Final answer:
When a listener moves towards a moving sound source due to the Doppler effect, they perceive a higher pitch because the sound waves are compressed, resulting in a higher frequency. Conversely, moving away from the source causes the listener to perceive a lower pitch as the sound waves stretch out and the frequency decreases.
Step-by-step explanation:
Understanding the Doppler Effect with Moving Sound Sources
When a listener moves towards a moving sound source, there is a change in the observed frequency of sound. This phenomenon is known as the Doppler effect. As a sound source approaches a listener, the sound waves compress, leading to a shorter wavelength and higher frequency. This means the listener will perceive a higher pitch. Conversely, as the sound source moves away, the sound waves stretch, resulting in a longer wavelength and lower frequency, causing the listener to perceive a lower pitch.
The change in pitch as the listener and sound source move closer together or farther apart is noticeable in everyday experiences, such as hearing the siren of a passing ambulance or a train whistle. For example, as a police siren moves towards you, the pitch of the siren you hear increases.
It's important to note that this effect is relative. If both the source and the listener are moving, the observed frequency change will depend on the relative speed between them. The greater their relative speed towards each other, the greater the increase in perceived frequency. Likewise, the greater their relative speed apart, the more substantial the decrease in perceived frequency.