When two oscillators are tuned to very nearly – but not quite – the same frequency, the difference between them causes an interference pattern known as beating. When the difference in frequency is below the audio rate, this can sound like a tremolo applied to the loudness of the combined sound. When the difference is in the audio range, a third tone appears that is the difference between the two – which can be quite annoying.
Analog oscillators in particular are known to have imperfections in the way their pitch tracks a control voltage, causing this beating to appear at different notes – particularly higher ones. What some may consider an imperfection has become a sought-after artifact by many to create a more “organic” sound; you can purposely detune one oscillator’s pitch compared to another to create this effect.
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