Final answer:
There are two Doppler shifts that occur during the relevant exam, which can be difficult to measure due to their small size compared to the original frequency. Beat frequency is used instead for its ease of measurement.
Step-by-step explanation:
During an exam concerning the Doppler effect, typically two Doppler shifts occur. When considering ultrasound, which operates at an original frequency of 2.50 MHz, it is challenging to measure the echo frequency accurately to detect small shifts in the frequency. Therefore, the beat frequency, the difference between the observed frequency and the source frequency, is used as it simplifies the measurement. The beat frequency is expressed as fB = | ƒobs − ƒs |, negating source frequency variances as they would affect both the observed and the source frequencies. An instance where one might rely on the Doppler shift in everyday life is when driving a car or walking near traffic, where it helps in perceiving the speed of approaching or receding vehicles based on the change in pitch of their sound.