3) The speed of the person divided by the speed of sound in air (about 300 m/s) determines the Doppler shift. The round trip distance for the sound is actually increasing/decreasing at twice the rate of the person's movement. So if the person is moving at 1 m/s the Doppler shift is 2/300. Multiply this figure by 40 kHz and you have a shift (40,000*2/300), about 270 Hz. (1 m/s is probably the fastest you'll see someone move indoors; 0.05 m/s is probably a good lower bound, or 13 Hz.)