This computer model illustrates the Doppler effect ? a physical phenomenon that occurs when the sound source and (or) the listener move with respect to the medium. The relation of frequency of the sound waves fL (perceived by the listener) to the frequency of the sound waves fS (emitted by the source) is explained by the formula fL = ((v + vL)/(v + vS))fS (*), Where v is the speed of sound in the medium (v>0) , and vS and vL are the velocities of the source and of the listener in respect to the medium. The movement of the listener towards the source while the source is moving in the same direction is considered to be the positive direction of vS and vL. Modify the velocities and the directions of the source and the listener?s motions, as well as the frequency fS of the sound emitted by the source, and observe the result. The speed of sound in the medium is assumed to be constant and equal to v = 340m/s, which corresponds to the speed of sound in air under standard atmospheric pressure at 20?C. Computer calculates the frequency fL of the sound as perceived by the listener, and the change of the frequency Δf = fL - fS Note that in this model, the velocities of the source and of the listener are given in terms of projections onto the horizontal axis X, and are directed from left to right.