The same effect is observed when we are passed by a fast-moving hooting car in the street, or when the observer is moving fast in relation to a stationary sound source (e.g.

• 因为音调的高低是由频率（每秒振动的次数）决定的，所以开过来的机车汽笛的音调就比机车和观测者相对静止时间听起来就越变越低了。