When a sound source is moving, several notable effects occur with respect to the waves in front of it. These effects are commonly referred to as the Doppler effect.
The Doppler effect is the change in frequency or wavelength of a wave due to the relative motion between the source of the wave and the observer. It applies to all types of waves, including sound waves. Let's consider two scenarios: when the source is moving towards the observer and when the source is moving away from the observer.
Moving source towards the observer: When a sound source moves towards an observer, the waves in front of the source get compressed. As a result, the wavelength of the waves appears shorter, leading to an increase in the perceived frequency. This increase in frequency results in a higher pitch of the sound. For example, if a siren on a moving vehicle approaches you, you initially hear a lower pitch, which gradually increases as the source gets closer.
Moving source away from the observer: Conversely, when a sound source moves away from an observer, the waves in front of the source get stretched out. This lengthening of the wavelength leads to a decrease in the perceived frequency. Consequently, the pitch of the sound appears lower. For instance, when a vehicle with a siren moves away from you, you initially hear a higher pitch, which gradually decreases as the source moves farther.
It's important to note that the speed of sound remains constant in a given medium, regardless of the motion of the source or the observer. However, the apparent frequency and pitch experienced by the observer can be influenced by the relative motion between the source and the observer.
In summary, when a sound source moves towards an observer, the waves in front of it get compressed, resulting in a higher perceived frequency and pitch. Conversely, when a sound source moves away from an observer, the waves get stretched out, leading to a lower perceived frequency and pitch.