FOI has investigated whether the sounds recorded in Hårsfjärden bay on 12 October 1982 originated from a submarine or a surface vessel. The conclusion is that it was probably the taxi boat Amalia – which was in the area on that day – which was the source of the sounds including the propeller cavitation noise on the so-called 3:47 recording. The analysis supports its conclusion by a number of facts, for example that the sound recording includes a frequency identical with Amalia’s engine ignition frequency, that Amalia had a damaged propeller blade, the similarity of the engine rpm and the similarity between her propeller noise and that found in the 3:47 recording. At the same time there are particular features of the 3:47 recording, so-called asymmetric sidebands, which FOI has not yet been able to explain.

The assignment also included analysis of a ping signal from a recording made on the previous day and some increases in background noise on that same recording. The ping signal could have come either from one of the Swedish Navy’s own helicopter sonars or from some other sonar – the investigation does not exclude either of these alternatives. The elevated background noise levels were probably caused by rain.

“It looks as if it would be difficult to extract any more information from these two recordings,” says Erland Sangfelt, the FOI scientist responsible for the analysis. Factors such as the quality of the recordings and the lack of documentation place limits on what is possible.

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