Final answer:
The claim that the observed frequency becomes infinite at the speed of sound is false; it actually results in the accumulation of sound waves and possibly a shock wave, not an infinite frequency.
Step-by-step explanation:
The statement that the observed frequency becomes infinite when the source is moving at the speed of sound is false. When a source of sound is moving at the speed of sound, the sound waves in front of the source pile up because they cannot propagate away from the source fast enough. However, the frequency does not become infinite; instead, a large pressure difference is created, which can result in a shock wave. This is a principle demonstrated in the Doppler effect, where the observed frequency increases as the source approaches the observer but not to infinity. The observed frequency would only theoretically reach infinity if the source could travel at an infinite speed, which is not possible according to the laws of physics.