automatic whistle

例句：

1. 1. The automatic whistle is used to signal the start of a race.
   自动哨子用来发出比赛开始的信号。
2. 2.The automatic whistle was invented to help people communicate in emergency situations.
   自动哨子被发明出来，以帮助人们在紧急情况下进行沟通。
3. 3.The automatic whistle was used by the police to alert the public of an impending danger.
   警察使用自动哨子来警告公众即将到来的危险。