Final answer:
The statement that the observed frequency becomes infinite when the source is moving at the speed of sound is false. Instead, a shock wave occurs, not an infinite frequency.
Step-by-step explanation:
Observed Frequency and the Speed of Sound
When discussing the observed frequency of a sound as the source approaches an observer, it's important to understand the phenomenon known as the Doppler Effect. According to the Doppler Effect, the observed frequency does indeed increase as the sound source moves towards an observer. However, the statement that the observed frequency becomes infinite when the source is moving at the speed of sound is false. When a source moves at the speed of sound, a large number of waves are indeed piling up at the observer's location, but this creates a pressure difference known as a shock wave, not an infinite frequency. An infinite frequency would require wave crests to arrive at the same instant, which is physically impossible.