A portable siren emitting a steady note of frequency 1000 Hz moves directly towards a high, rigid wall with a speed of 20 m s⁻¹ through still air
(vsound=340m s−1). An observer standing 50 m behind the siren (on the line of motion) remains at rest with respect to the air and hears both the sound coming directly from the siren and the sound reflected from the wall.
(i) Just before the siren reaches the wall, find the frequency of the direct sound received by the observer.
(ii) Find the frequency of the sound that reaches the observer after reflection from the wall.
(iii) Hence determine the number of beats heard per second by the observer at that instant.