Beat frequencies are generated by two overlaid waves at different frequencies. If for e.g. you play back a 400hz wave and a 406hz wave, your ear will hear a wave at 6 hz.

For the ear to hear a beat frequency does not require a playback mechanism capable of reproducing a carrier at that frequency. Specifically, to hear a 6hz (cycles per second) frequency using ~400hz carrier waves, any speaker or headphone will suffice. Here, i show yuu.

http://pupnik.de/beat_frequency_6hz_at_400hz.mp3