Two wavelengths apart, different frequencies
The following animation shows the patterns of constructive and destructive interference that are produced by two point sources of equal power, rhythmically expanding and contracting in phase and with different frequencies. The two point sources are separated by two wavelengths.
It can be seen that sources of different frequency produce a spatial pattern that changes at the beat (difference) frequency of the sources. To see this try placing your mouse pointer at a point in the field and see how the pattern around it changes with time.