A burglar alarm in a house emits sound at a fixed frequency of 3.1 kHz. On a windy day a person cycles past the house while the alarm is sounding. The wind is blowing at a velocity of 16.2 m/s towards the south east and the cyclist is cycling due south with a velocity of 6 m/s (so the house is due north of the cyclist's position). What frequency does the cyclist hear?