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I have a copy of Principles of underwater sound by Urick, 3rd edition. In the chapter on The noise background of the sea: ambient noise level, on pages 215/216, it says

"The analysis of data in the frequency band 20 to 100 Hz revealed periodicity of 12 and 24 hours, with a maximum of noise at noon and midnight, local time, at all locations, independent of longitude. Although the change in level was small (normally 1.5 to 5 dB), it was undoubtedly real, but the origins of these changes in noise, which are apparently free from biological activity and synchronized to the mean solar day at diverse locations, remains a mystery."

The book was published in 1967, and my copy printed in 1983. I'd be fascinated to find out if there has been any new research into this. I did a little googling but couldn't find any references to this issue.

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  • $\begingroup$ Would it be possible to try and give the question a bit more focus? For example, you seem to be specifically interested in man-made background noise in passive sonar applications(?). Personally, I do not see any mystery with noon and midnight traffic. $\endgroup$
    – A_A
    Jan 4, 2021 at 17:35
  • $\begingroup$ That's right, I'm working in the field of passive sonar. I assume the background is man-made (see my answer), but it still is confusing to me. Why do you not see any mystery with noon and midnight traffic? If it was shipping traffic, I'd have expected constant traffic either at all hours, or maximal during a period of the day (eg 0900-1700). $\endgroup$
    – Dan Pollard
    Jan 5, 2021 at 8:40

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Some further googling seems to have shed some light: it appears that shipping traffic is highest at these hours, as demonstrated by Figure 3 in

http://cetus.ucsd.edu/Publications/Publications/McKennaIslandsSymp2009.pdf.

Why that is, I'm not sure. Also, this publication isn't about global shipping patterns, but only that in a certain region on the west coast of the USA - but it certainly goes some way to explaining the noise detected by sonar.

I know this was a very niche question, but hopefully this is helpful to someone who may stumble on my question in the future.

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  • $\begingroup$ This is a very good reference. $\endgroup$
    – Jay Patel
    Jan 15, 2021 at 14:30

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