When a wave source is approaching an observer, such as a moving sound source, the observed frequency of the waves does indeed change. This phenomenon is known as the Doppler effect, and it applies to both sound waves and other types of waves, such as light waves.
The Doppler effect causes a perceived shift in frequency when there is relative motion between the source of the waves and the observer. The key point to understand is that the frequency shift occurs due to the relative motion, not because the waves themselves have changed at the source.
For sound waves, if a sound source is moving toward an observer, the waves are compressed as the source moves closer. This compression leads to an increased frequency and a higher perceived pitch. Conversely, if the sound source is moving away from the observer, the waves are stretched out, resulting in a decreased frequency and a lower perceived pitch.
The reason the perceived pitch changes is because our ears detect the frequency of sound waves as pitch. When the waves are compressed or stretched due to the relative motion, the frequency changes, and thus the perceived pitch is affected.
It's important to note that the Doppler effect can lead to a change in frequency and pitch, but it does not alter the fundamental properties of the sound itself. The actual pitch produced by the sound source remains the same, but the observed pitch by the listener is affected by the relative motion.
In summary, the observed change in frequency and pitch of a wave source approaching an observer is a result of the Doppler effect. It occurs due to the relative motion between the source and the observer, causing the compression or stretching of waves and altering the perceived pitch, even though the waves themselves have not changed at the source.