A source of sound S emitting waves of frequency 100 Hz and an observer O are located at some distance from each other. The source is moving with a speed of 19.4 m/s at an angle of 60° with the source-observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer is: (velocity of sound in air 330 m/s)

The apparent frequency is given by the Doppler effect formula for a moving source:

f' = f * (v / (v - vs * cosθ))

where:

f' is the apparent frequency
f is the source frequency (100 Hz)
v is the velocity of sound (330 m/s)
vs is the velocity of the source (19.4 m/s)
θ is the angle between the source's velocity and the line joining the source and observer (60°)

Substituting the values:

f' = 100 Hz * (330 m/s / (330 m/s - 19.4 m/s * cos(60°)))

f' = 100 Hz * (330 m/s / (330 m/s - 19.4 m/s * 0.5))

f' = 100 Hz * (330 m/s / (330 m/s - 9.7 m/s))

f' = 100 Hz * (330 m/s / 320.3 m/s)

f' ≈ 103 Hz