音响信号(声音，口哨，枪声等)。

Use audio signals (that is, voice, whistle, and weapons fire).

youdao

当用号角或口哨发出信号时，大战就可以开始了，在任何位置都行，能持续几分钟到几个小时。

The fighting starts when a signal such as an air horn or whistle blows, and can last anywhere from a few minutes to a few hours.

youdao

如果无口哨的话，只使用可视信号。

If a whistle is not available, use the visual signal alone.

youdao