The Doppler Effect With Sound
You, the person hearing the sounds, are called the observer or listener and the thing emitting the sound is called the source. As mentioned in the introduction, there are two situations which lead to the Doppler effect:
When the source moves relative to a stationary observer.
When the observer moves relative to a stationary source.
In points 1 and 2 above there is relative motion between the source and the observer. Both the source and the observer can be moving at the same time but we won’t deal with that case in this section.
Definition: Doppler effect
The Doppler effect is the change in the observed frequency of a wave when the source or the detector moves relative to the transmitting medium.
The Doppler effect occurs when a source of waves and/or observer move relative to each other, resulting in the observer measuring a different frequency of the waves than the frequency that the source is emitting. The medium that the waves are travelling through, the transmitting medium, is also stationary in the cases we will study.
The question that probably comes to mind is: “How does the Doppler effect come about?”. We can understand what is happening by thinking through the situation in detail.