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

音响信号(声音，口哨，枪声等)。

youdao

Arnold directs the labs wordlessly, with hand signals and the occasional whistle.

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The whistle sounds, as it has since the 1800s, and it signals that men and women from large cities and small will be stopping, however briefly, in their town, at their station.

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