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
The researchers pointed out that low-frequency processes, such as fluctuations in stream flow and groundwater levels, would not usually be apparent to human populations.
研究人员指出，低频过程诸如河流流量和地下水水位的波动，通常不会对人类产生明显影响。
3
What is needed is a system that USES low-frequency radio waves.