A fire truck is approaching a stationary observer at 40 m/s. If the frequency of the fire truck siren is 1500 Hz, what frequency will the observer hear? The speed of sound is 340 m/s.

Answer 1

Let

f0 = 1500 Hz be the frequency generated,

c = 340 m/s be the speed of sound,

v_s = 40 m/s be the speed of the truck (the source of sound),

v_r = 0 m/s be the speed of the observer (the receiver of the sound).

The source of the sound approaching the receiver causes

the receiver to hear higher frequency f according to the formula.

f = f0(c+v_r)/(c-v_s)

Substituting actual numbers

f = 1500(340+0)/(340-40) = 1700Hz