How can they travel?

My book is called Thousand Mile Song because of the fact that certain, very low sounds, under 50Hz, which are made by the largest whale species, blue and fin whales, are able to travel thousands of miles because of the peculiar physics of underwater sound.

Scientists have tracked whales from such distances but we do not really know if the whales themselves do listen to each other from so far away. The constant presence of shipping noise in the oceans today makes it much harder for such tones to carry as far as they once did when the oceans were silent. Does this harm the lives of whales? It may do more chronic damage than the more publicized sonar explosions set off by the US Navy.