Dolphins emit clicks of sound for communication and echolocation. A marine biologist is monitoring a dolphin swimming in seawater where the speed of sound is 1518 m/s. When the dolphin is swimming directly away at 9.1 m/s, the marine biologist measures the number of clicks occuring per second to be at a frequency of 2040 Hz. What is the difference (in Hz) between this frequency and the number of clicks per second actually emitted by the dolphin?