Audio Frequency is abbreviated as AF. Alternating current has the frequency that will produce audio waves, when it applied to the transducer like the headphone, headset or loudspeaker. These audio waves are within the range of human hearing. The frequency range of AF is generally from 20 Hz to 20,000 Hz.
All the telephone circuits operate at AF signals and the frequency range is from 200 Hz to 3000 Hz. Telephone line modem is an AF device. AF device converts binary digital data into the analog signals. AF devices can act as transmitter over the telephone circuit. In this way AF device converts incoming AF signals into the binary digital data.