When you listen to the same material through different headphones, you’ll hear differences that are due in part to “sweetening.” Sweetening refers to the EQing of the headphones to make the music sound better. In open-backed headphones and many earbuds, for instance, the bass frequencies may be emphasized to counter the natural leakage of bass through the open back or ear canal.
Most general-listening, consumer headphones are sweetened in some way. There are two common sweetening modes: Free Field (FF) and Defined Field (DF). The first simulates an open listening environment without reflection, and the latter simulates an enclosed listening environment such as a room. For critical listening such as monitoring a mix, you don’t want any sweetening at all, but rather a flat frequency response that lets you compare and set levels precisely.
