An ambulance traveling at a speed 20 ~m ~s ⁻¹ emits a sound of frequency 540 Hz from its siren. Sunanda is driving a car which approaches the ambulance from the opposite direction at a speed of 20 ~ms ⁻¹ . What will be the change in detected frequency by Sunanda, as she crosses the ambulance? (Given, the speed of sound in air is 340 ~m ~s ⁻¹ .)