A source of sound moves away with velocitv of sound from a stationary observer. Then the frequency of the sound heard by the observer :
When a source of sound moves away from a stationary observer, the frequency heard by the observer decreases due to the Doppler effect. The observed frequency is given by f' = f * (v - v_s) / (v + v_s), where f is the source frequency, v is the speed of sound, and v_s is the speed of the source. As the source moves away with the speed of sound, the observed frequency is halved.