Ring Modulator
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Ring modulation takes two separate signals and multiplies them. A ring modulator will typically take the input signal from an instrument and mix it with a second signal generated by an internal oscillator of which you can vary the frequency.  

When two sine waves with frequencies A and B are ring-modulated, the output will also contain the frequencies A+B and B-A. If frequency B is not a multiple of A, these additional frequencies are inharmonic; e.g. ring-modulating sine waves at 1000Hz and 1250Hz will add the frequency 2250Hz, which is neither a multiple of 1000Hz, nor of 1250Hz. When more complex sounds are ring-modulated, sums and differences of all the harmonic frequencies are added.  

Moderate frequencies generate eerie, bell-like tones that sound awesome beneath oceans of reverb. Higher frequencies produce ear-shredding shrieks. Meanwhile, ultra-low frequencies (under 10Hz, say) can create pretty, pulsing tremolo effects. But you always need to be careful when you adjust them, as the volume can get very quickly out of control.