Question

What frequency is received by the ambulance after reflecting from a wall near the person watching the oncoming ambulance (as in the previous problem). The ambulance is moving at 109 km/h and emitting a steady 821-Hz sound from its siren

Answer 1

Answer:

900.48925 Hz

979.9785 Hz

Explanation:

$v_a$ = Relative velocity of ambulance =$109\ km/h=\dfrac{109}{3.6}$

$v_w$ = Velocity of wall = 0

v = Velocity of sound in air = 343 m/s

From doppler effect we have

$f=f'\dfrac{v+v_w}{v-v_a}\\\Rightarrow f=821\dfrac{343+0}{343-\dfrac{109}{3.6}}\\\Rightarrow f=900.48925\ Hz$

The frequency of sound is 900.48925 Hz

When the wall acts like a source

$f=f'\dfrac{v+v_a}{v-v_w}\\\Rightarrow f=900.48925\dfrac{343+\dfrac{109}{3.6}}{343-0}\\\Rightarrow f=979.9785\ Hz$

The frequency of sound is 979.9785 Hz