All basic sets contain the following: computer; audio interface; and headphone amplifier. The computer contains the software, which runs the listening program. The audio interface converts theaudio signal from digital to analog. The headphone amplifier separates the bone and air conduction channels. The differences in the basic sets allow for a variation in use however, each selection in the basic set series allows you to perform a complete listening therapy program, including both passive and active listening and communication work.