A low-pass filter would filter everything out assuming the threshold is below the two frequency. The beat frequency of the two tones would indeed by 7kHz, but i

A low-pass filter would filter everything out assuming the threshold is below the two frequency. The beat frequency of the two tones would indeed by 7kHz, but if you think of the resulting waveform, the contingent parts are smaller, faster oscillations that that would be suppressed by the filter.