Final answer:
A high-pitched unhearable frequency could indeed become audible due to the Doppler effect if the source moves toward an observer at a speed that causes a sufficient shift in frequency into the audible range. Conversely, as the source moves away, the shift would be toward lower frequencies, potentially moving an originally audible frequency out of human hearing range.
Step-by-step explanation:
The question of whether a high-pitched, unhearable frequency could be heard due to the Doppler effect involves an understanding of sound waves and the perceived change in frequency when there is relative motion between a source of sound and an observer.
The Doppler effect refers to the alteration in the observed frequency of a sound because of this motion. When the source or the observer moves towards the other, the observed frequency increases (shift to higher pitch), and when they move apart, the observed frequency decreases (shift to lower pitch).
Now, if the frequency of the sound produced by a source, such as a motorcycle or race car, is initially above the human hearing range (ultrasonic), as it moves towards an observer at a high enough speed, the Doppler shift could potentially bring the frequency into the audible range for humans. The opposite is also true; a sound initially within the human hearing range could be shifted to an unhearable frequency as the source moves away quickly enough.
As an example, the characteristic sound of a motorcycle or ambulance siren shifting from higher to lower pitch as it passes by a stationary observer demonstrates the Doppler effect. The abruptness and magnitude of this pitch shift are more pronounced the closer and faster the source moves relative to the observer.