1. 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. 这需要的是一种利用低频无线电波的系统。
4. It is designed to detect the low-frequency radio pulses emitted by lightning strikes. 这一系统旨在感应雷击产生的低频射电脉冲。
5. Besides, the improving of observation condition of low-frequency events is discussed. 讨论了改进(超)低频事件的观测条件问题。
