Passive Sonar research (1951 – 1957)
From 1951 on, work was also done on passive sonar equipment. There was intensive cooperation with France in passive sonar trials and experiments. Firstly, the development of the Passive Distance Indication (PAI). Hydrophones were mounted at four fixed positions alongside the longitudinal axis of a submarine. Given the slow speed of sound in water, the propellor noise of a target does not arrive simultaneously at the four hydrophones. The delay lines W1 and twice W2 are tuned until the signal processing unit seemingly receives the sound at the same moment. The amount of delay set by delay line W2 indicates the direction of the target. With a flat sound wave front, the time delays of the sound at the two bases zero delay-W2 and W2-W1 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 self-produced noise of various Navy vessels. After many measurements at sea, this work was taken over by the Royal Netherlands Navy as part of the regular ship maintenance activities. The laboratory’s underwater acoustics group continued to be involved in various aspects of these measurements. TNO also advised on the design and installation of a measuring track at sea for measuring the ship’s noise. For instance, TNO 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 the advanced models OLA-2 and OLA-3.
Each of these models with an equal performance meant a significant reduction in volume and weight when compared to the predecessor version. However, no industrial production followed although the laboratory-developed systems were used in the Royal Netherlands Navy submarines for some time.
An attempt was made to determine the revolution speed of the propeller of a passing ship by displaying the sound of the propeller on an electron tube. Two hydrophones are mounted on the end of a rotatable rod mounted in the centre of the submarine. The rod was turned in such a way that both hydrophones have an equal distance to the noise source. The direction of the source is then perpendicular to the bar.