Occurs when a radio meteor is illuminated by an unmodulated radio wave, the reflected wave being shifted in frequency because of the Doppler effect. The difference between the frequencies of the transmitted and reflected waves can be transformed into an audio signal, the frequency of which changes because of the changing motion of the meteor relative to the receiver. Whistling meteors get their name from the sound they produce in radio receivers, not because they are directly audible (although some meteors are).