A car traveling at 22.1 m/s honks its horn as it directly approaches the side of a large building. The horn produces a long sustained note of frequency f0 = 235 Hz. The sound is reflected off the building back to the car's driver. The sound wave from the original note and that reflected off the building combine to create a beat frequency. What is the beat frequency that the driver hears (which tells him that he had better hit the brakes!)? Assume the speed of sound in air is 340 m/s.