CHAPTER 18 Sound System Equalization Sound System Engineering by Don and Carolyn Davis The original one-third octave band rejection filter set utilizing summing circuitry was first used by one of the authors in 1967 and the patent 3,624,298 was filed in March 1969 and issued in November 1971. Since its inception the basic problems in its correct use have been two fold: first, the ability to design a sound system capable of benefiting from the use of an equalizer, and second, the attempt to equalize the unequalizable. These problems are with us more than 40 years later. SYSTEM CRITERIA