Early equalizers were fixed and integrated into the circuits of audio receivers or phonograph playback systems. The advent of motion picture sound saw the emergence of variable equalization. Notably, John Volkman’s external equalizer design from the 1930s featured a set of selectable frequencies with boosts and cuts, and is sometimes considered to be the first operator-variable equalizer.
Throughout the 1950s and 1960s, equalizers grew in popularity, finding applications in sound post-production and speech enhancement. The Langevin Model EQ-251A, an early program equalizer with slide controls, was a precursor to the graphic equalizer. One slider controlled a bass shelving filter, and the other provided peaking boost/cut with four switchable center frequencies. Each filter had switchable frequencies and used a 15-position slide switch to adjust the gain. Cinema Engineering introduced the first graphic equalizer. It could adjust six bands with a boost or cut range . However, with graphic equalizers, engineers were still limited to the constraints imposed by the number and location of bands.
By 1967, Saul Walker introduced the API 550A equalizer, whose bandwidth is inherently altered relative to the amount of signal boosted. This EQ, like others of its time, featured a fixed selection of frequencies, and variable boost or cut controls at those frequencies. In 1971, Daniel Flickinger invented an important tunable equalizer. His circuit, known as `sweepable EQ’, allowed arbitrary selection of frequency and gain in three overlapping bands.
In 1966, Burgess Macneal and George Massenburg began work on a new recording console. Macneal and Massenburg, who was still a teenager, conceptualized an idea for a sweep-tunable EQ that would avoid inductors and switches. Soon after, Bob Meushaw, a friend of Massenburg, built a three-band, frequency adjustable, fixed-Q equalizer. When asked who invented the parametric equalizer, Massenburg stated “four people could possibly lay claim to the modern concept: Bob Meushaw, Burgess Macneal, Daniel Flickinger, and myself… Our (Bob’s, Burgess’ and my) sweep-tunable EQ was borne, more or less, out of an idea that Burgess and I had around 1966 or 1967 for an EQ… three controls adjusting, independently, the parameters for each of three bands for a recording console… I wrote and delivered the AES paper on Parametrics at the Los Angeles show in 1972… It’s the first mention of `Parametric’ associated with sweep-tunable EQ.”
- Bohn, D.A. Operator adjustable equalizers: An overview. In Proc. Audio Eng. Soc. 6th Int. Conf.: Sound Reinforcement; 1988; pp. 369–381.
- Reiss, J.D.; McPherson, A. Filter effects (Chapter 4). In Audio Effects: Theory, Implementation and Application; CRC Press: Boca Raton, FL, USA, 2015; pp. 89–124.
- Flickinger, D. Amplifier system utilizing regenerative and degenerative feedback to shape the frequency response. U.S. Patent #3,752,928 1973.
- Massenburg, G. Parametric equalization. In Proc. Audio Eng. Soc. 42nd Conv.; 1972.