**Solution:**

Given,

Velocity of source (us) = 20 m/s

Velocity of an observer (uo) = 0

Frequency of the horn 9f) = 512 Hz

Velocity of the sound in the air (v) = 330 m/s

Change in the pitch = ?

Now, using the Doppler's effect formula, we have

\(f_1'=\frac{v\pm u_o}{v\pm u_s}\times f\)

In the condition, when the observer approaches the source, we have,

\(f'=\frac v{v+u_s}\times f=\frac{330}{330+20}\times512\)

∴ f' = 482.74 Hz

Hence, the change in pitch = f - f' = (512 - 482.7) Hz = 29.26 Hz