Your car horn happens to have the unusual property of emitting a pure tone at a frequency of 640. Hz. You drive at 26.0 m/s directly toward a high wall, and sound the horn briefly. After a short time, you hear the echo of the sound, after it was reflected by the wall. Assume the speed of sound is 340 m/s.
What is the frequency of the echo, as perceived by you as you drive the car?