A listener at rest with respect to the air and the ground hears a sound of frequency $$f_1$$ from a source moving towards him with a velocity of $$15\ \text{m s}^{-1}$$ towards East. If the listener now moves towards the approaching source with a velocity of $$25\ \text{m s}^{-1}$$ towards West, he hears a frequency $$f_2$$ that differs from $$f_1$$ by $$40\ \text{Hz}$$. The frequency $$f$$ of the sound produced by the source is, taking the speed of sound in air as $$340\ \text{m s}^{-1}$$,