See also RF (radio frequency).
AF (audio frequency) (also abbreviated af or a.f.) refers to alternating current ( AC ) having a frequency such that, if applied to a transducer such as a loudspeaker or headset, will produce acoustic waves within the range of human hearing. The AF range is generally considered to be from 20 Hz to 20,000 Hz.
All telephone circuits operate with AF signals in a restricted range of approximately 200 Hz to 3000 Hz. A telephone-line modem is an AF device that converts binary digital data into analog signals that can be transmitted over the telephone circuit, and also converts incoming AF signals into binary digital data.