You are walking along a small country road one foggy morning and come to an intersection. while you are crossing, you hear an ambulance siren. as a paramedic, you know that the siren sounds at a frequency of 5.47 khz. but, as an amateur musician with perfect pitch, you can tell that the frequency you are hearing is actually 5.99 khz. is the ambulance moving toward or away from you?
Since the sound you hear from the siren has a HIGHER frequency than the sound actually coming out of the siren, you know ... from your knowledge of the Doppler effect ... that the distance between you and the ambulance is decreasing.
Just from the sound alone, you can't tell whether it's the ambulance moving toward you, or you moving toward the ambulance. Either motion would produce the same effect, because the distance between is decreasing. You'd need to find some other way to tell which one is producing the motion. (at least MOST of it)
