Beats
When two waves of similar frequency and slightly differing frequencys superpose, they two interfere with each other (two maximas canceling e.t.c.), but because the freqencys are different the point at which this occurs is always changing. So for example if a man is in position relative to two source of sound waves of similar amplitude and frequency, he will hear a oscillations in intensity of the sound with time the frequency of this oscillation is the frequency of one wave minus that of the other, remember that a negative sign here has no meaning.