This focusing is the main reason that even relatively weak sounds in the ocean, especially low-frequency ones, can often travel thousands of miles.
正是由于这种集声，即使是海洋中相对微弱的声音，尤其是低频声音，也能传播数千英里。
2
These higher-frequency sounds do not travel as far, and so may better conceal the individuals producing them, who are especially vulnerable to predators in their ground nests.
Most reported that their companies had instituted travel restrictions including limits on the frequency of travel (57%), on the cost of airline tickets (57%) and accommodation (51%).