Blue whale song is built from very low tonal units, most of them below 100 hertz, produced by males and repeated in long patterned sequences. At those frequencies sound travels efficiently through deep water, which is why the same call types can be logged for decades by fixed hydrophone arrays originally installed to listen for submarines.
Comparing archived tapes with modern recordings of the same call type shows a persistent decline. In the north-east Pacific, the tonal frequency of the well-studied B call has fallen by roughly thirty per cent since the 1960s. Smaller declines, on the order of a fraction of a hertz per year, have been reported for populations in the Southern Ocean, the Indian Ocean and the Atlantic.
The cause is genuinely open. Proposals include recovering population density allowing quieter and therefore lower calls, changes in ocean background noise, and sexual selection favouring deeper voices. The trend is often reported as a response to ship noise alone, which the data do not establish.