Results from the commission HKV 23 321:61522, MTRL ref 252:3301

In our commission we were asked to find out if the sound source in the 3:47 recording from October 12, 1982, originates from a submarine or a surface vessel. In this work we recorded and analysed sound from the taxi boat Amalia which, as we now have been informed, was present in the area on October 12, 1982. The 3:47 recording contains strong, asymmetric sidebands in the demon signal from the propeller. We know that they can arise from an underwater sound source, through scatter of sound against a moving sea surface. We do not know if and how they could be formed from a surface source like a vessel. However, we can not exclude that possibility. Thus, even if we have not proven that Amalia causes asymmetric sidebands we still judge that the origin of the cavitation noise in the 3:47 recording is Amalias propeller. This conclusion is supported by the following facts:  Amalia sounds similar to the source in the 3:47 recording  Amalia has similar demon as the source in the 3:47 recording  Amalia has the same RPM as the source in the 3:47 recording  Amalia was present in the area south of Mälsten, October 12, 1982  Amalia has a fault in one of the propeller blades  The frequency of Amalia's EFR (Engine Firing Rate) is present in the 3:47 recording In the commission we should also analyse a ping signal from October 11, 1982 and make a judgement of its origin. The conclusion is that the ping was transmitted either from our own Sonar 204 or from a foreign sonar of some sort. Our technical investigation does not exclude any of the two alternatives. The faults that we have investigated, which could explain the observed ping signal, do not seem too plausible but can not be excluded. We should also judge on the origin of some increased noise levels in the recording. Our conclusion is that they are due to rain.