1.1. An ambulance siren emits two pure sounds. The lower of the two sounds has a frequency of 650 Hz. It is travelling towards a stationary observer at 13.4 m s⁻¹. The speed of sound in air is 340 m s⁻¹.

Calculate the change in frequency, Δf, between the source frequency and that heard by the observer.

1.2. The ratio between the two frequencies emitted by the ambulance is 0.722. 

(i) Determine the second frequency as heard by the observer.

[4]

(ii) Explain the effect a change in speed of the ambulance will have on the ratio between the two observed frequencies.

[2]

1.3. As the ambulance approaches a red light at which a number of cars are stopped, it changes its siren to a single monotone sound which the car drivers observe as 700 Hz. The ambulance slows further to 5.3 m s⁻¹.

Observed wavelength can be calculated using the following equation:

${ \lambda }_{0}={ \lambda }_{S}(1\pm\frac{{v}_{s}}{v})$

Calculate the wavelength of sound emitted by the ambulance.