"Our best guess is individual ships are radiating more acoustic energy than they did back in the 60s," Hildebrand says. "The ships are bigger, faster, they are carrying more cargo, and all of that requires more propulsion power. Some fraction of that propulsion power gets radiated out as acoustic energy."
McDonald, who is a consultant to a number of companies and government agencies, says it's possible to reduce the noise level by such things as using rubber engine mounts, and changing the propellers, but it would be hard to force the industry to do that without strong evidence that the noise is having an adverse impact.
That's going to be hard to do, but history would suggest that the problem should not be ignored.
Gray whales migrate from the Arctic to Baja, Calif., each year, and historically, they have been particularly fond of one lagoon where they breed and calve.
"In the late 60s or early 70s, there was a salt works installed in the lagoon (to extract salt)," Hildebrand says. "There was a fairly well documented population of animals that would come into the lagoon and use it for calving. When the salt works started up, they abandoned the lagoon.
"All the time the salt works was active, the animals avoided the lagoon. The salt works failed for economical reasons. After it failed, it took about five years for the animals to start to come back and reoccupy that lagoon. So there's a case where you can document exclusion from a preferred habitat."
Did the whales stop procreating? Hardly. Their numbers have grown significantly over the years, so once again, it's hard to measure the impact, even of an industrial activity.
But there's no question that the ocean has changed through pollution, over-fishing, and now, we know, noise.
"It wasn't a pristine environment in the 1960s," when the Navy monitored the area, Hildebrand says. "So there's probably another 10 decibels or so to get back to the primordial state. So it's 20 decibels above the conditions that these animals evolved in, and that's a big number."