Question

A siren emitting a sound of frequency 100 HZ moves away from you towards a cliff at a speed of 10m/s. The frequency of sound, you hear coming directly from the siren, is [take speed of sound in air =330m/s]

• A. 32/33x1000 HZ
• B. 33/34x 1000HZ
• C. 33/32x1000HZ
• D. 34/33x1000HZ

Answer 1

Answer: (B)

33/34x 1000HZ

We take the wave speed v as 330 m/s. We take our source speed $V_S$ as $10m/s.$ We take our speed$V_D$to be zero. Finally we take our source frequency f to be 1000 Hz. Since the siren is moving away from us, we will perceive a lower pitch from the Doppler effect thus well have an addition on the bottom rather than a subtraction:

$F_1=f\times \left(\frac{V}{V+V_s}\right)$

$F=100Hz\times \left(\frac{300}{300+10}\right)$

$F=\frac{33}{34}\times 1000Hz$