Beats are a phenomenon of importance in acoustics and radio resulting from the interference of two wave-trains of similar frequency in which a new periodicity is set up in the aggregate amplitude having frequency equal to the difference of the two constituent frequencies. Beats between two musical notes of similar pitch can often be heard as an unpleasant throbbing; beating between two ultrasonic tones may result in an audible tone.
In the case of two turbine aircraft engines that are rotating at nearly the same frequency but not precisely at the same frequency, four frequencies are generated. These are: (f1) the rotational frequency of turbine one, (f2) the rotational frequency of turbine two, (f1 + f2) the sum of turbine rotational frequencies one and two, and (f1 - f1) which is the difference or "beat" frequency of turbines one and two. The difference of the two frequencies is the lower frequency and is the one that is "felt" as a beat or "wow" in this case.