Answer:
Doppler effect is the apparent change in frequency emitted by source heard by a listener when there is relative motion between the two. The apparent frequency increases when the source and listener approach each other and decreases when the source and the listener move away from each other.
Frequency and pitch are related. Higher the frequency, higher would be the pitch.
Thus, when a fire engine's siren approaches, a higher pitch is heard because of the Doppler effect.There is only increase in the apparent frequency. The emitted waves compact increasing the frequency perceived by a listener.
