 Constant k filters were invented by George Campbell . He published his work in 1922 , but had clearly invented the filters some time before , as his colleague at AT & T Co , Otto Zobel , was already making improvements to the design at this time . Campbell 's filters were far superior to the simpler single element circuits that had been used previously . Campbell called his filters electric wave filters , but this term later came to mean any filter that passes waves of some frequencies but not others . Many new forms of wave filter were subsequently invented ; an early ( and important ) variation was the m @-@ derived filter by Zobel who coined the term constant k for the Campbell filter in order to distinguish them . 
