Final answer:
You should approach a helicopter from the front after being waved in by the pilot or medic. Standing on the sidewalk, the observed frequency of an ambulance siren matches the actual frequency when it's coming towards you.
Step-by-step explanation:
When approaching a helicopter, first wait for the pilot or medic to wave you in. Then approach from the front of the craft. This is the safest direction as the pilot has a clear line of sight and can signal you effectively. The rear and top are the most dangerous areas due to the tail rotor and downdraft from the main rotor, respectively.
If you are standing on the sidewalk facing the street and an ambulance drives by with its siren blaring, the frequency that you observe most closely matches the actual frequency of the siren when it is coming toward you. This is due to the Doppler effect, which causes the frequency to appear higher as the source approaches.