Expression for Apparent Frequency #8211; Consider a source of sound and an observer are moving with velocities v<sub>s</sub> and v<sub>o</sub>, respectively. The source is provided a sound of frequency f and the frequency of sound heard by the observer be f . f = v #8211; v<sub>o</sub>/λ = v #8211; v<sub>o</sub>/v #8211; v<sub>s/gt;subgt; f