THE DOPPLER EFFECT
When a source of sound waves and a detector are not in relative motion, the
frequency that the source emits matches the frequency that the detector receives.
However, if there is relative motion between the source and the detector, then the
waves that the detector receives are different in frequency. For example, if the
detector moves toward the source, then the detector intercepts the waves at a rate
higher than the one at which they were emitted; the detector hears a higher