Equal loudness contour circuit using an operational amplifier
Jack Duane Grimes, Sheri L. Doran · IEEE Transactions on Audio and Electroacoustics · 1970
Without contour compensation, music sources appear to have lower bass and treble response at lower volume levels. This occurs because our ears are less sensitive to the extreme frequencies at low-sound levels (Fletcher-Munson effect). This correspondence describes a circuit that compensates for this effect and is compatible with today's integrated circuit amplifier designs.