Answer:
42,500 Hz
Explanation:
The Doppler equation describes how sound frequency depends on relative velocities:
fr = fs (c + vr) / (c + vs),
where fr is the frequency heard by the receiver,
fs is the frequency emitted at the source,
c is the speed of sound,
vr is the velocity of the receiver,
and vs is the velocity of the source.
Note: vr is positive if the receiver is moving towards the source, negative if away.
Conversely, vs is positive if the receiver is moving away from the source, and negative if towards.
Given:
fs = 40,000 Hz
c = 340 m/s
vr = 0 m/s
vs = -20 m/s
Find: fr
fr = fs (c + vr) / (c + vs)
fr = 40,000 (340 + 0) / (340 − 20)
fr = 42,500 Hz
