The two signals which are &quot;interfering&quot; are added together. The amplitude of the resultant signal varies sinusoidally, as the instantaneous phase difference between the two signals goes from 0-&gt;2pi. One way of describing the signal would be that it is a separate tone (sitting in the middle of the two frequencies) being amplitude modulated by a signal at the beat frequency... which is what you hear and why you &quot;hear&quot; the beat frequency) I went with this way of describing the signal because you were talking about &quot;an arbitrarily high frequency signal getting louder and softer 440 times per second&quot; which is the definition of amplitude modulation.<p>Counterintuitively, there is no frequency component generated at the beat frequency when you sum a 1kHz and 1.001kHz signal, its easy to test that out with matlab, octave, scipy&#x2F;numpy&#x2F;matplotlib, etc. Generate the two signals, add them together, and look at the Fourier transform, you&#x27;ll see two components, one at 1kHz and one at 1.001kHz (assuming you take a long enough window to have that type of resolution) and no component at the beat frequency. A third sinewave doesn&#x27;t just jump out of nowhere when you add two separate sinewaves together.<p>If you take the sum of those two signals and run them through an ideal brickwall highpass filter at 999Hz so there are no frequency components below 999Hz, you&#x27;ll still &quot;hear&quot; the beat frequency because it isn&#x27;t a separate spectral component, its just the two signals slowly going out of phase, cancelling eachother out, and then going back in phase and boosting the amplitude.