(b) A vehicle travels at constant speed around a wide circular track. It continuously sounds its horn, which emits a single note of frequency 1.2kHz. An observer is a large distance away from the track, as shown in the view from above in [Fig. 4.1]. [Fig. 4.2] shows the variation with time of the frequency f of the sound of the horn that is detected by the observer. The time taken for the vehicle to travel once around the track is T.