1. The whistle sounds because of the vibration of the air in the whistle. The whistle mouth mainly makes the airflow impact the opening gap of the whistle, causing the air in the whistle to vibrate.

2. The sound frequency of whistle is influenced by many factors, among which the most important factor is the shape of whistle, and the largest size of whistle is the main factor. When blowing a whistle, the speed of air flowing through the whistle mouth will also affect the sound frequency of the whistle. When the air speed is high, the sound frequency is higher than when the air speed is low.

For a whistle with a fixed shape, blowing the whistle with a small amount of air can reduce the volume and make it less harsh. Changing the size of the air outlet can also change the frequency of the whistle.