Both those problems can be avoided at a stroke, says James Sabatier, a physicist at the University of Mississippi, if you listen for the ultrasonic signals associated with footsteps. Ultrasonic microphones are not as badly affected by wind noise, says Sabatier. He also proposes an ultrasonic sonar system that broadcasts a signal and listens for Doppler shifts that indicate echoes from a moving pair of feet. The strategy is also used by bats to detect the flutter of insect wings.

In addition to footsteps, Sabatier points out that the movement of a walking person's arms towards and away from the ultrasonic source should produce the same effect. The inventors hope to interest groups involved in various security applications such as border control.