Answer: Frequency will be greater than 450Hz.
Step-by-step explanation:
Frequency is defined as the number of oscillations of wave happening in 1 second.
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More the time period, less will be the frequency and vice-versa.
In the given question, an ambulance is emitting a frequency of 450 Hz as ambulance move towards a man.
When ambulance is coming near to the man, time required for the ambulance to reach man is decreasing and as we know, frequency is inversely related to time period. Therefore, frequency greater than 450 Hz will be heard by the man.