Frequency modulation

Frequency modulation refers to using the output of a low-frequency oscillator to continually alter (modulate) the frequency of another oscillator. This example provides the user control of the amplitude and frequency of both the "carrier" oscillator (the one we hear directly) and the "modulator" oscillator (the effect of which we hear indirectly). The output of the modulating oscillator is added to a constant (the main frequency), thus causing the carrier frequency to fluctuate up and down around that central frequency.

Linear frequency vs. linear pitch

As expressed in Fechner's law, our subjective sensation of a phenomenon is often proportional to the logarithm of the empirical measurements of the intensity of the physical event that evokes that sensation. One example of that in musical contexts is that our sensation of changes in musical pitch are proportional to the logarithm of the change in the measured fundamental frequency of a tone.