Bats use the Doppler effect as a directional guide and to detect insects. As it flies at a speed of 4.80 m/s, a bat emits a brief pulse of 57.0 kHz ultrasound in the forward direction. A nearly stationary insect in front of the bat reflects sound back to the bat. Find the frequency of the sound detected by the bat. (Bats are particularly sensitive to 61.8-kHz sound.)