In the context of the Doppler effect, what occurrence is associated with moving sound sources?

The Doppler effect describes how the frequency of sound waves changes in relation to an observer based on the movement of the sound source. When a sound source approaches an observer, the sound waves in front of the source are compressed, resulting in an increase in frequency. This phenomenon is perceived as a higher pitch by the observer. As the sound source moves away, the sound waves are stretched out, leading to a decrease in frequency, which is perceived as a lower pitch.

In this context, the correct answer states that the frequency of the sound waves increases as the source approaches, which accurately reflects the behavior of sound in relation to the Doppler effect. The second choice about amplitude remaining unchanged is not relevant to the Doppler effect; while it may remain relatively constant, the focus of the Doppler effect is primarily on frequency changes. The assertion that the speed of sound does not vary with the movement of the source overlooks the fact that the medium's speed of sound remains constant; however, the frequency observed changes based on the source's movement. Therefore, the key aspect being tested is the relationship between the movement of the sound source and the resulting change in frequency, which is captured correctly in the chosen answer.