A pair of speakers driven at certain frequencies can display interference in a few ways. If both are set to the same frequency and amplitude (volume), they can together create spatial interference patterns, where they seem to grow louder or quieter depending on the listener's position relative to the speakers. If instead their frequencies are slightly mismatched, their sound waves interfere to create a beat frequency.
The plots below demonstrate how beat frequency arises when two sound waves are created with slightly different frequencies.
