What is Different situation in Apparent Frequency?

Different situation in Apparent Frequency #8211; (i) If source is moving towards listener but listener is at rest, then v_s is positive and v_o ∴ f = v/v #8211; v_s times; f, i.e. f gt; f (ii) If source is moving away from listener but listener is at rest, then v_s is negative and v_o = 0. ∴ f = [v/v #8211; (-v_s) times; f = (v/v + v_s) times; f i.e. f lt; f (iii) If source is at rest and listener is moving away from source, then v_s = 0 and v_o is position. ∴ f = (v #8211; v_o/v) times; f i.e. f lt; f