 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 . 
 Constant k filters <a href='uninflect' title='1'><mark>were</mark></a> invented by George Campbell . He published his work in 1922 , but <a href='been_done' title='2'><mark>had</mark></a> 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 <a href='null_genetive' title='3'><mark>'s</mark></a>filters <a href='uninflect' title='4'><mark>were</mark></a> far superior to the simpler single element circuits <a href='null_relcl' title='5'><mark>that</mark></a><a href='been_done' title='6'><mark>had</mark></a> been used previously . Campbell called his filters electric wave filters , but this term later came to mean any filter <a href='null_relcl' title='7'><mark>that</mark></a><a href='uninflect' title='8'><mark>passes</mark></a> waves of some frequencies but not others . Many new forms of wave filter <a href='uninflect' title='9'><mark>were</mark></a> subsequently invented ; an early ( and important ) variation was the m @-@ derived filter by Zobel <a href='null_relcl' title='10'><mark>who</mark></a>coined the term constant k for the Campbell filter in order to distinguish them . 
Constant k filters was invented by George Campbell . He published his work in 1922 , but done clearly invented da filters sum time before , as his colleague at AT & T Co , Otto Zobel , was already makin improvements to tdadesign at dis time . Campbell  filters was far superior to thdaimpler single element circuits done been used previously . Campbell called his filters electric wave filters , but this term later came to mean any filter pass waves of some frequencies but not others . Many new forms of wave filter was subsequently invented ; an early ( and important ) variation was theda@-@ derived filter by Zobel coined the term constant k for the Campbell filter in order to distinguish them .