the other dimensions. Improvements in all aspects of sonar and data processing technology were tested and sometimes successful. This resulted in an improvement of the efficiency of the energy transfer between transmitter and transducer, and of automatic gain control. Mechanical arrangements were replaced by electronic circuits and various embodiments for the visual presentation of the signal were tested.
Passive sonar research in the period 1951 – 1957
From 1951 on, work was also done on passive sonar equipment. There was intensive cooperation with France in sonar trials and adaptations. Firstly, the Passive Distance Indication (PAI). Alongside the longitudinal axis of a submarine, hydrophones were mounted at four fixed positions. Given the slow speed of sound in water, the screw noise of a target does not arrive simultaneously at the four hydrophones. Adjustable delay lines W1 and twice W2 are set so that the noise arrives simultaneously at the signal processing post. The amount of delay set by W2 indicates the direction of the target. With a flat sound wavefront, the time delays of the sound at the two bases are the same. A deviation to this arises from the curved wavefront. From this deviation, the position of the target can be calculated.
Subsequent experiments with the PAI became useless as a result of the Royal Netherlands Navy’s decision to purchase its sonar equipment abroad.
In 1955, a start was made with measuring the own produced noise of various vessels with their sonar. After a number of measurements at sea, this work was taken over by the Royal Netherlands Navy as part of the ship management routine in later years. The laboratory’s underwater acoustics group was involved in the various aspects of these measurements. TNO also advised on the design and installation of a measuring track for measuring ship noise and supplied the necessary hydrophones to the Navy.
In the late 1950s, the electronic part of an underwater listening device (OLA) was made. Improved insights and new technical developments resulted in models OLA-2 and OLA-3.
Each of these models meant a significant reduction in volume and weight when compared to its predecessor, at equal performance. This, however, did not lead to industrial production, although the laboratory developed systems were used by the submarines of the Royal Netherlands Navy for some time.
An attempt was made to determine the revolution of the propeller of a passing ship by displaying the sound of the propeller on an electron beam tube. In the centre of the submarine, two hydrophones are mounted on the end of a rod, which is rotatable about the centre in the angular direction. This angle is set so that the hydrophone has equal distance to the noise source. The direction of the source is then perpendicular to the bar.
Transducer research (1958 – 1964)
Much work was made by the laboratory in developing transducers and hydrophones. Just after the Second World War, not much information could be found in public sources. Reliable theories were missing so that most constructions came about empirically. In addition, the use of modern ceramic materials had to be studied because these materials provided advantages over piezoelectric crystals or 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 a 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 that is searching for submarines transmits sonar signals, then those signals can be captured by a submarine before the ship detects 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 the high-frequency signals come from target-seeking torpedoes. The device 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) together receive the frequency band from 20 to 40 kHz. With the triangle of six elements (numbers 1 to 6) the frequency band from 10 to 20 kHz is received, and all ten hydrophone elements (numbers 1 to 10) together process the lowest is received band from 5 to 10 kHz. The hydrophone consists entirely of titanium and is resistant to any depth underwater where a submarine can dive.
In 1958, in parallel to the work mentioned above, 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 intention of this large research and development venture was to gain experience with the fundamental problems associated with such a design. The transducer, which was not intended as a pre-production model, was constructed from 216 hexagonal, mutually supporting elements, divided over 36 columns of six elements each, placed in a cylindrical shape.
In 1964, the panoramic transducer 216TP5R was completed. This panoramic transducer could be broadcast and received around a corner angle (= chart angle) of 360 degrees. The resonance frequency was 5 kHz and the total weight was 2800 kg. The measurements performed with this transducer have contributed significantly to the knowledge needed to assist the 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 column 2 and 11, D column 4 and 9, D column 6 and 7, D column 5 and 8, D column 3 and 10. The length of the delay lines D indicate a relative measure of the delay.
The laboratory produced 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 of the Navy in Hoek van Holland after initial measurements took place at the Roeleveense Plas in Nootdorp.
Sonar measurement station Roeleveense Plas (Nootdorp) (1953 – 1961)
During the development of transducers, the measurement station in the Waalhaven, Rotterdam no longer met the requirements:
- the water was not deep enough on the spot,
- the background noise was too high due to the proximity of ship traffic and ports, and
- the distance between the laboratory in The Hague and the port in Rotterdam was impractically large.
In 1953, a new raft was constructed using the same, old pontoons. This raft was anchored in the Roeleveense Plas, a triangular artificial freshwater lake with sides of 300 and 400 meters near Nootdorp next to the highway A12. The quiet environment and the large depth (18.5 metres) benefited the quality of measurements, especially for frequencies below 500 Hz. This raft was also made suitable for measurements on sonar domes (a “dome” is a streamlined envelope of the transducer that serves to reduce the noise of the water flow). Although this measuring station initially met all the requirements, after a number of years it turned out that a replacement was needed. The development of the sonar technique led from the searchlight sonar with a single beam to the panoramic sonar with a combination of a number of fixed beams in a single transducer as described above. The second element in development was the use of lower frequencies. Both factors combined resulted in transducers and domes becoming considerably larger and heavier. Transporting it to the raft could therefore no longer be done per rowing boat as before. Around 1960 it was decided to construct a larger raft (6 x 14 metres) with a fixed fifty meters long pontoon bridge connected to the bank. The new setup was designed to be able to test objects with maximum dimensions of 2 x 2 x 4 metres and a weight of 5 tonnes. To this end, a rail system was installed on the bridge and the bank. The bottom of the pontoon had a 2 x 4 metre hole. A 13-metre high lifting column above the hole could lift objects up to 1.80 m high from the trolley and could lower them up to 6 meters below the water surface. This raft was put into use in 1961.
Thanks to the van der Heem & Bloemsma documentation centre, website http://www.vanderheem.com/index.html for some photo references to the industrial versions of the developed sonar equipment.